Sugarcane

ACKNOWLEDGEMENT

PCI Agricultural Services are grateful to the South African Sugar Association Sugarcane Research Institute (SASRI), for permission to use the information contained in their Junior Certificate Course in Sugarcane Agriculture, as the basis for this module.

1. INTRODUCTION

Sugarcane is a giant perennial grass of the genus Saccharum and is grown in tropical and sub‐tropical climates under a wide range of soil conditions. It is indigenous to South and Southeast Asia and was thought to have been cultivated in New Guinea around 6000 BC. Crystallised sugar was reported 5000 years ago in the Indian sub‐continent and in the 8th Century, Arab traders introduced sugar to the Mediterranean lands. Sugarcane was one of the early crops taken to the Americas by the inhabitants of the Canary and Madeira islands.

Sugarcane is one of the very few plants that stores its food reserves in the form of sucrose (sugar) and not as starch. The sucrose is stored in the long fibrous stem which contains 10% ‐ 14% sucrose  on a fresh weight basis. Sugarcane is the plant from which sucrose can most easily and cheaply be extracted in its purest form and provides most of the world’s refined sugar. The crop is of great importance providing approximately 65% of all the sugar produced in the world, the rest being produced from sugar beet, which is grown in cooler more temperate climates. South Africa is a large producer of cane sugar, after countries such as Brazil, Cuba, India, Australia, United States, Philippines and China, and has an annual production of 2.2 million tons.

• EXTERNAL STRUCTURE

The sugarcane plant is a tall perennial grass that reaches maturity within 8 ‐ 24 months. It consists of a number of unbranched stalks that store the sucrose for which the crop is grown. When these sucrose‐bearing stalks are harvested, new ones grow from the cut bases to produce ratoon crops.

The stalk

The stalks are solid, tall and slender and carry 2 rows of leaves. These are divided into joints by nodes, which are where the leaves attach and the buds form. The section between the nodes is known as the internode and the primary aim of the farmer is to cultivate long, thick, sucrose‐rich internodes.

Figure 1: Sugarcane stalk

The leaf

The leaves are arranged alternately, a single leaf arising from each node. They consist of 2 distinct parts; a lower part or sheath, and an upper part or blade.

Trash

Trash is formed when the leaf ages and dies and farmers aim to remove as much of the trash as possible before harvest and milling to reduce handling costs.

The bud

The bud is situated in the root band which is the central section of the node between the leaf scar and growth ring. There is usually 1 bud per node which is situated on alternate sides up the stem.

The root system

There are 3 main types of sugarcane root:

·         Sett roots

Sett roots develop from the root eyes in the root band of the node. They are thin and much branched and provide the developing plant with water and nutrients until the developing shoot roots take over this function.

·         Shoot roots

These develop from the root eyes on the lower nodes of young germinating shoots and are thick, white and fleshy. They are less branched than sett roots and grow faster. Shoot roots take up water and nutrients and provide anchorage for the stalk.

Figure 2: The sugarcane roots

·         Mature roots

The root system of established cane is developed from the root bands of shoots after the initial development of the shoot roots.

There are 3 kinds of roots found in established cane (see the figure below):

Figure 3: The Root System

·         Superficial roots:

These spread out at a shallow level, are unbranched until they have completed extending, after which they branch vigorously. They supply the cane stool with most of its water and nutrients under moist conditions but due to their shallow nature their use is limited in dry conditions.

·         Buttress roots:

White and succulent, growing downwards at an angle which provides anchorage for the plant. They absorb a certain amount of water, but not many nutrients.

·         Rope roots:

Grow straight downwards in strands of 15 ‐ 20 roots. They provide anchorage as well as supplying water to the stem proving especially useful in times of drought.

THE INFLORESCENCE (TASSLE)

When the sugarcane plant reaches a certain stage of development, normally 12 ‐ 24 months, under suitable conditions the growing point may start to produce a flower instead of leaves.

Figure 4: The Inflorescence

• VEGETATIVE PROPAGATION

Sugarcane can be propagated either vegetatively (seedcane or setts) or by means of true seed. In commercial practice vegetative propagation is used, as it is simple in operation and guarantees uniformity of variety. The production of true seed is a more complicated process and normally only undertaken by plant breeders in the development of new varieties.

APICAL DOMINANCE

Normal, growing sugarcane displays the characteristic of apical or top dominance. The growth of buds on the stem is suppressed by the presence of auxins, a class of hormone, in the growth tip. If this tip is damaged or removed, the dormant buds below will begin to develop.

Cane is propagated vegetatively using stem cuttings, or setts. In order to utilise the sugarcane stalk  to produce setts, the effects of apical dominance need to be removed. This is done by simply removing the top of the stalk. The stalk is then cut into short sections (setts) containing between 1 ‐ 5 buds.

CANE SETTS

These are sections of the cane stalk containing buds. Removal of the stalk top eliminates the apical dominance effect and reduces the effect of bud growth suppression. A hot water treatment at 50°C for 2 hours will remove apical dominance and ensure more uniform germination.

In order to entirely eliminate the effects of apical dominance, single bud setts would be ideal, but losses to natural causes such as disease or drying out will result in undesirable gaps in the planting row, so for practical purposes setts of 3 ‐ 5 buds, prepared from young mature stalks, are used.

SETT GERMINATION AND STOOL DEVELOPMENT

The germination rate depends on the cultivar, seed stalk quality, position of the node on the stalk and the soil and climatic conditions.

Primordial: An organ, structure, or tissue in the earliest stage of development   Tiller: A lateral shoot from the base of the stem, esp. in a grass or cereal.

Each cane sett bears root primordia (root eyes) and buds that are capable of producing shoots. The setts also contain sufficient reserves of moisture and nutrients to

support initial root and shoot development. Setts are dipped in fungicide to avoid disease infestation. Setts are laid end to end in the planting furrow ensuring adequate bud germination and shoot development.When the setts are planted, the buds germinate, giving rise to primary shoots.The root primordia develop and give rise to set roots. If whole stalks are planted, apical dominance will ensure that only the uppermost nodes will produce shoots, unless the stalks have been subjected to the necessary heat treatment prior to planting.

TILLERING

The primary shoot, once established, starts to tiller, which is   the

term to describe the development of secondary shoots from the buds at the base of the primary shoot (see the figure below). Tertiary shoots will then develop from the secondary shoots. These tillers form their own root systems and the process will continue until limited by such factors as light, space, restriction of root development or lack of nutrients. The final product is a clump of shoots, all supported by their own root systems.

Only a proportion of the tillers formed will develop into mature canes.

The sett roots will gradually cease to function as the shoot roots take over.

Figure 5: Sugarcane stems

RATOONING

Cane is harvested when the stalks are mature, but preferably before lodging takes place.

Canes are cut at or just below ground level, as the highest levels of sucrose are at the base of the stalk.

The stool buds are slightly below ground level and provide the foundation for the new crop which will be supported by the existing root system until the new shoot roots are able to take over. The regeneration of harvested cane is similar in pattern to the germination of the setts.

• GROWTH AND DEVELOPMENT

EARLY STAGES

During the early stages of growth the sugarcane plant is entirely dependent upon the food and moisture reserves contained in the set. The roots of the sett support the plant until the primary shoot has developed about 6 leaves, after which shoot roots develop from the first node and tillers begin to form.

Cane yield is dependent upon the number of stalks per hectare, their length and thickness. Therefore, good even germination and heavy tillering is essential for a good yield.

A good yield can be ensured by, correct spacing between rows, efficient planting methods and correct planting time, effective weed and pest control, adequate and timeous supply of water and the necessary temperature requirements being met.

A typical yield is 100 ‐ 120 tons of cane per hectare. This equates to 10 ‐ 12 tons of sucrose per hectare at the standard 10% sucrose content. The early growth stages form the foundation for high yields and are a critical phase in the production cycle.

FULL COVER

Full cover is when the cane leaves meet in the inter‐rows and the maximum amount of light is intercepted by the leaves, with virtually no direct sunlight reaching the ground.

This ensures maximum growth and also shade out any weeds, eliminating competition for moisture, nutrients and space. At full canopy the number of green leaves remains relatively constant and the stalks will elongate rapidly under conducive growing conditions. Sucrose will steadily accumulate in the stalks, with the upper parts containing less sucrose than the middle and lower sections.

At a later stage of growth the rate of both stalk elongation and sucrose accumulation will decline. Once the plant has reached a certain size it can proceed to the reproduction (flowering) phase.

FLOWERING

Flowering is mainly initiated according to day length and sugarcane will begin to form flowers when the day length is 12.5 hours. In South Africa this occurs between March 5th and March 17th. The flowers are most obvious during July and August. Certain varieties of cane, such as N14 and NCo 293 are more likely to flower than other varieties. The varieties NCo 376 and N19 hardly ever produce flowers.

In addition to day length there are other factors that influence the onset of flowering. The stalks must have several mature internodes, night temperatures should be at 21°C, water supply should be adequate and the plant should not be under any stress.

HANDLING FLOWERED CANE

Flowering has both beneficial and detrimental effects on cane and sucrose yields.

BENEFICIAL EFFECTS

Flowered stalks are slightly heavier than the average non‐flowered stalks in fields where up to 50%  of the stalks have flowered. Flowering induces natural ripening by restricting the number of internodes being formed and channelling the photosynthate into sucrose. In South Africa flowered fields yield more sucrose than non‐flowered fields if harvested before the end of September.

      Eldana: Also known as the African sugar‐cane borer. They bore the stems of their host plant DETRIMENTAL EFFECTS

Estimates of cane yields can be inaccurate, due to the height of the cane being exaggerated by the long, low‐sucrose tops, which are discarded.

Artificial cane ripeners can be ineffective as the flowers prevent the ripening material from settling on the leaves.

Flowered cane must produce side shoots for the stalk to remain alive. The flower dies after a while. The overall sucrose content decreases as the side shoots grow as they contain little sucrose. Side shoots negatively affect the output of the cane cutters. Flowered stalks do not produce side shoots when infested with eldana. The stalk die resulting in a loss of yield. In South Africa from October onwards, non‐flowered stalks contain greater amounts of sucrose than flowered stalks.

MANAGEMENT OF HARVEST

If the percentage of flowering throughout the field is greater than 25% harvest must be before the end of September.

If flowering is less than 25% in one part of the field only the part that has flowered should be harvested.

If flowering is less than 20% the cane can be left as stand‐over and be harvested the following year.

• CLIMATE AND GROWTH

Sugar cane is ideally grown in the humid regions of the tropics, in the sub‐tropics and in warm temperate regions of the world. In Southern Africa sugarcane is most successfully grown in the hot, humid, low‐lying regions, such as Kwa‐Zulu Natal and Lowveld regions (Mpumalanga) in South Africa, South‐Eastern Mozambique and Zimbabwe.

Rainfall is the main factor limiting the growth of sugarcane in South Africa. During the growing season the plant requires a relatively high and well‐distributed rainfall of around 1 200 mm per annum.

Sugarcane grows best at mean daily temperatures between 30°C and 35°C. Lower temperatures will slow germination, lead to reduced stalk elongation and lower yields.

Light intensity and day length influence stem elongation, stalk thickness and the amount of tillering. In poor light intensity, stalks are thinner and longer and the leaves yellowish in colour.

• SUCROSE AND THE FARMER

The farmer should aim to produce maximum yields of sucrose per hectare and not necessarily maximum yields of cane per hectare. To obtain high sucrose yields the cane yield must also be high, but if the sucrose content is low the farmer will not achieve maximum profits. Due to the high costs of harvesting and transporting it is obviously more profitable to cut 8 tons of cane to produce 1 ton of sucrose than to cut 10 tons for the same yield. Over‐fertilisation is a waste of money if it only produces high cane tonnages with low sucrose yields.

The top part of the stem where the green leaves are attached is said to be immature and it is in this section that growth is taking place and the sucrose content is low compared to the rest of the stalk. When the cane stalk is short the amount of immature to mature stalk is high. As the stalk grows the proportion of immature to mature stalk decreases which results in a higher average sucrose percentage for the whole stalk.

• VARIETIES AND BREEDING

The objectives of the SASRI breeding programme are to produce the most profitable sugarcane varieties for the industry’s climatic and environmental conditions, in terms of recoverable sugar per hectare and for these varieties to have adequate resistance to the pests and diseases of economic importance.

The task of selecting and breeding a new variety takes between 12 ‐ 15 years and involves the detailed study of the main characteristics of the variety, namely:

Estimated recoverable sucrose content:

• Cane yield;
  • Sucrose yield;
  • Ratooning ability;
  • Resistance to diseases; smut, mosaic and leaf scald;
  • Resistance to eldana;
  • Erectness and resistance to lodging; and
  • Proneness to flowering.

SUGARCANE VARIETIES IN SOUTH AFRICA

The varieties of sugarcane bred by the South African Sugarcane Research Institute, or imported by SASRI for propagation for planting commercially in South Africa are known as released varieties and are proclaimed in various issues of the Government Gazette. The cultivation of any variety not gazetted is illegal.

A full list of released varieties is available from the South African Sugarcane Research Institute at Mount Edgecombe, or PCI Agricultural Services in Johannesburg.

CHOICE OF VARIETIES

The choice of varieties for planting in any particular area is firstly restricted to the varieties which are permitted for planting in that area and, secondly to those which are best suited and recommended for that particular area. These variety lists are reviewed annually and updated recommendations are available from the South African Sugarcane Research Institute.

The varieties most widely planted in South Africa are NCo376, N12, N14, N19, N23, N25 and N27.

ACKNOWLEDGEMENT:

PCI Agricultural Services are indebted to the South African Sugar Association Sugarcane Research Institute (SASRI) for permission to use the information contained in their Junior Certificate Course in Sugarcane Agriculture as the basis for this module.

1. LAND PREPARATION

FIELD DRAINAGE

In order to grow a healthy crop the soil needs to have a correct balance between air and water. If there is too much air the crop will suffer from drought, with too much water the crop will drown. Surplus water should be able to move freely through the soil profile.

Detailed information regarding field drainage is outside the scope of this module and is presented in a separate, dedicated module.

LAND PREPARATION

In sugarcane production the main objectives in land preparation are the complete removal of unwanted vegetation including residual material (stools) from the previous crop, and the preparation of a suitable seedbed. These operations should always be carried out with soil conservation uppermost in mind.

Eradication of the previous crop is necessary to control diseases such as ratoon stunting disease (RSD), smut and mosaic and to interrupt insect and nematode pest life cycles.

The 4 recommended methods of stool eradication are as follows:

·         Mechanical

The old stools are lifted out of the ground and left on the soil surface to dry out and die.

·         Chemical

The most commonly used chemical for killing sugarcane is glyphosate, which is relatively environmentally friendly and leaves no long‐term residue in the soil. Chemical stool eradication allows for minimum tillage to be utilised at planting, which improves long‐term sustainability by reducing soil erosion.

·         Hand hoeing

This is a very effective method of eradicating unwanted cane stools, but is very labour intensive, requiring roughly 40 man‐days per hectare.

COMBINATION TILLAGE

Excellent results can be achieved by combining chemical and mechanical methods. The old crop is sprayed with glyphosate and left to die off in the ground. A couple of weeks later a stool plough fitted with a depth wheel or skid is deployed to sever the stool from its root system at a depth of 80 mm. The soil is not turned over and the old stool remains in place, reducing the risk of erosion.

Follow‐up operations to completely eradicate volunteers will be necessary in all of these methods and the planting of a green manure crop between two cane crops is strongly recommended.

MINIMUM TILLAGE

Experiments have demonstrated that deep soil disturbance is not necessary in land preparation and SASRI trials shows that cane planted directly into the old interrow will yield as well or better than cane planted in the conventional manner. It is sound economics to prepare a fine tilth only where it  is required; in a narrow band of approximately 200 mm in the old inter‐row which can be achieved

using discs in light soils and rotary hoes in heavier soils.

Minimum tillage will substantially reduce soil erosion, conserve soil structure and organic matter, prevent moisture and nutrient loss, reduce preparation costs and increase yields.

A well nurtured crop and associated healthy root system is the best way to restore the physical properties of misused soil.

Maximum tillage (ploughing and harrowing) is not recommended by SASRI on highly erodible soils. In South Africa no cultivation at all is permitted on these soils on slopes greater than 20%.

Detailed practical information regarding land preparation, mechanical stool eradication, chemical application rates, minimum tillage techniques and seedbed preparation is available on request from the South African Sugarcane Research Institute and PCI Agricultural Services.

• PLANTING

It is vitally important that the planting operation is carefully planned well ahead of time as it influences the potential productivity of the field for an entire crop cycle, which could be as long as  10 ‐ 15 years. Precise planting dates will vary between bioclimatic regions and is primarily a function of soil temperature, available moisture and the likely incidence of mosaic disease. Detailed planting dates for individual areas should be sought from the South African Sugarcane Research Institute or from PCI Agricultural Services.

ROW SPACING

Optimum row spacing depends on factors such as variety, bioclimatic region, land slope, soils, irrigation and harvesting technique. Experiments have shown that where moisture stress is not too severe, sugarcane yield increases as the distance between the rows decreases, within certain limits. In practice, a row spacing of 1 metre is as close as field equipment will conveniently allow.

Table 1: Row Spacing

Close row spacing	Between 1.0 m and 1.1 m
Intermediate row spacing	Between 1.2 m and 1.3 m
Wide row spacing ‐	1.4 m

BIOCLIMATIC REGION

In the cooler growing areas the growth rate is comparatively slow, the time to canopy is longer and the weed problem period is consequently longer. Closer row spacing is more practical.

In the hotter arid regions where sugarcane is grown under irrigation and leaf canopy is more rapid, wide row spacing is optimum.

In the coastal lowlands of South Africa an intermediate row spacing is more common.

Land slope:

The erosion hazard is greatest on steeper slopes; therefore a quick canopy is required to protect the soil. This would indicate close row spacing.

Soil type:

Where soils are shallow and rainfall is low, close row spacing will cause undue moisture stress therefore intermediate row spacings would be optimum.

On the more erodible soils closer row spacing should be used.

Variety:

Where varieties with erect leaves are planted, row spacing should be closer to speed up canopy closure.

It is most important that rows are equidistant regardless of the row spacing selected. Parallel rows are essential for efficient mechanical weeding, spraying, harvesting, etc.

Tramline planting:

This should only be considered in mechanical harvesting situations to accommodate the necessary machinery and equipment. The rate of canopy closure would be very slow in the wide interrow with subsequent weed control problems.

ROW SPACING AND CROP CHARACTERISTICS

With close row spacing the competition for light results in tall, thin stalks that are pale in colour with a marked tendency to lodge. Stalk populations reach a peak at canopy stage but subsequently stabilise at a lower level as a result of stalk mortality due to the intense competition for light. Stalk populations at wide row spacings take longer to peak and have a less pronounced peak but end up with a final population only slightly lower than that of close row spacing.

The planting furrow

The depth of the planting furrow, measured from the original soil level to the bottom of the furrow, should be approximately 100 mm. The deeper furrows lower the soil temperature and slow the germination.

For herbicides to function efficiently the fields should be left as smooth as possible after planting, which is impossible if the furrows are too deep. Improved germination is obtained if the planting operation is completed quickly and drying of the soil and avoided. Ideally the furrow should not be left open for more than a few hours, particularly in heavier soils.

The germination will improve with a fine seedbed with minimal air pockets.Fertiliser should be placed in a band in the bottom of the furrow immediately before planting, and distributed evenly in order to ensure even growth. The fertiliser should always be covered with a layer of soil to prevent direct contact with and damage to, the germinating buds.

SEEDCANE

Seedcane quality is vitally important to ensure profitable and sustainable crop production over the lifespan of the planted field. Seedcane should be free from diseases and pests and have varietal purity and germination capacity. Young, well grown seedcane, 9 ‐ 12 months of age, from carefully managed nurseries established with heat‐treated stock, is ideal. Seedcane should be topped below the growing point to ensure quick and uniform germination.

Approximately 2 to 3 hectares of nursery are required to plant 100 hectares of commercial cane.

Seedcane treatment:

Seedcane should be subjected to hot water treatment (50°C for 2 hours), ensuring the elimination of Ratoon Stunting Disease (RSD).

Autumn and winter planted cane is slow to germinate and more prone to fungal attack than summer planted cane. SASRI recommends that all autumn, winter and early Spring planted cane is dipped in  a fungicide solution, such as Benlate (7.5 g/10 litres water), prior to planting.

Sandy soils which are prone to nematode infestation should be treated at planting with a recommended nematicide, such as Temik (20 kg/ha) or diafuran (30 kg/ha).

Detailed information regarding recommended fungicides and nematicides and their application rates is available from SASRI and PCI Agri.

Seed rate:

A rough guide is 8 ‐ 9 tons of seed cane per hectare, depending on the variety. The setts should overlap by 30% in the furrow and more if conditions are unfavourable.

Sett length:

Seedcane is usually cut into setts of approximately 40 cm long. Whole stalks may be used but it is necessary to chop these into setts in the furrow. This process will disrupt the apical dominance and encourage the buds to germinate but may spread RSD (ratoon stunting disease) if it is present. To ensure that the seedcane is free from disease the cane knives are dipped frequently in a disinfectant solution.

Planting with entire unchopped stalks is only likely to succeed if relatively young cane is used, as the effect of apical dominance appears to be less marked in young cane. The growing point must be cut off to stimulate germination of the buds.

Trash removal:

When using whole stalks and chopping in the furrow trash need not be removed, but when using setts and dipping, it should be removed to prevent wastage of fungicide.

The farmer should never attempt to economise with seedcane, as good germination with no gaps  is essential to obtain maximum yields for the duration of the crop cycle.

COVERING

The depth of soil covering the setts should be between 30 mm and 50 mm and must be uniform to ensure even germination. Covering with too much soil will delay germination.

Light consolidation of the soil over the row facilitates good sett/soil contact.

PLANTING METHODS

Hand planting:

The conventional method of planting entails a furrow being drawn by a tractor using a potato‐ridger type implement. The seed cane is placed manually into the furrows and covered either by tractor or by hand. Fertiliser hoppers are often attached to the ridger to combine the two operations but fertiliser may also be applied by hand.

In sandy soil on flat fields, roughly 12 to 15 workers will be required to plant 1 hectare in 1 day. If filter cake or water is applied to the planting furrow a further 15 workers will be needed.

On steep lands where hand ridging is needed an additional 15 ‐ 20 workers will be required to   plant

1 hectare in 1 day.

Therefore, on steep land, where hand ridging is needed and filter cake is applied, about 45 ‐ 50 workers will be necessary to plant 1 hectare per day.

High labour costs and the time spent in planting are disadvantages of the hand planting methods.

Machine planting:

Has many advantages, but can only be used on relatively flat terrain.

An area of 1 ‐ 2 hectares per day can be planted by machine, using 1 tractor driver and 3 machine operators. Further 2 ‐ 3 workers will be needed to prepare the seedcane.

Advantages of machine planting:

• Labour saving;
• Time saving;
• Furrows are opened and closed immediately, avoiding drying out of the soil which results in better germination;
• Consolidation of the soil over the sett by a spring loaded press wheel which also improves germination;
• Uniform depth of planting and covering; and
• Less seedcane used; about 6.5 tons per hectare as opposed to approximately 10 tons per hectare with traditional hand planting.

Disadvantages of machine planting:

• A malfunctioning machine will cause gaps in the cane line or uneven fertiliser distribution;
• Careful planning and close supervision are necessary;
• Operators need to be trained;
• The land should be flat and cleared of stones; and
• Blades need to be sterilised regularly reducing the risk of spreading RSD.

Planting aids:

In rain fed areas and where machine planting cannot be practised, seedcane germination is seldom as high as required. A number of different planting aids have been successfully tested:

Filter cake:

It is common practice to apply filter cake at a rate of 40 tons per hectare over the setts for Autumn, Winter and early Spring planting. Filter cake contains more than 70% moisture when fresh and has high water retention capabilities, drying out to about 60% moisture after 3 ‐ 4 weeks.

This ensures the setts remain in a moist environment even when planting in the dry season.Filter cake will also provide all of the phosphorous and half of the nitrogen required at planting, so fertiliser applications need to be adjusted accordingly.

Water planting:

As little as 2.5 litres of water per metre of open furrow, just prior to closing, substantially improves germination and extends the planting season.

In heavier soils the amount of water can be increased to 3 ‐ 4 litres per running metre.

Whatever planting system is used, the aim must be to achieve a full stand of cane with no gaps so that the full yield potential of the crop may be achieved.

• RATOON MANAGEMENT

A ratoon is the growth that regenerates from a harvested cane crop. The first crop is referred to as plant cane and successive regenerating crops as first, second, third, etc., ratoons.

The management of ratoon crops is similar to that of plant cane. The basic differences are determined by the provision, or otherwise of trash mulch.

Trash comprises all the above‐ground parts of sugarcane other than the harvestable stalks and stubble. A cane field is said to be trashed when all of this material is left on the soil surface to form mulch. It is more convenient and cheaper for most South African farmers to burn their trash at harvest time but this practice markedly affects the long term sustainability of sugarcane farming, particularly on steep slopes and erodible soils.

THE EFFECTS OF TRASHING OR BURNING

Growth characteristics:

Studies have revealed that the growth characteristics of sugarcane are markedly influenced by the provision of a trash blanket, showing slower initial growth and tillering and a lower population of taller and thicker stalks than ratooned cane on burnt ground.

Cane yield:

Over the long term an overall yield increase of approximately 9 tons per hectare can be expected from fields provided with a mulch blanket.

Soil temperature:

Soil temperatures monitored under a trash blanket remain relatively constant and slightly lower  than in bare soil, although frost will be more likely in colder areas.

Soil characteristics:

A gradual increase in soil organic matter is likely to occur as soil organisms break down the dry matter and incorporate it into the soil structure.

Advantages:

Increased cane yield, reduced surface water run‐off, soil erosion, surface capping and compaction, evaporation, suppression of weed growth and a favourable environment for soil organisms.

Disadvantages:

Lower cutter output and difficulties with cane handling. Delays in ratoon regeneration and possible exacerbation of existing eldana infestations.

MANAGEMENT OF RATOON FIELDS

Trashed fields following harvest:

• Gleaning of cane stalks from under the trash blanket to avoid losses;
  • Spreading of trash evenly over the harvested land;
  • Application of fertiliser top dressing as soon as the cane rows are visible;
  • Weed control by hand or herbicide application; and
  • Scouting for diseases in the young ratoon cane and eldana borers in older cane.

Burnt fields following harvest:

• Scattering of burnt cane tops as a mulch blanket evenly over the field;
  • Application of herbicides to control emerging weeds;

• Fertiliser application as per recommendations;
  • Erodible soils and steep slopes should not be subjected to cultivation; and
  • Scouting for diseases in the young ratoon cane and eldana borers in older cane.

ADDITIONAL OPERATIONS IN RATOON FIELDS

Prevention of compaction:

Due to the high usage of transport machinery over the growing areas during the removal of the harvested cane crop, compaction is extremely difficult to avoid. However, the repair of compacted soil prior to the plough‐out of the field is extremely difficult and steps should rather be taken to prevent or limit compaction from occurring.

Suggested steps are as follows:

• Ensure that irrigated fields are well dried off prior to harvest, as wet soil will compact far more easily than dry soil;
  • Harvest fields grown on compaction‐prone soils during the dry season;
  • Ensure that fields are adequately drained;
  • Increase the soil organic matter by incorporating a green manure crop and leaving a mulch blanket of cane trash. Position trash on traffic lanes;
  • Ensure that infield transport does not pass over the cane rows;
  • Use lightweight cane haulage vehicles fitted with large, high flotation tyres at low pressures;
  • Use walking beam axles on trailers and weight transfer hitches on tractors;
  • Limit wheel slip; and
  • Ensure that trailers being mechanically loaded are kept moving and not loaded in one spot.

Cultivation:

Weeds in the interrow can be effectively controlled with tine or disc cultivators, taking care to avoid excessive loosening of the soil and subsequent erosion. Care should also be taken to avoid root damage.

Recommended herbicides are a safer and more economical solution.

A wise philosophy in ratoon management is to reduce the number of field operations to the bare minimum for the sake of economy and sustainability.

ACKNOWLEDGEMENT:

PCI Agricultural Services are gratefulto the South African Sugar Association Sugarcane Research Institute (SASRI) for permission to use the information contained in their Junior Certificate Course in Sugarcane Agriculture as the basis for this module.

1. PESTS

It is important to remember that any pest is only of economic importance above a  certain  population density. Control measures are usually designed to lower the population to a density at which it is no longer an economic pest.

SUGARCANE INSECT PEST CONTROL

There are four main approaches to the control of any pest:

Ecological:

This involves altering the situation to make it less suitable for the pest, for example:

• Trash caterpillar ‐ burn the trash at harvest rather than keeping it in the field.
  • Mosaic disease ‐ which is carried by the maize aphid. Avoid planting maize near cane fields.
  • Numicia (Green Leaf Sucker) ‐ The nymph stage is unable to fly, so it is recommended to harvest when the maximum numbers of nymphs are present.
  • Eldana borers ‐ avoid having stand‐over cane.

Biological:

This involves encouraging naturally‐occurring and introduced enemies of the pest, mainly parasites, predators and harmful fungi. Examples are:

• Numicia eggs are attacked by 2 known parasites.
  • Ant populations attack eldana eggs and young larvae.
  • Trash caterpillar outbreaks are often controlled by a fungus.

Chemical:

This involves insecticides, pheromones, attractants and repellents.

Insecticides should always be selectively applied with great caution to avoid destroying the natural enemies of the pest and worsening the situation in the long run.

Integrated Pest Management:

Utilising a combination of the above methods for the greatest effect, for example:

• Careful timing of selective insecticide application to strike the pest at its most vulnerable stage without adversely affecting its natural enemies.

Insecticides:

Classified according to their type of action.

Stomach:

These kill when they are eaten by the pest, e.g. sevin, malathion, sodium fluorosilicate, et al.

Contact:

Kill on contact with the pest, e.g. sevin, malathion, dieldrin, oil sprays, et al.

Systemic:

These are taken up by the plants and kill the insects that feed on the poisoned sap, e.g.; rogor, metasystox, et al.

Insecticides may be persistent and remain effective for a number of years (dieldrin), or non‐ persistent and last for only a few days after application (malathion).

An insecticide may be broad spectrum, in that it will kill a wide range of insects (dieldrin, malathion), or specific or narrow spectrum, only targeting one insect type (Pirimor – aphids).

Insecticides are made in various different formulations:

• Dusts, which are applied dry;
• Wettable powders, which are mixed with water before application;
• Emulsifiable concentrates which are mixed with water or applied as an ultra‐low volume (ULV) concentrate; and
• Baits which incorporate an attractant, such as bran.

Points to consider when choosing an insecticide:

• Presence of natural enemies: This may suggest the use of a narrow spectrum, non‐ persistent insecticide;
• Mammalian toxicity: This could favour the use of ‘softer’ products such as sevin or malathion, rather than more toxic substances like dieldrin or parathion;
• Formulation: Should be chosen to suit the prevailing circumstances and preferences of the farmer; and
• Persistence: Days to harvest and safety periods should be considered.

All insecticides used in South Africa are registered with the Department of Agriculture and every  year a booklet is published, namely A Guide to the use of Pesticides and Fungicides, which provides detailed information regarding agricultural pests and diseases and the appropriate chemical control procedures.

Care should be taken when handling insecticides and the safety, handling and treatment instructions on the label should be studied and adhered to.

Sugarcane pests are classified according to their habits or mode of attack, i.e. leaf eaters, leaf suckers, borers and soil pests.

LEAF EATERS

Trash Worms:

Also known as ratoon worms, are the larval stages of approximately seven different moth species, with Mythimna phaea being the most responsible for damaging young ratoons.

Figure 3: Lifecycle of the Trash worm

These caterpillars lie dormant under the post‐harvest trash blanket during the day and feed on the leaves of the young ratoons at night.

There are many natural enemies of this pest but no specific insecticide has yet been registered for its control. Broad spectrum insecticides should be applied judiciously to avoid widespread destruction  of beneficial insects.

Army Worm:

These larvae attack the leaves of young cane before moving on in search of other food.

Figure 4: Lifecycle of the Army worm

They can be controlled naturally by diseases and parasites and it is therefore important not to disrupt the natural predation by injudicious spraying of insecticides.

Red Locust:

Not presently an economic pest of sugarcane in South Africa, but recent outbreaks in Mozambique, Zimbabwe and Swaziland show that there is still potential danger from this pest. Its  breeding grounds are northern Zambia and southern Tanzania, from where it periodically swarms and migrates. Swarms are normally controlled by aerial spraying.

Figure 5: Lifecycle of the Red Locust

LEAF SUCKERS

Green Leaf Sucker (Numicia):

Adults are bright green, about 6 mm long, with flattish wings.

They move by hopping in short, jerky flights. Adult females lay eggs in a row of punctures on the lower side of the leaf midrib. The nymphs that hatch from the eggs are paler than the adult and approximately 2 mm – 3 mm in length.

Figure 6: Lifecycle of the Numicia

Both adults and nymphs feed by sucking sap from the leaf poisoning the plant. The first symptom to be seen is a weakening of the leaf tissue followed by buckling and drooping of the leaves. Later, the leaves become a blotchy yellow colour and often die at the tips and edges. The growing point may  be affected and the top of the stalk becomes soft and flabby.

Cane grown under irrigation in low rainfall areas is most liable to attack as it forms an oasis amongst the surrounding dry grass. Populations are normally kept in check by natural enemies but a severe outbreak can cause considerable losses in yield and a marked reduction in sucrose content.

Nymph numbers can be reduced by burning cane prior to harvest and insecticides should only be applied in extreme cases. Information regarding recommended insecticides should be obtained from SASRI and PCI Agri.

Leaf Hopper:

Adults are about 6 mm long with brown to black wing markings. Nymphs are smaller, relatively broader and wingless. Eggs are laid in batches on the upper surface of the leaf midrib, where they cause clear red blotches. Occasionally batches of eggs are laid on the stems which may cause distortion. Black, sooty mould may develop on the honeydew produced by the leaf hoppers.

Figure 7: Lifecycle of the Leaf Hopper

Yield losses may result from heavy infestations, but the threat of Fiji disease spread by leaf hoppers in other countries, is far more important. Natural control is usually adequate and parasites of the eggs, nymphs and adults have been recorded. In extreme cases advice regarding insecticide applications should be sought from SASRI or PCI Agri.

Sugarcane Aphid:

Adults may be winged or wingless and are soft‐bodied and pale in colour, about 1 mm in length. Females produce young directly without mating or laying eggs. The young are wingless and some remain this way for their entire life span. Others develop two pairs of membranous wings in the  adult stage. Aphids are sap suckers and excrete honeydew on which sooty mould sometimes develops. In heavy infestations the leaf stomata may become blocked by the fungus, thereby slowing growth. As soon as the aphids disappear, clean new leaves are formed and no permanent injury results. Natural biological control is usually adequate, the population being kept in check by various predators, including ladybirds.

Maize Leaf Aphid:

Winged and wingless forms exist, with a life cycle similar to the sugarcane aphid. They are distinguishable by their dark green colour and size, approximately 2 mm in length.

They are best known as a pest of maize, but are also found on sorghum and some wild grasses. They do not like sugarcane and will only survive on it for a few days.

Their importance as a pest lies in their ability to spread mosaic disease from maize to sugarcane. Natural control is sufficient but should be supplemented by keeping maize their principal host plant away from cane fields.

Sugarcane Mealybug:

Soft‐bodied, egg‐shaped pink insects up to 5 mm long. Found in clusters between leaf sheath and stalk where they suck the juice from the nodes.

Males are rare and females can produce fertile eggs without mating. The eggs hatch within an hour of being laid and the active nymphs move upward towards the younger joints.

They are spread to new plants by ants which look after them for the sake of the honeydew that they produce.

A heavy infestation can cause significant stunting of the cane.

A wide variety of natural enemies occur, including a parasitic fungus, caterpillars, maggots and wasps.

STEM BORERS

Eldana Borer:

The caterpillar of an indigenous brown moth it has become a widespread pest of most varieties in South Africa. A characteristic feature of the borer is that it will move backwards or forwards with equal ease. In serious infestations the stem may become completely hollowed out.

Figure 8: The Lifecycle of the Stem Borer

Cane of all ages may be attacked but the most serious damage occurs in cane of 15 months or more (carry‐over or stand‐over cane). Under drought conditions eldana damage may become increasingly severe. Secondary red rot disease may occur. Control by natural enemies is not always   satisfactory.

The egg and larval stages, before boring begins, are the most vulnerable but very few parasites have yet been recorded.

Ants are known to destroy both eggs and young larvae therefore, the use of insecticides have been discouraged to date.

The following eldana borer control measures have been recommended by SASRI:

• Plant into clean land that is free of plant residues;
  • Use clean seed material and inspect to confirm before planting;
  • Avoid susceptible varieties;
  • Harvest as young as possible, cutting at ground level;
  • Remove all substantial stalk residues, leaving only trash or tops;
  • Avoid stressing the cane during the growth period;
  • Avoid excess nitrogen application; and
  • Pre‐trash the maturing crop, especially if milling is to be delayed.

Stalk Borer (Top Grub): Sesamia.

Larvae attack the cane stems, forming burrows in the stem tissue. In mature cane burrows are usually found in the lower nodes.

The life cycle of sesamia is thought to be similar to eldana.

The larvae frequently attack young plant cane when the stools are about 50 cm tall. They damage the growing point causing a symptom known as dead heart, which often leads to the death of the shoot. Borer attack at this stage does not have a marked effect on yield.

Damage to mature cane does not result in significant yield losses except in cane grown at higher altitudes.

Natural control normally prevents serious outbreaks. Although the use of insecticides is not recommended as a rule they are used in high altitude areas.

SOIL PESTS

White Grubs:

The larval stages of 3 types of chafer beetles are recognised as pests of sugarcane.

Figure 9: The Lifecycle of the White Grub

All species cause damage by feeding on cane roots.

Natural controlling effects are unpredictable and unreliable so it is advisable to treat a white grub presence at planting with a suitable soil insecticide. Advice on recommended insecticides can be obtained from SASRI and PCI Agri.

Termites:

Usually feed on fungi which they cultivate on vegetable matter. They do not normally harm cane although setts may suffer attack when planted in sandy soil during a drought. Standard pre‐planting dipping will provide some protection.

NEMATODES

The roots of most plants in practically every soil type throughout the world are host to several species of plant‐parasitic nematodes. These are microscopic worm‐like creatures whose feeding in small numbers has little effect on the root system of their host. However, in large numbers nematode damage will restrict root growth affecting nutrient and water uptake which limits shoot growth. Roots damaged by nematodes are also more susceptible to harmful bacteria and fungi.

The nematode species Pratylenchus, Paratrichordorus, Xiphinema and Meloidogyne are the only species regarded as important pests of sugarcane in South Africa. SASRI research indicates that these species are widespread in sandy soils and can result in suppressed root growth, poor tillering and shoot development and a subsequent reduction in the yield. Nematodes can be controlled, but not eliminated, by applying a nematicide to the soil.

Symptoms of nematode damage:

Above ground symptoms can easily be confused with nutritional or moisture stress:

• Uneven growth;
  • Stunted growth;
  • Reduced tillering;
  • Shorter internodes; and
  • Spiky leaves.

Below ground symptoms are more diagnostic:

Sparse root system; Short, stubby lateral roots and; Swellings or galls on roots. Detailed advice regarding nematode infestations, recommendations related to soil types, application rates, application methods and timing and nematicide handling precautions, should be obtained from SASRI or PCI Agri.

• DISEASES

Diseases can be found in most sugarcane fields in Southern Africa and more than 20 have so far  been identified by SASRI researchers. The most serious of these are Ratoon Stunting Disease (RSD), Smut, Mosaic, Rust, Red Rot and Leaf Scald.

Sugarcane is prone to infection by pathogenic organisms because of several characteristics: Sugarcane is vegetatively propagated, meaning that the stalks of the seedcane are planted and not the true seed. Seedcane can be infected with systemic diseases which will be transferred to the crop. Sugarcane production in Southern Africa is a monoculture, and fields carry the same crop for long periods being ratooned and cultivated on a perennial basis. This situation is favourable for the incubation, overwintering and spread of diseases.

Much of the sugarcane crop in South Africa is grown without irrigation and often suffers from moisture stress, which increases the risk of disease.

DEFINITION OF TERMS

Resistance:

The ability of a plant or variety to withstand the attack of a pathogen and to remain free from the disease caused by that pathogen.

Susceptibility:

This is the inability of a plan or variety to defend itself against attack by a disease organism or overcome the effects of invasion by a pathogen.

Tolerance:

The ability of a plant or variety to endure invasion or infection by a pathogen without showing many symptoms or serious damage.

Systemic:

The term applied to a disease which is found in every part of the plant, from the leaves to the  roots.

Examples of systemic diseases are RSD, smut, mosaic and yellow leaf syndrome (YLS). Systemic diseases re‐develop in an affected field after cutting and are also spread by the planting of infected seedcane.

The most important sugarcane diseases are systemic.

Certain pesticides are said to have systemic activity, which means that they are translocated within the plant after application and uptake.

Vector:

An insect that is the agent of transmission of a viral disease.

Several aphid species are vectors of the mosaic virus and some species of leaf hopper are vectors of YLS and sugarcane streak virus.

PRINCIPLES OF DISEASE CONTROL IN SUGARCANE

Three conditions are necessary for a pathogenic disease to occur; host susceptibility, environmental conditions and the presence of an infectious pathogen.

Control of a disease is achieved by addressing one or more of these factors:

Host susceptibility:

Is reduced by the breeding of resistant sugarcane varieties.

Environmental conditions:

Can be influenced by changing planting dates or irrigating to reduce stress.

Infectious pathogens:

Measures such as the removal of rogue diseased stools, the application of fungicides ensuring the quality of the seedcane.

No single measure is the complete answer for the control of major sugarcane diseases, and an integrated control programme is rather recommended.

The main factors in an integrated disease control programme are as follows:

• Plant good quality, disease‐free seedcane;
• Eliminate volunteer plants before replanting;
• Inspect cane fields and remove rogue and diseased plants;
• Eradicate severely contaminated fields;
• Plant resistant varieties where possible; and
• Efficient crop management.

The main components for the integrated control of the more important diseases are shown in the following table:

Table 2: Components for Integrated Control of Important Diseases

Table 3: Disease Affecting Varieties of Sugarcane

The SASRI Bulletin, Sugarcane Diseases in Southern Africa (2003), is recommended as a source of detailed information and as a guide to disease identification.

The following diseases are discussed in approximate order of economic importance:

RATOON STUNTING DISEASE

RSD is common in all areas and causes a greater overall loss in yield than any other sugarcane  disease in South Africa. Losses are more severe when the crop is under stress. Yields of affected  fields decline progressively with successive ratoons and some varieties are more severely affected than others.

Symptoms:

Diseased stools become stunted, sometimes giving affected fields an uneven  appearance, particularly in ratoon crops. Growth can be noticeably poor under dryland conditions.

Red‐brown dots or streaks at the base of the nodes may be seen when mature stalks are sliced lengthwise.

Control:

• Plant only healthy seedcane and disinfect cane knives and harvester blades;
  • Eradicate volunteer plants completely before replanting; and
  • Although widespread and often damaging, RSD can be controlled relatively easily.

SMUT

Smut is the most important fungal disease of sugarcane in South Africa and is most common in poorly grown cane. Losses in yield increase with successive ratoons and can be very severe in susceptible varieties.

Symptoms:

Dark brown, whip‐like structures usually develop from the tops of infected shoots and stalks. Severely infected stools degenerate into clumps of grass‐like, unmillable shoots.

Control:

• Plant resistant varieties;
  • Plant healthy, disease‐free cane. Rogue affected fields and plough out severely infected fields; Eradicate volunteers before replanting; and
  • When seedcane of susceptible varieties is being heat treated add Bayleton fungicide to the treatment.

MOSAIC

Mosaic is the most important virus disease of sugarcane in South Africa. Severe outbreaks are largely restricted to the cooler areas of the southern coastal hinterland and high altitude inland areas. Mosaic is capable of causing severe yield losses in several important varieties.

Symptoms:

Characteristic mottling of leaves with dark green patches on a lighter green background is more distinctive on younger leaves particularly at the base of the leaf blades. Infected stools tend to have a yellow‐green appearance and may be severely stunted. In some varieties symptoms may be seen on the young internodes of the stalk.

Spread:

Transmitted by several species of aphid. Maize and several grass species act as hosts of mosaic.

Control:

• Plant resistant varieties;
  • Plant healthy seedcane;
  • Control weeds effectively; and
  • Avoid planting and cutting susceptible varieties between mid‐October and the end of January

RUST

Rust is widespread and severe infections are likely to reduce yields significantly. Trust is most severe in young cane during or after prolonged spells of cool, wet weather.

Symptoms:

Orange to brown pustules on the lower surfaces of the leaves. Severely infected leaves may die prematurely and a severely infected field will take on a general orange‐brown colouring. Rust is most conspicuous on young plants.

Spread:

By wind‐blown and rain‐splashed spores.

Control:

• Plant resistant varieties.

YELLOW LEAF SYNDROME (YLS)

Since its re‐appearance in 1994, YLS has become very common in most areas of South Africa. It is often associated with poorly growing or stressed cane, particularly in wet soils in Winter, and may cause yield losses.

Symptoms:

A yellow to yellow‐red colouration of the under surface of the leaf midribs that may extend into the lamina. Most clearly seen on leaves 3 to 6 from the top of the stalk and most frequently seen in maturing cane in the cooler months.

Control:

• Plant YLS tolerant varieties and avoid seedcane with conspicuous symptoms.

RED ROT

Red rot is widespread and common in South Africa. It is most likely to be severe in the cooler areas, particularly in old or stand‐over cane or following frost, drought or borer damage. It can cause a reduction in sucrose content and very severe losses due to premature stalk death.

Symptoms:

An internal red discolouration of the stalk tissues, with characteristic white blotches and elongated red lesions on the leaf midrib. Rotting often occurs at the nodes, affecting the buds. When the disease is advanced cavities may form within the stalk, often containing a grey fungal mycelium. Seriously rotted stalks may die and become ‘mummified’.

Spread:

By wind and rain splash.

Control:

• Avoid susceptible varieties and plant healthy seedcane;
• Avoid stand‐over cane; and
• Cut affected cane early.

LEAF SCALD

Leaf scald is endemic in northern irrigated areas and sporadic outbreaks have occurred elsewhere. Potentially a serious disease.

Symptoms:

Blotchy leaf chlorosis and narrow, sharply defined white lines on the leaves. The leaves wither  (scald) and curl inwards. Red streaks at the nodes within affected stalks or shoots. Side shoots, often with other symptoms, develop at the base of affected stalks. In susceptible varieties growing under stress, stalks, whole stools, or patches of cane, may suddenly wilt and die, possibly without showing other symptoms.Symptoms of smut and leaf scald may often be seen on the same plants in the northern irrigated areas.

Spread:

By rain splash and irrigation water on implements and planting infected seedcane.

Control:

• Plant resistant varieties;
  • Use healthy seedcane;
  • Sterilise cane knives with Jeyes Fluid, especially during seedcane preparation; and
  • Most released varieties are resistant to leaf scald.

PINEAPPLE DISEASE

Fairly common where cane is planted under cool, dry or excessively wet conditions that delay germination. All varieties may be affected and heat‐treated seedcane is particularly susceptible. Economic losses may occur following poor germination in affected fields.

Symptoms:

Failure to germinate or weak growth after planting. Infected setts rot and have a red, eventually black discolouration and a characteristic fruity smell.

Control:

• Treat seedcane with a fungicide, especially when planting in the cooler months and after hot water treatment; and
  • Avoid planting when germination is likely to be delayed.

SOUR ROT

This disease can cause substantial sucrose losses particularly in susceptible varieties following a severe drought late in the growing season.

Symptoms:

Infected stalks rot and have an orange internal discolouration and a characteristic sour odour. The rind of infected stalks turns orange and later black. Coiled black masses of fruiting structures from pustules break through the surface of the rind of affected stalks. Pustules may also be visible on leaf sheaths, midribs and lower areas of the leaf blades.

Control:

• Affected cane should be harvested as soon as possible and over‐mature cane avoided.

POKKAH BOENG

Caused by a systemic fungus, Pokkah Boeng is widespread and can occur on most varieties but usually causes little damage.

Symptoms:

Chlorosis and distortion at the base of young leaves. The spindle may not unfurl properly and the growing point may become distorted and could possibly die.

Control:

• No control measures are necessary at present; and
• Potential new varieties showing susceptibility are discarded during the selection process.

BASAL STEM ROT

Widespread, but only common in the cooler, southern and inland areas of South Africa. Severe damage can occur in small patches but this disease is generally of minor importance.

Symptoms:

Reddish‐brown rotting at the base of the stalk with a white fungal mycelium around and between the basal leaf sheaths. In addition, a brown to green secondary fungus is often seen on the basal leaf sheaths. Infected shoots are often spiky and stunted with brown‐orange leaves causing patches of poor growth. Infected stools are weakened and may die leaving gaps in the row.

Control:

• Plant resistant varieties where this disease is known to be a problem; and
• Ridge up around infected stools.

Several other sugarcane diseases occur in South Africa but are of minor economic importance. These include Sheath Rot, Brown Spot, Ring Spot, Chlorotic Streak and Streak.

Information on these diseases can be found in the SASRI bulletin, Sugarcane Diseases in Southern Africa (2003), or can be obtained directly from SASRI or PCI Agri.

Various non‐pathogenic disorders may cause physical damage to the sugarcane plant, including  frost, lightning, herbicides and mites. A disorder known as Banded Chlorosis can result from sudden spells of cold weather, but is of little economic importance.

RATOON CHLOROSIS

This is caused by iron deficiency induced by high soil alkalinity and can be controlled with ferrous sulphate foliar sprays.

SOOTY MOULD

Is a superficial black fungal growth that develops on the sticky secretions left by aphids, leafhoppers and mealybugs. Since the fungi do not infect the cane they do not cause any direct damage.

ACKNOWLEDGEMENT:

PCI Agricultural Services are gratefulto the South African Sugar Association Sugarcane Research Institute (SASRI) for permission to use the information contained in their Junior Certificate Course in Sugarcane Agriculture as the basis for this module.

1. IRRIGATION

The principles of irrigation are covered in detail in the PCI Agri and the SASRI Junior Certificate course Irrigation modules and will only be mentioned briefly as related to sugarcane agriculture.

PRINCIPLES AND DEFINITIONS

Irrigation is the supply of water to the crop to meet the moisture requirements necessary for the maintenance of uniform growth. In the sugarcane growing areas irrigation is designed to supplement the natural rainfall in order to supply the crop with its total water requirement.

Water requirements of sugarcane:

The total water requirements of a field of cane are affected by the evaporation of water from the  soil in which the cane is growing. Local climatic factors of temperature, radiation, humidity and wind will control the transpiration rate, or water use, of the sugarcane plant. The daily amount of water used in evaporation and transpiration is known as evapotranspiration and is measured to determine the total amount of water required by the sugarcane crop.

Canopy factor:

The water use of a growing cane crop will vary according to the amount of leaf cover, or canopy, as this will have a direct bearing on the rate of evapotranspiration.

Stage of growth	Germination	¼ canopy	½ canopy	¾ canopy	Full canopy
Evapotranspiration as a percentage of Class A pan evaporation.	15% ‐ 40%	55%	70%	80%	85% ‐ 100%

AMC:

Available Moisture Content ‐ Also known as Available Water Capacity.

This is the water that is potentially available for use by the crop. Estimated by determining the texture of the soil. The higher the clay percentage the more water can be held in the soil. AMC is specified as millimetres per metre and is the water content of the soil between Field Capacity (FC) and Permanent Wilting Point (PWP).

TAM:

Total Available Moisture, or Total Available Water.

To estimate the total storage capacity available to the plant the available rooting depth must be determined. Once this is known the total available moisture can be calculated by multiplying the Available Moisture Content by the Effective Rooting Depth (ERD) of a particular soil.

FAM:

Freely Available Moisture. Only about 60% of the TAM is freely available to the sugarcane plant the balance being held too tightly in the soil for the plant to easily extract. Soil moisture should  therefore not be depleted below 60% of TAM and irrigation should be applied when this level is reached.

FC:

Field Capacity is the maximum amount of water that a soil can hold against the pull of gravity.

PWP:

Permanent Wilting Point is the point at which the plants can no longer extract water from the soil profile without wilting.

ERD:

Effective Rooting Depth is the depth of a soil in which approximately 85% ‐ 90% of the crop roots are found.

FACTORS AFFECTING THE DESIGN AND OPERATION OF AN IRRIGATION SCHEME

Crop water requirements:

These will vary according to the following factors:

• Season and region;
• Crop canopy or leaf cover; and
• Rainfall.

Soil conditions:

The total available moisture of the soil, dependant on texture and depth, is partly available for use by the plant when there is no rain or irrigation. Of this reservoir of water, 60% can be used up by the crop before irrigation is required. The size of the reservoir and the crop’s daily water requirements will determine the interval between irrigations.

The infiltration rate, measured in millimetres per hour (mm/hr), is the rate at which the soil will accept water infiltration. This will vary with soil type and is increased by the presence of organic matter in the soil. The application rates of irrigation systems should not exceed the infiltration rate of the soil or run‐off will occur.

The land slope of an irrigated field will limit the type of system that can be utilised as well as the application rate of overhead irrigation systems. The steeper the slope, the slower the application rate must be in order to avoid run‐off.

The quality of the irrigation water should not be overlooked and the water supply should  be regularly analysed to check for the build‐up of harmful salts. Saline or brak soils and sodic areas develop more easily under irrigation with poor quality water and waterlogged soils.

CONTROL

The sugarcane plant requires varying amounts of water according to its stage of growth, the climatic region and the season.

Irrigation scheme design is generally based on the peak demand of the crop and for the remainder of the season it will be necessary to reduce the amount of water applied in accordance with the crop’s reduced requirements, in order to avoid wasting water and possibly promoting waterlogging.

Sprinkler irrigation control consists of 2 main phases:

• Ensuring that the correct amount of water is delivered at the correct pressure to a full set of correctly operating sprinklers, i.e. system maintenance.

• Knowing where to irrigate and when to start and stop irrigating, i.e. irrigation scheduling.

System maintenance:

Any irrigation system is subjected to leakage, wear and damage and regular system checks and maintenance are necessary in order to avoid deterioration in design specification performance.

Irrigation scheduling:

Knowing where to irrigate, when to start and when to stop depends on a knowledge of the amount of available moisture in the soil. This can either be determined exactly by the use of moisture measuring instruments, or estimated using a soil moisture profit and loss account, (SOMPLA).

SOMPLA is a system of recording theoretical balance of soil moisture, with crop water consumption being debited to the account and rainfall and irrigation being credited.

These figures are recorded on the Irrigation Control Schedule.

Procedure for using the Irrigation Control Schedule:

• From the field area, calculate the number of days needed for the irrigation equipment to completely irrigate the field (irrigation cycle).
• From the TAM given, calculate the soil moisture deficit at which irrigation should commence (usually not less than 60% of TAM).
• From the canopy factors table, determine the daily evapotranspiration of the crop in mm per day.
• On a daily basis, subtract the appropriate evapotranspiration from the available moisture brought forward from the previous day. Continue subtracting until rainfall interrupts the programme or irrigation is required and applied (60% depletion).
• The rainfall and irrigation figures should be added to the available moisture provided that the TAM of the soil is not exceeded, in which case any further rainfall should be ignored as it is presumed lost to run‐off or deep percolation.

The following calculation procedure should always be followed:

• Available moisture, day 1
• Minus Daily evapotranspiration
• Plus Rainfall
• Plus Irrigation
• Equals Available moisture, day 2

Stop irrigating when rainfall and effective irrigation exceeds the TAM.

In practice it is not enough to depend entirely on a single method of scheduling irrigation. Soils are seldom uniform over large areas and the TAM could vary considerably within a field.

The use of a soil auger or the digging of test holes assists the farmer with deciding when it may be necessary to override his theoretical schedule.

SYSTEMS

The  three  basic  methods  of  irrigation  used  in  the  South  African  sugar  industry  at  present are,

overhead, surface and drip irrigation.

Within these groupings there is a wide range of systems used to apply water.

Overhead Irrigation

This involves water under pressure flowing through a system of pipes and being distributed through sprinklers, jets or perforated pipes in droplet form over a given area of crop land. Overhead systems are relatively expensive to install, operate and maintain, but can supply water uniformly over a large area.

Several different types of overhead systems are currently in use in South Africa:

Hand Moveable Systems

Consist of portable aluminium or light steel pipes to which risers and sprinklers are fitted at regular intervals. Water is supplied through a permanent, normally underground, mainline pipe.

Each sprinkler distributes water in a circle that reaches to the adjacent riser, resulting in a 100% overlap and even distribution. The lateral pipes are disconnected and moved manually to the next cycle position.

Rotating Boom and Centre Pivot Systems

These are tractor‐drawn booms or self‐propelled sprinkler lines supported on wheeled chassis or towers. Centre pivot irrigation systems have a long large diameter pipe carried on motorised towers about 50 metres apart and rotating about a central pressurised water supply point. Spray nozzles or rotating sprinklers are mounted on the pipe to distribute the water. A ten tower unit covers about  85 hectares and will normally take 24 hours to complete one circuit. Centre pivots apply water very uniformly and are extremely stable enabling efficient use on undulating land.

Large Sprinklers

These are commonly referred to as ‘big guns’ and are high capacity, high pressure sprinklers which cover circles of between 60 ‐ 180 metres in diameter. They are normally mounted on large tripods so that they can operate in tall cane.

Semi‐solid Set and Dragline System

An increasingly popular sprinkler irrigation system that consists of a permanent network of main lines and laterals. The main lines are usually located underground and the laterals can be either underground or on the surface. The laterals provide sufficient coverage to irrigate the entire field  and only the sprinklers need to be moved along the laterals. To minimise the costs involved in burying laterals, some growers make use of what is termed the ‘dragline’ system, where flexible portable hoses are connected to the risers on the lateral so that a single riser can be used to irrigate three or more points in succession.

Permanent Systems

In this design all the components, other than the risers, are located underground and the system  will irrigate the entire area at one time. The risers remain in position permanently and are not moved.

Floppy Sprinklers

This system is being used more frequently in the South African sugar industry. It consists of a permanent network of underground main lines and laterals with a ‘floppy’ sprinkler on top of an aluminium riser typically on 12 m x 14 m spacing. This unique South African designed sprinkler comprises a silicon tube in a plastic mounting. When water is applied to the system the tube ‘flaps’  in a specific way distributing water evenly over the crop. There are no moving mechanical parts and each sprinkler is fitted with a unique flow controller allowing accurate water application over a wide pressure band. The system applies water more precisely than conventional impact sprinklers at a lower pressure with resultant energy savings.

Surface Irrigation

This irrigation method involves applying water to the crop by allowing it to flow over the surface of the field in furrows either in or between the cane rows. Newly planted cane is normally irrigated in‐ row for the first few months then changed to inter‐row when the cane is fully established.

Subsequent ratoon crops are most successfully irrigated in the inter‐row. Surface irrigation should not be used for shallow soils with underlying impervious layers or on slopes of more than 6%.

Surface irrigation is a labour intensive operation.

Application techniques are influenced by the infiltration rate of the soil.

Due to the soil at the furrow head receiving water first, the penetration is deeper at this point. However, infiltration rates decrease rapidly after the initial wetting so that the water moves to the far end of the furrow.

Application techniques are therefore aimed at ensuring that distribution through the profile is equalised throughout the length of the furrow.

·         Outflow Method

With this technique, excess water is allowed to flow out of the field furrow, being collected in a tail‐water ditch. Careful design and metering is necessary to limit waste.

·         Cut‐back Method

This technique releases a large initial flow of water which is then cut back so that it does not exceed the intake rate of the wetted furrow as a whole.

·         Pulse or Surge Method

The water is released in surges to successively wet the furrow in increments and therefore obtain a more uniform result.

Delivery Methods vary in sophistication and have a direct bearing on the overall system efficiency. The most basic technique of conveying water from the supply canal to the irrigation furrow is by opening the wall of the of the supply canal with a shovel, allowing the water to flow directly into the furrow.

Hand‐operated siphons are also used to transfer water from the supply canal to the furrow.

Spile pipes are short lengths of pipe that run through the wall of the supply canal at a fixed level above the raised canal bottom and discharge water into the cane furrows. These are used with level or stepped ditch layouts and require engineering expertise at installation.

Gated pipes are lightweight, large diameter pipes which are laid in long lengths across the cane  rows. Water is supplied under low pressure through underground mains and discharged into the cane rows through adjustable gates or nozzles. This is a sophisticated flexible system but  is  expensive and complicated to operate.

An alternative delivery method is to supply water at low pressure through underground mains, risers and hydrants to lightweight pipe manifolds with 3 to 5 branches. The branches all discharge at the same time and at a pre‐determined rate into individual furrows. Flow rates are adjustable at the hydrant making the system flexible and easy to operate.

Drip Irrigation Systems

Although relatively new to the South African sugar industry, drip irrigation is used extensively in a number of high value crops as well as in sugarcane in Hawaii and very possible that these systems may become popular in South Africa.

Drip irrigation involves frequent or continuous feeding of water directly to the root system of the crop through a network of small diameter (12 mm to 20 mm) pipes containing specially designed emitters with discharge rates of up to 4 litres per hour.

The dripper pipes are installed between or adjacent to the cane lines, either on the surface or buried 300 mm below. Surface driplines have to be removed from the field before harvest.  Subsurface pipes can be left in place during harvest and can be used to irrigate several successive crops.

Drip irrigation should be applied every day.

Due to the continuous replenishment of the water supply the cane plant is not subjected to moisture stress resulting in higher expected yields.

Care should be taken when placing the dripper lines, relative to the plant roots, as the lateral movement of the water is restricted to approximately 400 mm either side of the pipe.

• FERTILISATION

Practical decisions regarding fertilisation of the sugarcane crop should not be taken without first determining the type of soil, its natural properties and the nutrients which it already contains.

These factors are determined by the use of soil analysis and fertiliser recommendations based on annual estimates of crop removal of nutrients from the soil. For best results with ratoon crops leaf analysis should also be undertaken.

Once the levels of soil and leaf nutrients have been analysed these results should be compared with the known threshold values for sugarcane crops in a specific area. Extensive research has  established the minimum levels of nutrients required for optimum yields. Once the difference between actual nutrient levels and threshold levels is known, an accurate fertiliser recommendation can be made.

Detailed information regarding the properties of soil, plant nutrition and soil chemistry is fully covered in the PCI Agri Soil Science and Agronomy and Plant Nutrition modules and in the SASRI Junior Certificate course Fertilisers and Soils modules.

PLANT NUTRITION

The following macro and micro nutrients are important in the production of sugarcane:

Nitrogen (N)

Essential for growth and photosynthesis.

Excess N leads to excessive top growth, lower sucrose levels and often increased pest and disease damage.

Average recommended applications under South African conditions and soils are 120 kg ‐ 140 kg N/ha for plant cane and between 140 kg and 180 kg N/ha for ratoon crops.

Plant cane should receive one third to one half of the recommended rate in the planting furrow and the remainder top dressed at the stage of rapid stalk elongation, approximately 6 weeks after planting.

Ratoon cane should receive its entire N as soon after harvest as possible.

Symptoms of N deficiency are the yellowing of leaves from the base of the plant upwards with the tips and margins dying off prematurely together with poor growth.

Source: Calcino (1994)

Phosphorous (P)

Essential for early growth, root development and especially important for the plant crop. Phosphorous recommendations depend on the level of available P in the soil, and range from 20 kg P/ha up to 70 kg P/ha in later ratoons.

Deficiency causes stunted growth with slender leaf blades often bronzed which die back from the tips.

Potassium (K)

Assists with translocation and storage of sugar and proteins. Regulates opening and closing of the stomata. Potassium fertiliser recommendations depend on the level of available K in the soil and will vary according to clay content and levels of Ca and Mg present.

Average annual potassium applications range between 150 kg K/ha and 250 kg K/ha.

Deficiency worsens the effects of drought on the crop and renders it more susceptible to frost and disease damage.

Deficiency results in depressed growth and causes yellow to brown discolouration of the lower leaves, with scorching of the outer edges (firing) and red discolouration of the upper surface of the leaf blade midrib.

Source: Tiwari (2006)

Calcium (Ca)

Provides strength to cell walls and the plant as a whole.

Deficiency causes reduced growth. The younger leaves become hooked and the spindle dies off at the tip.

Magnesium (Mg)

Essential for photosynthesis.

Deficiency causes pronounced red flecking and is most intense on older leaves.

Sulphur (S)

Essential for cell metabolism and protein formation.

Deficiency causes reduced growth, leaf blades turn light yellowish green, but do not die back from the tips as with N deficiency.

Silicon (Si)

Important for P nutrition and water use. Also alleviates aluminium (Al) and manganese (Mn) toxicities. Improves resistance of the crop to pests, diseases and frost.

Zinc (Zn)

Deficiencies most frequently found in sugarcane grown on sandy soils. High soil pH can also cause uptake problems.

Uptake of macronutrients

The sugarcane plant takes up large amounts of macronutrients during its growth cycle, mainly nitrogen and potassium. See Figure 1.

Figure 10: Approximate macronutrient content (kg) in a 100 t/ha sugarcane crop

FERTILISERS

Inorganic fertilisers may be grouped into 4 categories:

Straights, or single nutrients, containing nitrogen (N), phosphorous (P) or potassium (K).

Nitrogen is applied mainly to sugarcane as urea or LAN but sometimes as ammonium sulphate (AS) or ammonium sulphate nitrate (ASN). Urea is the cheapest form of N, but may be lost to the atmosphere if applied to the surface of sandy, low organic matter, soils.

LAN has a lower N concentration and is not as volatile.

Ammonia gas is the most concentrated form of N but specialised equipment is required to inject it into the soil. AS and ASN have the advantage of also supplying sulphur (S) used for crop growth. The disadvantage is that they cause acidity in the soils and are more costly per unit of nitrogen.

Superphosphate has been widely used in the past to supply crop phosphorous needs, but the newer compound fertilisers MAP and DAP are now more cost‐effective forms of P application.

Potassium fertilisers include potassium chloride (KCl) and potassium sulphate.

Compounds consist of chemical compounds containing more than one nutrient.

Three important P fertilisers are available as compounds with nitrogen, namely; mono‐ammonium phosphate (MAP), di‐ammonium phosphate (DAP) and ammoniated supers (AMP).

Blends are mixtures of single‐nutrient or compound fertilisers.

Blends supply two or more of the macronutrients and often include small amounts of zinc.

Blends do not have specific names, but are simply identified by three numbers, which refer to the ratios of N, P and K contained.

The figure in brackets after the ratio indicates the total concentration of nutrients in the product, for example:

Blend 2:3:4(38) contains 2 parts N: 3 parts P: 4 parts K and these three nutrients in total make up 38% of the entire fertiliser bag contents.

Table 4: Commonly‐used Fertilisers and their Compositions

PRODUCT	LAN BASED	N%	P%	K%	S%
Single‐Nutrient (‘straights’)
Urea		46
LAN (Limestone ammonium nitrate)	Y	28
ASN (Ammonium sulphate nitrate)		27			13
AS (Ammonium sulphate)		21			24
Ammonia gas		82
Superphosphate (‘supers’)			10.5		±7
Potassium chloride (‘potash’)				50
Potassium sulphate				40	±18

Compound
MAP+0.5%           ZN           (mono‐ammonium phosphate)		11	22
DAP+0.5%ZN (di‐ammonium phosphate)		18	20
AMP+0.5%ZN                                 (ammoniated superphosphate)		6	14		±5

Blends*
2.3.2 (22)	Y	6.3	9.4	6.3
2.3.3 (30)	Y	6.7	10	13.3
2.3.4 (38)		8.4	12.7	16.9
3.2.1 (25)	Y	12.5	8.3	4.2
1.0.1 (48)		24	0	24

3.1.4 (45)		16.9	5.6	22.5
4.1.6 (38)	Y	13.8	3.5	20.7
4.1.6 (45)		17.1	4.3	25.6
5.1.5 (45)		20.9	4.2	20.9
2.0.3 (49)		19.6	0	29.4
1.0.2 (39)	Y	13	0	26
2.0.3 (38)	Y	15.2	0	22.8
4.1.0 (42)		33.6	8.4	0

*A very wide range of blends are marketed. Presented here are some commonly used ones. Y‐ Products contains nitrogen carrier LAN

Micro‐nutrient Fertilisers

Zinc (Zn) is the most widely deficient micro‐nutrient in South Africa, therefore most fertiliser compounds and blends will contain 0.5% or 1.0% zinc. Deficiencies of other important micro‐ nutrients will best be detected by leaf analysis.

The most common are iron (Fe), copper (Cu), manganese (Mn), boron (B), and molybdenum (Mo).

FERTILISER APPLICATION; TIMING AND PLACEMENT

Once the correct amount of fertiliser and application rates have been chosen, the farmer must decide on the most effective time of application and the best placement for efficient nutrient use by the plant.

Nitrogen (N):

Nitrogen can be lost in numerous ways so it should be applied at a time when the plant roots are sufficiently mature to be able to utilise the nutrient, but long enough before harvest to ensure that cane quality is not affected. (Figure2). It is important that N should only be applied when growth commences. On sandy soils it is preferable to apply N in two doses, to ensure efficient uptake and minimal loss.

Figure 11: Correct time in the crop growth cycle for N fertiliser application

Less N is generally required for plant cane than ratoon cane as some N is released from the soil during land preparation. About one third of the recommended amount should be applied in the furrow at planting with the balance top‐dressed later. Too much N in the furrow at planting could result in sett burn.

Phosphorous (P)

Unlike nitrogen Phosphorous P is not mobile in the soil and therefore less prone to losses. Its application is therefore easier to manage than N. Phosphorous is important for the plant crop and this application is usually sufficient for the plant and first ratoon crops. At planting all of the required P should be applied in the furrow with subsequent ratoon crop applications broadcast in the one application with N and K.

Potassium (K)

Is also less subject to losses than N. Up to 100 kg K/ha can be placed in the furrow at planting and the remainder applied as a top dressing. In ratoon crops all the required potassium is broadcast in one application.

FERTILISER APPLICATION METHODS

The most common methods of applying fertiliser are discussed below:

Tractor‐Mounted Fertiliser Spreaders

These are more commonly used where large areas need to be covered. Fertiliser is poured into a bin mounted on the three‐point linkage at the back of the tractor. A distribution point, operated by the tractor PTO, spreads the fertiliser in a broad band behind the tractor. The distributor can either be of the oscillating pipe type or a spinning disc. These spreaders require careful calibration as the rate  and distribution depends on the speed of the tractor.

Mayfield Granular Fertiliser Applicator

Widely used in the South African sugar industry particularly on steep sloping ground inaccessible to machinery. The Mayfield is a knapsack‐type applicator with a pipe for fertiliser release. An adjustable calibration nozzle allows for the release of fertiliser, as required. The applicator has a single or  double row configuration and can be adjusted to broadcast fertiliser in a broad swathe rather than  in narrow bands.

Figure 12: The Mayfield Applicator

Source: Mayfield (2006)

Wheelbarrow Applicator

This is a bicycle‐like implement with a small fertiliser bin and distributor. The fertiliser is released in a band according to the calibration by the operator which adjusts the opening of the slot below the bin.

Figure 13: The Wheelbarrow Fertiliser Applicator

Source: Rona (2013)

Fertigation

Where irrigation water is applied with dripper systems the option of applying soluble fertiliser through the dripper lines exists. This combination of fertilisation and irrigation is known as fertigation and is a very efficient method of applying fertiliser accurately with minimal losses. Fertigation makes it possible to apply the fertiliser in numerous split applications improving the efficiency of uptake by the crop. Fertigation specialists should be consulted with regard to  application rates, mixtures and suitable types of fertiliser.

Direct Injection of Nitrogen

Anhydrous ammonia, a gaseous carrier of nitrogen, is applied by direct injection into the soil using purpose‐designed equipment. Correct soil moisture status is critical to avoid gas losses.

Tin and string method

One of the simplest and most reliable methods of applying fertiliser is by the tin and string method which can be used on small and large areas alike. In this method a tin of a certain size is filled with fertiliser. A worker then allows it to dribble out evenly over a certain distance, which is marked out by the length of string. The size of the tin and the length of the string should be calibrated for each fertiliser type and application rate. This method can be very effective over large areas when a group of 4 to 6 workers are used.

Aerial application

Urea is commonly applied to large areas using fixed‐wing aircraft. Although cost‐effective, the accuracy of application can be dependent on prevailing weather conditions.

Figure 14: Aerial Fertiliser Application

Source: thescientistgardener.blogspot

LIME AND GYPSUM

Although soils in humid areas are often naturally acidic, certain agricultural processes promote soil acidification. These processes include:

• The use of ammonium and urea fertilisers;
• The export of calcium, magnesium and potassium in harvested crops; and
• The accelerated breakdown of organic matter through cultivation.

Crops’ growth on very acid soils is often poor, due to aluminium toxicity, Ca and Mg deficiencies, micronutrient deficiencies and reduced microbiological activity.

The application of lime or gypsum is sometimes required where soil acidity problems are encountered.

Lime

Lime is used to correct acidity problems which occur in the topsoil. Agricultural limes are mainly calcitic (calcium carbonate) and dolomitic (magnesium carbonate).

The major effects of lime on soil properties are as follows:

• Increase in soil pH;
• Decrease in exchangeable acidity;
• Increase in Ca and Mg levels; and
• Stimulation of soil fauna, e.g.; earthworms.

Gypsum

Calcium sulphate ‐ The average chemical composition of gypsum is 25% calcium and 16% sulphur. The major effects of applying gypsum are as follows:

• Improvement in subsoil acidity;
• Addition of calcium and sulphur; and

• Improvement of poor soil properties due to salinity and/or sodicity

ORGANIC PRODUCTS

Products such as manures, compost, filter cake, condensed molasses solids (CMS), vinasse, molasses, green manures, flyash and cane trash all contain nutrients in varying amounts and are widely used  on sugarcane where available.

Due to wide variability, organic products should always be analysed to obtain accurate nutrient content before use, thereby ensuring correct application rates. In addition to supplying essential nutrients organic products generally improve the physical properties and health of the soil, including water infiltration, water holding capacity and aeration. They also act as a source of food for soil organisms such as microbes, fungi and earthworms. Organic products are generally bulky and costly to handle and transport.

Poultry and Animal Manures

This is an excellent source of nitrogen, phosphorous and potassium. In general, poultry manure  tends to have higher concentrations of N and P compared to other farmyard manure. When using poultry manure, caution should be taken to avoid a build‐up of P in the soil, as the P and N levels in the manure are very similar. It is therefore recommended that poultry manure is applied at rates  that satisfy the P requirements and that the N requirement should be supplemented by other fertilisers. Manures are best applied in the furrow at planting.

Filter Cake

A by‐product of sugar production and is a mixture of sugar juice, starch, wax, gums and pectins. It is used as a source of P and N and, in strongly acidic soils. The calcium content can help to overcome  Ca deficiencies. Used in the furrow at planting.

Flyash

Is the residue left in the boilers after bagasse has been burnt to generate steam in the sugar mill and contains nutrients such as K, Ca, Mg, S and Si (silicon).

Molasses, Vinasse and CMS (condensed molasses solids)

Liquid by‐products of sugar and ethanol production and all contain a variety of nutrients, including K, N, P, Ca, Mg and S.

Molasses is a thick, syrupy by‐product produced when sugarcane is processed to make sugar. It is used to make alcohol and a residual by‐product of this process is called vinasse. Concentrating vinasse produces CMS.

Bagasse

Is the fibre left over after the juice has been squeezed out of the sugarcane stalks. Although not a good source of nutrients is an excellent organic supplement for maintaining and restoring the physical properties of soils.

Sewage Sludge

Used in some sugar producing areas as it is high in nutrients but can also contain heavy metals and human pathogens so should be used with caution. Its use in South African agriculture is strictly regulated.

ACKNOWLEDGEMENT:

PCI Agricultural Services are gratefulto the South African Sugar Association Sugarcane Research Institute (SASRI) for permission to use the information contained in their Junior Certificate Course in Sugarcane Agriculture as the basis for this module.

1. WEED CONTROL

A weed is any plant growing where it is not wanted. SIMPLE CLASSIFICATION

Broadleaf weeds e.g. Blackjack (Bidens pilosa), pigweed (Amaranthus spinosus)

Grasses e.g. Guinea grass (Panicum maximum), Digitaria sanguinalis

Sedges e.g. Yellow flowered watergrass (Cyperus esculentus), purple flowered watergrass (Cyperus rotundus)

EFFECT OF WEEDS ON THE SUGARCANE CROP

Weeds compete for light, moisture and nutrients. In sugarcane the aim is to control weeds until the crop reaches full canopy after which the shading effect of the crop will naturally suppress weed growth.

Competition for moisture is very important especially during periods of water stress and some weed species present strong competition for the available moisture supplies. Weeds will also take up some of the fertiliser applied to the cane, reducing the amount of nutrients available.

Weeds can harbour insects and diseases. Panicum maximum is the alternate host for Numicia, the green leaf sucker and Sorghum bicolor and Setaria species carry mosaic disease. Weeds can hinder mechanical operations.

They can physically obstruct the movement of water in irrigation channels and drainage ditches. Weeds on the edges of fields can create fire hazards. Trials conducted by SASRI have shown that weeds can affect yields substantially. Significant increases in income will result from effective weed control programmes.

METHODS OF WEED CONTROL IN SUGARCANE

Preventive control

Field verges should be kept free of weeds to prevent the spread of weed seeds, such as Bidens  pilosa, runners, like Cynodon dactylon, and tubers, for example Cyperus rotundus, into the fields.

Physical Control

Natural elements such as water and light are used to control weed growth. The aim is to obtain full canopy as soon as possible by ensuring good even germination and vigorous early growth. The use of trash blankets in ratoon crops will effectively suppress early weed growth giving the cane plants a head start.

MECHANICAL CONTROL

Hand weeding

This method is the labour‐intensive removal of weeds by hand. This prevents the weeds present in the cane row from seeding. Care must be taken to remove the complete root system.

Hand hoeing

Labour‐intensive system and can prove ineffective if the hoe enters the ground too deeply which simply transplants the weed plants. Care must be taken not to damage the cane roots. The growth of Cyperus rotundus is actually encouraged by hoeing through stimulation of the tubers.

Mule drawn cultivators:

Useful on steep slopes or when the cane is too high for tractor operations, but it can be fairly slow. Tractor drawn cultivators are very efficient and cost effective but have disadvantages:

• Only weeds in the interrow are controlled;
• The exception is the ‘finger weeder’ that is used in plant cane, which has conventional tines that cultivate the interrow and ‘fingers’ that pass through the cane row to pluck out young weeds;

Figure 1: The Finger Weeder

Source: farm5.staticflickr

• Difficult to use in areas where there are open drains;
• Cannot be used on steep slopes;
• Cane may be damaged if the rows are not parallel;
• High fuel consumption; and
• Soil disturbance encourages germination of weed seeds.

CHEMICAL CONTROL

The use of herbicides is a major contribution to weed control but should not be relied upon completely. The ideal system should be based on herbicides, interrow cultivation and spot weeding or spot spraying of problem areas to ensure full control.

Advantages of Herbicides:

• Control of weeds in the plant row by using selective herbicides;
• One application will give a long period of control.
• Mechanical cultivation will last for 3 weeks where herbicide will last for 10 ‐ 12 weeks;
• Large areas can be treated quickly ensuring that weeds stay within the boundaries of the easily‐controllable stage; and

• In favourable conditions, herbicides are very effective.

Disadvantages of Herbicides:

• If incorrectly applied herbicides can result in poor weed control and possible damage to the crop, increasing production costs unnecessarily; and
  • Although labour requirements are lower, management inputs are greater.

HERBICIDES

Herbicides can be classified in different ways.

Selectivity, according to the type of weed they control, for example:

• Grasses, controlled by alachlor and metolachlor;
  • Broadleaf weeds, controlled by MCPA and atrazine (post‐emergent);
  • Watergrass, controlled by EPTC (pre‐emergent) and Servian (post‐emergent); and
  • Some herbicides control both grasses and broadleaf weeds, such as diuron, ametryn, paraquat and glyphosate.

Point of uptake by weeds:

• By foliage – post‐emergence
  • By roots       pre‐ or post‐emergence
  • By shoots – pre‐emergence
  • By seeds –  pre‐emergence

Note: The terms ‘pre‐emergence’ and ‘post‐emergence’ apply to the stage of growth of the weed only and not to the sugarcane crop.

Pre‐emergence Herbicides

These should be applied to the bare soil immediately after planting or, in a ratoon crop immediately after harvesting burnt cane or interrow cultivation.

It is important to know what weeds are expected so that the correct herbicide can be selected. Records should be kept of weeds that emerge as a future management aid.

Pre‐emergence herbicides require moist soils to allow the herbicides to penetrate into  the  seed zone.

Post‐emergence Herbicides

Are most effective when applied to the foliage of young weeds with attention to the growth stage, in order to avoid damage.

As a general rule, apply as early as possible;

The volume of water used is important with post‐emergence sprays with good leaf coverage essential. Up to 250 ‐ 300 litres water per hectare should be used.

Timing of Applications:

• Cyperus species – Early flowering stage, as most of the tubers will already have germinated.
  • Grasses – 3 ‐ 4 leaf stage, just prior to tillering.
  • Broadleaf weeds – Less than 100 mm in height, pre‐flowering.

• A surfactant (wetter or sticker) should be added to post‐emergence sprays to increase its effectivity. Some herbicides have a surfactant already mixed with the chemical, so it is important for the label to be checked before use.

TOXICITY OF HERBICIDES

A chemical may not only be toxic when taken in through the mouth but also after absorption  through the skin or inhalation of contaminated air.

All labels should be read carefully and the necessary handling precautions taken.

CROP DAMAGE BY HERBICIDES

Although all herbicides are tested for their phytotoxic effects on sugarcane before registration, some herbicides may affect cane growth. Herbicides sprayed before the emergence of the cane are usually far less damaging than those sprayed onto cane foliage. The later the cane growth stage, the greater the damage will be. Until sugarcane reaches a leaf height of approximately 400 mm, full cover applications of post‐emergence herbicides can be made, as the crop recovers quickly from any damage. For cane that is taller than 400 mm, the spray should be directed onto the interrow, away from the leaves. Only one full cover application of a post‐emergence herbicide should be made during a single crop cycle.

Visual symptoms of herbicide damage may appear as one or more of the following:

• Germination failure;
• Inhibition of tillering;
• Stunted growth;
• Swollen root bands;
• Sideshooting;
• Chlorosis (yellowing) of leaves;
• Necrosis (death) of leaves; and
• Spiky leaves.

APPLICATION OF HERBICIDES

Accurate application and even distribution are of vital importance when using chemicals for weed control. Spray equipment should be accurately calibrated and in good working order. Speed,  pressure and nozzle height should be kept constant.

METHODS OF APPLICATION

Aerial

The advantage is that large areas can be covered in a short time, but spray drift on to neighbouring crops is a problem. Only a limited number of herbicides are registered for aerial application.

Tractor Mounted Sprayer or Boom Sprayer

More labour efficient than knapsack sprayers and the output rate is easier to control.

This method is difficult to use in smaller fields and only when the cane is low enough to pass under the tractor.

An alternative system is the use of manually operated lances on lines taken off the tank to supplement or replace the boom.

Knapsack

Can be used in small areas and steep slopes on cane of any height, and wet soil conditions without concerns of compaction.

The system is labour intensive and slow and has proved difficult to maintain  accurate  walking speeds.

Controlled Droplet or Low Volume Applicator

The system uses less volume of water per unit area and applies a constant droplet size but is unsuitable for narrow row widths due to constant swathe width.

One operator can spray approximately 2 hectares per day.

Sprayer Operation Checks:

• Nozzles – check that all nozzles are the same type and output rate and are not blocked or damaged. Nozzles should be replaced each year.
  • Agitation – when using wettable powder formulations, adequate tank agitation is essential to avoid settling of solids.
  • Filters – must be kept clean.
  • Pipelines – check and replace perished or damaged pipes to avoid blockage of nozzles.
  • Water – should be clean and free from floating grass or dirt. When using paraquat, avoid muddy water as this will inactivate the chemical.
  • Mixing – follow the correct procedure:
    • Half fill the tank with water
    • If using wettable powder, first mix the required amount of powder with a small quantity of water to make a slurry or paste. Liquid formulations do not need pre‐mixing.
    • Add the herbicide to the tank.
    • Top up with water.
    • Turn on the agitator system.
  • Pressure – Operate all sprayers at the pressure recommended by the manufacturer for each specific nozzle.

• HARVESTING

CROP ESTIMATES

A method of estimating tons of cane per hectare is to calculate the stalk population per hectare (number of stalks per metre x length of row per hectare) then multiplying this by the average mass  of selected average stalks.

For example:

Assume 125 stalks in 10 metre row, or 12.5 stalks per metre Average stalk mass is 1.1 kg

If 1.3 metres is the row spacing then length of row per ha is 10 000/1.3    = 7 692 m of row/ha

SELECTION OF FIELDS FOR HARVEST

Efficient harvesting requires erect, uniform, tall, heavy cane. This normally occurs when there is 1.5 to 2 metres of stalk and the yield ranges from 80 ‐ 120 tons/ha.

Traditionally the most mature fields are harvested early and progressively younger cane is cut as the season progresses, however there are many other factors which need to be taken into consideration when planning the harvesting programme. For example:

• Fields to be replanted;
  • Fields with high levels of eldana borer;

• Fields requiring deep underground drainage systems;
• Planning for fire protection or strip cropping;
• The best fields should be harvested during the high sucrose period in the Spring;
• The youngest fields to be harvested in any season on any farm, should be cut during the high sucrose period; and
• Frost damaged, badly lodged or drought stricken fields should be harvested early before further deterioration occurs.

TASKS AND METHODS OF HARVESTING

The harvesting operation is broken down into the following distinct parts:

• Burning or trashing;
• Base cutting;
• Topping;
• Stacking or windrowing; and
• Loading.

A fair day’s task in erect, burnt cane, yielding 100 tons/ha on relatively level land is 5 tons per man cut, topped and stacked. In green cane about 3.5 tons per man can be expected per day.

On steep land these figures will be reduced by 30 ‐ 40%.

If a man is only cutting, topping and trashing unburnt cane, a good average task is 4.5 ‐ 5 tons per day. The task for the stacker would be twice this amount. In burnt cane, using a short‐handled knife an average task is about 9 tons per man per day cut, topped and windrowed. The correct use of the long‐handled Australian cane knife should increase this amount to 10 ‐ 11 tons. With correct training some cutters have achieved 15 ‐ 20 tons cut, topped and windrowed in a day.

Figure 15: Cane knives

CHEMICAL RIPENERS

In some areas chemical ripeners are used as a management tool, to improve the sucrose content of immature sugarcane at the beginning and end of the milling season.

The most commonly used chemicals are Ethepon and Fusilade Super, which can be used singly or in combination. Advice should be sought regarding the timing, product, rate of application and variety compatibility before embarking on a chemical ripening program. This advice is available from SASRI consultants and extension officers.

CANE QUALITY

The processing of sugarcane for the extraction of sugar begins in the field.

The variety of cane, the soil in which it is grown, the degree of maturity at harvest, the cultural practices used, including fertiliser application, irrigation, chemical ripening and topping height all combine to produce a raw material of varying quality. The sugar miller assesses cane quality on the amount of recoverable sugar per ton of cane crushed. High sucrose, high purity and low fibre are the three most important factors contributing to high recovery of sugar. The grower is mainly concerned with the yield of sucrose per hectare per unit of time.

CANE CONSTITUENTS

Harvested sugarcane consists of juice and fibre. The juice contains water, sucrose and soluble impurities. The fibre consists of the bagasse, the outside rind of the cane, the dead leaves and tops and insoluble extraneous matter, like soil. The soluble solids in cane juice, sucrose and soluble impurities (non sucrose) are referred to as brix. Cane purity is another measure of cane quality and  is expressed as:

Sucrose % x 100 Brix %

Figure 16: The Constituents of Cane

FACTORS INFLUENCING SUCROSE CONTENT

Season

Sucrose and purity are low during active growth but increase when photosynthesis continues with restricted growth. Seasonal changes in rainfall patterns will cause shifts in the peak sucrose periods. Seasonal changes in sucrose and purity are likely to be more marked in young cane.

Varieties

All varieties peak in sucrose and purity at more or less the same time, but some have lower sucrose content at the beginning or end of the season.

Height of Topping

Sugarcane contains a long green top during the months of rapid growth, December to March but, as the season progresses towards September and October; the cane ripens nearly to the growing point at the top of the stalk. During the early and late parts of the season it is therefore necessary to top the cane lower than during the optimum period.

Trash

Sucrose content is reduced in proportion to the amount of trash present as trash contains no  sucrose and has a high fibre content.  As little trash as possible should be sent to the mill.

Base Cutting

Cane should be cut cleanly at ground level as the stumps will then send up strong, vigorous shoots. Buds that develop above ground level do not develop their own root system so remain small often breaking off and are eventually crowded out by stronger shoots. Cutting above ground level wastes cane and recoverable sucrose thereby reducing the quality of the ratoons.

Cutting the cane below ground level does not affect the quality of the ratoon crop but soil and roots adhering to the cane, lowers its quality and makes sucrose extraction more difficult. This also increases the wear and tear on mill equipment.

Fertiliser application

Generally, fertiliser applications which stimulate growth will lower the sucrose content particularly if the cane is cut during periods of rapid growth or is still young. Over‐application of nitrogen tends to lower sucrose content.

For optimum quality, cane needs enough nitrogen, phosphorous, potassium and trace elements, with no excess of any one nutrient.

Irrigation

Irrigation lowers sucrose content if cane is cut during a period of rapid growth. Controlled irrigation and drying‐off can lead to an increase in sucrose content.

Drying‐off is practised to inhibit vegetative growth. As the growth rate declines, less of the sugar formed by photosynthesis is used in building new tissue and more is stored as sucrose. As ripening proceeds, the sucrose percentage in the stalks gradually increases while the percentage of impurities declines.

CLIMATIC FACTORS AFFECTING QUALITY

Drought

Initially the drying‐off effects of drought conditions can hasten ripening and improve sucrose percentage, but if the drought is prolonged it can seriously depress sucrose and increase fibre content, eventually leading to the death of the stalks.

Frost

Cane deterioration will depend on the severity of frost and weather conditions following the frost period. If the growing point has been killed it is advisable to cut millable cane as soon as possible after a frost. Side‐shooting after frost damage will lower the sucrose content.

Flowering

Flowering reduces cane quality as the stored sucrose is used up by the flower. In addition, the top of the stalk becomes light and pithy and any further growth has to come from side shoots.

Wind

Excessive wind can cause cane to lodge which leads to deterioration and subsequently a reduction in the sucrose content.

Flooding

Cane planted in flood plains subjected to periodic flooding, can be drowned or dirtied which leads to deterioration and the loss of sucrose content.

Temperature

High temperatures will stimulate rapid growth while low temperatures speed up ripening. A combination of climatic conditions can either stimulate or supress flowering.

TIMING AND MATURITY

Under normal circumstances it is advisable to cut the cane of highest ‘relative maturity’ first, which  is the cane that indicates the least potential to further increase in sucrose percentage. Relative maturity is indicated by a difference in purity or sucrose between the top section of the stalk and   the middle or bottom sections. The bigger the difference, the less mature the cane. Young cane should be cut as close as possible to the optimum sucrose period of September to October. Older cane should be cut at the beginning or end of the season.

QUALITY LOSS DUE TO CANE DETERIORATION

Cane starts to deteriorate from the time it is harvested whether it is unburnt, burnt and cut immediately, or burnt and left standing in the field, which is why the cane should reach the mill as soon after cutting as possible. Rates of deterioration vary considerably depending on weather conditions, being most rapid in the hot, humid summer months.

Deterioration causes a decline in purity and a fall in sucrose percentage, although this may be obscured by a drying out of the cane simultaneously. Unburnt cut cane dries out more slowly than burnt cut cane; however purity of juice from unburnt cane generally declines more rapidly than from burnt cut cane due to rapid conversion of sucrose to other sugars.

When compared with cane that is burnt and cut immediately, cane that is burnt and left standing shows a more rapid decline in recoverable sugar percentage. This emphasises the importance of burning only enough cane to meet the requirements for one day’s harvesting.

Chopped cane, whether burnt or unburnt generally deteriorates more rapidly than whole stalk cane again emphasising the importance of transporting cane to the mill as soon as possible after harvesting.

• PROCESSING

Sugar cane is processed by milling. This involves the passing of the cane stalks between rollers which squeeze out the juice containing the sugar. The juice is heated in large pans to produce brown (raw) sugar. This heating process evaporates the liquid and the sugar crystals remain in the pan. The  brown sugar can then be further refined in a fairly complicated process to form white sugar.

An important by‐product of sugar production is molasses – a sweet, thick, black liquid. Molasses is used as a stock feed and provides energy in both cattle and sheep rations. Molasses can be fermented and produces carbon dioxide gas and alcohol. The carbon dioxide gas is used in the manufacture of soft drinks and welding. The alcohol is known as cane spirit and forms the basis of various blends of liquor as well as industrial spirit.

Ethanol is an important by‐product of the sugar production process and is widely used as a biofuel alternative to oil‐based fuels.

Bagasse is another by‐product of the milling process and is the residual dry fibre after the extraction of the juice. Bagasse is used as a fuel source for the boilers in the sugar mills, in the manufacture of paper and paperboard products, as mulch for the cane fields and a raw material for chemical production.

Another by‐product, filtercake, is a mixture of sugar juice, starch, wax, gums and pectins, and used  as an animal feed supplement, as a fertiliser and a source of sugarcane wax.