Wheat (Triticum aestivum) has been highly prized as a source of food since ancient times and it is thought that a wheat‐like plant was cultivated centuries ago, providing pre‐historic man with grains which he ground and baked. The grain was valued by the ancient Persians, Greeks and Egyptians and well‐preserved samples of wheat have been found in the Egyptian pyramids, which were built about 5 000 years ago.
The best climate for the growing of wheat is a cool, fairly moist growing season followed by a warm, dry period to allow the crop to ripen and be harvested. Very large areas of wheat are grown the northern temperate regions of America and Europe and it is a major cereal crop in the United Kingdom. In all those regions, the crop is grown throughout the summer and harvested in the autumn, but it is divided into spring wheat and winter wheat. The spring wheat crop is sown in the spring season as soon as the soil has dried out enough to plant the crop. The winter wheat crop is sown in the autumn season and the grain germinates and grows to the four leaf stage. The young plants then have a dormant period which lasts throughout the winter, and they begin to grow again as soon as the weather warms up in the spring. In the very severe winters of North America and Northern Europe, particularly Russia, a good covering of snow protects the young plants from being damaged by the heavy frosts which occur in those areas. A poor covering of snow usually means that the young plants are killed by the frost and the whole crop has to be re‐sown in the spring. Different varieties of wheat are used for the winter and Spring‐sown crops. The advantage of the Autumn‐ sown crop is that the young plants start to grow again as soon as the weather warms up, so the crop ripens early and yields are heavier than they are for the Spring‐sown crop.
In Southern and Central Africa, wheat is a cool season crop, as high temperatures with high humidity promote disease. In South Africa, wheat is mainly grown as a winter crop, both in the winter rainfall areas of the Western Cape, as well as in the summer rainfall areas of country.
In the winter rainfall areas the annual rainfall is between 350 mm and 400 mm, with optimum planting dates from the second half of April through May, with the onset of the rains. In the summer rainfall areas soil moisture conservation techniques are important to ensure adequate moisture from January onwards under dryland conditions. Wheat is grown under irrigation, mainly in the Vaalharts, Groblersdal and Brits areas.
For high yields, 450 – 650 mm of water is required during the growing season depending on the climate. Peak moisture demand occurs at flowering, where yield could be reduced by 6% per day as a result of moisture stress.
Growth commences at 5°C and light frost at piping or flowering will cause sterility. Cold encourages stooling and temperatures of between 21°C and 32°C are ideal from the piping stage until maturity. For optimum production, the wheat crop requires between 1 150 and 1 500 heat units for the period May to September. The cool winters in the South African Middleveld and Highveld are ideal for producing high yields.
Wheat requires well‐drained, fertile, loamy soils with a pH of 6.0 to 7.5. Soil temperatures below 5°C are unsuitable for seed germination. Rooting depth is approximately 900 mm and roughly 75% of water uptake occurs in the first 600 mm. Irrigated wheat has fewer soil condition limitations.
Cultivar choice is an important management decision and should be taken in consultation with local consultants and advisors, bearing in mind local climatic conditions, production restrictions, the possibility of drought and disease and optimum yield requirements.
The main cultivars grown in South Africa are Palmiet, Baviaans, Kariega, PAN 3404, PAN 3408, SST66, and SST825; however there are many varieties and each wheat growing country has developed varieties suited to their own particular requirements.
3. STAGES OF GROWTH
It is important to know the various stages of growth of the crop so that spraying with herbicides, insecticides, etc., can take place at the right time. The diagrams below show these stages, and these are the same for wheat, barley, oats and rye.
JOINTING OR SHOOTING
Table 1: Stages of growth of wheat‐in days.
|Warm Areas||Days||Cool areas|
At the Milk dough stage, the grain is formed but the inside is very soft and milky
At the Soft dough stage, the grain can be squashed flat between the fingers
At the Hard dough stage, the grain is hard but not completely dry
Wheat will grow well on most soils but does best on rich loam soil. Soil tillage techniques are of prime importance and unnecessary cultivations waste money, time and effort, losing valuable soil water in the process. Deep tillage (150 mm – 300 mm deep), minimum tillage (75 mm – 130 mm deep) or no‐till can be practised depending on the soil type, moisture availability, cultivar and previous crops planted.
Firm, smooth and well‐drained fields should be selected and cleared of weeds and stones. If possible planting where wheat was grown the previous year should be avoided.
TIME OF PLANTING
The crop is normally planted in June/July, depending on the area and cultivar being planted. It is important to remember that frost during the period when the flowers are pollinating can damage the crop and reduce yields. A good reason for keeping temperature records on a farm shows when frost is likely to occur during winter and by using these records the planting date of the crop calculated so the worst frosts will occur before the crop reaches the pollination stage.
The best method of sowing the seed is using a seed drill as it uses less seed, gives a better yield, an even germination and a more uniform growth of the crop. Seed can be broadcast by hand using a fertiliser spreader or it can be broadcast from an aircraft. After sowing the seed the land should be cultivated with a light harrow to level the soil and cover the seed.
The seed should be planted evenly and shallowly (20 mm – 40 mm) in a moist firm seedbed. Spacing between rows should be approximately 50 cm – 100 cm and 30 cm in the row, with the wider rows for dryland plantings and the narrow rows under irrigation. The planting density ranges from 100 kg ‐ 180 kg per ha depending on cultivar.
As the crop is either sown in narrow rows or broadcast, it is not possible to use mechanical cultivations to control weeds, after the crop has been sown, but cultivation of the seedbed before planting will kill any weeds that are beginning to germinate.
Once the plants have emerged from the soil, herbicide such as MCPA+ can be applied up to the jointing stage. This can be done by tractor or aerial spraying. Normally weeds are shaded out and not a serious problem and the inclusion of a legume crop in the rotation will greatly assist.
Wheat should be rotated with soyabeans, maize and again with wheat. Other crops that can be included in rotation with wheat are cowpeas, groundnuts and lupins.
Either flood or overhead irrigation may be used. The growing crop will take moisture out of the soil to a depth of 900 mm. The seedbed should be brought up to field capacity at planting. The critical time when water stress should be avoided is from the booting stage to formation of the grain in the flowering heads. Lack of water during this period will cause a severe drop in yield. A proper irrigation schedule should be drawn up using an evaporation pan and taking into account the texture of the soil. Irrigation scheduling is covered in detail in the Irrigation module. As a general guide, the
wheat crop should receive 8 to 12 irrigations at intervals of 10 ‐ 14 days applying a total of 400 – 500
- of water. If the seed is sown into a dry seedbed and then watered, a second watering should be given shortly afterwards to soften or break the crust formed on the soil surface by the first application. This is particularly important on soils which have a bad capping factor (T factor).
In clay soils all Nitrogen (N) should be applied at planting. On sandy soils the N requirement should be divided into 3 or 4 increments provided that the total N is applied before late tillering. By splitting the nitrogen dressing into three it will be available when needed by the growing plants, and the final dressing will help to increase the protein content of the grain. The top dressing of nitrogen can be put onto the crop in the form of granular fertiliser or dissolved in the irrigation water. The quantity of nitrogen applied depends on the amount of organic matter in the form of crop residues ploughed in during land preparation.
The following recommendations are given as a guideline and fertiliser consultants should be approached for detailed recommendations.
|Table 2: Nitrogen requirements for wheat|
|Yield (tons/ha)||Nitrogen (kg/ha)|
|4 to 5||90 to 130|
|5 to 6||130 to 160|
|6 to 7||160 to 180|
|Source: Manson et al, 2004|
Table 3: Recommended P for wheat (kg/ha)
|Yield||Soil P (mg/litre)|
|4 to 5||32‐40||24‐30||16‐20||12|
|5 to 6||40‐48||30‐36||20‐24||15|
|6 to 7||48‐56||36‐42||24‐28||18|
Source: Manson et al, 2004
|Table 4: Recommended K for wheat (kg/ha)|
|Yield||Soil K (mg/litre)|
|4 to 5||75||50||25||0|
|5 to 6||90||60||30||0|
|6 to 7||105||70||35||0|
|Source: Manson et a, 2004|
Although wheat is more tolerant of acid soils than barley, it does best at a soil pH of 5.5 – 0.5 and, if lime is required, should be applied and disked into the seedbed along with the fertiliser dressing.
Micronutrients iron, manganese, zinc, copper and boron are essential for growth and development of wheat and deficiencies should be addressed early in the season to prevent yield losses.
3. PESTS AND DISEASES
A variety of insects with different feeding habits, are found on wheat but are not all of economic importance. The decision to introduce control measures should be made for each pest, depending on threshold levels and on recommendations of a spray consultant.
Mainly oat aphid, English grain aphid and rose grain aphid. Mostly prefer high plant densities with damp conditions, typical of the winter rainfall and irrigated areas. Russian wheat aphid prefers dryer conditions and is more numerous in dryland plantings in the summer rainfall areas.
Generally only noticed once the larvae have reached the mid‐instar stage and careful scouting must be undertaken. Chemical intervention should be applied when pest populations reach 5 to 8 larvae per square metre.
Figure 1 and 2: Aphid (left) and an American Bollworm (right)
Source: davidsuzuki Source: www.nbaii.res.in
Grain chinch bug
Damage is more pronounced under warm dry conditions where plants are under stress.
Brown wheat mite
Symptoms are mottled leaves developing into yellow and bronze patches due to sap feeding.
Chemical controls are available.
Figure 3 and 4: A Grain Chinch Bug (left) and a Brown Wheat Mite (right)
Source: entomology.k‐state.edu Source: cals.uidaho.edu
They feed on seeds, roots and seedling stems. Partial control by seed treatment.
Black maize beetle
Adults feed on seedling stems, killing plants and reducing plant population. Seed treatment will reduce damage.
Figure 5 and 6: False Wire Worm (left) and a Black Maize Beetle (right)
Source: daff.qld.gov.au Source: teara.govt.nz
LEAFHOPPERS AND MAIZE STREAK VIRUS
Young plants become stunted with curled leaves with white longitudinal stripes. There are no chemicals registered for the control of leafhoppers in wheat but can be prevented by later planting dates and in areas away from maize fields.
The rusts are widespread and important diseases of wheat and other small grains which also affect many other wild and cultivated members of the grass family. Rust diseases affect wheat by reducing yield and affecting quality. To maximise profits farmers must understand the effects of these diseases on crop potential.
Rust pathogens survive on host plants during the non‐crop season and are dispersed by wind onto the crops the following season. Several research groups are dedicated to the development of rust‐ resistant cultivars but it is generally a long process, so the use of specific fungicides is recommended under the guidance of qualified technicians and consultants. Foliar fungicides are generally applied twice in each season.
Also known as black rust commonly occurs in the Western Cape of South Africa from September onwards. Symptoms occur on leaf sheaths, leaf blades, stems and spikes, showing as elongated oval‐ shaped pustules which may converge, leaving the stem with a red‐brown appearance. Chemical control is possible using propiconazole or tebuconazole.
Also known as brown rust is found mainly in areas where wheat and barley are grown. Orange‐ brown elliptical pustules sometimes surrounded by a yellow halo are scattered randomly on the leaves and can appear on the ears of grain. Fungicides with the active ingredient carbendazim together with one of the following: Flusilazole, propiconazole, cyproconazole, tebuconazole or triadimefon can be applied. Epoxiconazole, propiconazole and tebuconazole may be applied alone for effective control.
Figure 7: Rust on Wheat
Stripe Rust is also known as yellow rust. Symptoms consist of yellow to orange coloured pustules that develop in narrow stripes on the leaf sheaths and the inner surfaces of glumes and lemmas of the heads. Chemical control is similar to leaf rust but technical advice should be sought regarding specific chemicals.
Common disease of cereal crops worldwide. Symptoms are often seen on the leaves and include fluffy white pustules that become grey with age. These pustules can often be scraped off the surface of the leaf as the infection is very superficial. Later in the season black dots can be found embedded
|in the white pustules and are the fruiting bodies. The fungus survives the non‐crop season on host|
|debris or volunteer plants. The foliar application of carbendazim|
|together with one of the following: flusilazole, propiconazole,|
|cyproconazole, tebuconazole or triadimefon, is commonly used|
|for chemical control.||Alleviation: make suffering,|
|MAIZE STREAK VIRUS||deficiency, or a problem less|
|Also affects many grass species including wheat where it causes|
|wheat streak or wheat stunt. Symptoms include fine, linear,||Necrosis: the death of most|
|chlorotic leaf streaks, shortened tillers, leaves and spikes and||or all of the cells in an organ|
|excessive tillering. The virus is transmitted by leaf hoppers from||or tissue due to disease or|
|infected maize to healthy wheat. The disease can be avoided by||injury.|
planting in areas where affected maize and grasses have been removed and controlling the disease vector, the leaf hoppers but the planting of resistant cultivars remains the best control option.
FUSARIUM HEAD BLIGHT
A fungal disease is especially important where wheat is grown under irrigation. The disease is characterised by the discolouration of infected florets approximately 2‐3 weeks after flowering. The florets become light‐coloured and appear blighted with the entire wheat head being infected under high disease pressure. Infected kernels become shrivelled and contain much less starch and protein. The fungus survives primarily on crop residues and can affect maize.
SMUT AND BUNT
These fungal diseases infect wheat and other small grains and grasses. The fungi produce masses of black spores that completely replace the wheat heads, spikes and kernels. These diseases can be controlled by routine seed treatments by the seed supply companies.
FUSARIUM CROWN ROT
One of the most important soil‐borne diseases of wheat in the Western Cape of South Africa, but is also found in other small grains in other production areas particularly under dryland conditions. The disease is characterised by the honey‐brown discolouration of the lower parts of the tillers and necrosis of the crown tissue and sub‐crown internodes. The most characteristic symptom is the formation of whiteheads is dependent on water stress during the grain‐fill stage. The fungus survives primarily on crop residues between host crops making it a problem in conservation tillage and no‐till monoculture regimes.
Fusarium crown rot can be reduced by:
Implementing a rotation strategy with non‐host crops such as canola, lupins, medic, lucerne, etc.;
Control of grass weeds and crop residues; Alleviation of zinc deficiency;
Reduction of moisture stress; and Planting of resistant cultivars.
Occurs widely throughout all wheat‐producing areas and affects the roots, crowns and basal stems of small grain crops and grasses especially where wheat is under intensive cultivation. It favours areas where the soil pH is neutral or alkaline, moisture is abundant and soils are deficient in manganese and/or nitrogen. Infected plants ripen prematurely and are stunted. Symptoms are more apparent during heading when affected plants are uneven in height, die prematurely and discolour to the shade of ripe plants. A typical take‐all infection is characterised by the appearance of patches of white heads amongst the healthy plants. The heads that ripen prematurely are generally sterile and contain shrivelled heads. Diseased plants pull up easily and roots appear blackened and brittle. The lower stems may take on a black colour which is indicative of the disease. The disease is favoured by poorly drained soils, high seedling densities and high organic matter. It is more prominent in wet years or irrigated fields. Control can be achieved through:
Crop rotation or a one‐year break from wheat or barley;
Destruction of volunteer plants, grass weeds and removal of crop residues;
Ensuring that wheat plants have sufficient nutrients to promote healthy root growth; and Seed treatment.
Wheat should be harvested when completely ripe with no green colour in the straw and when the grain is hard and dry. Harvesting should commence at a grain moisture content of 16% reducing to 13% for optimum storage.
Traditional harvesting methods comprises of three operations:
Reaping, using sickles and scythes;
Threshing, which is the separating of the grain from the plant material; and Winnowing is the process of separating the threshed grain from the chaff.
These days the wheat crop is cut and the grain separated and cleaned by combine harvesters. The drum settings should be checked before harvesting begins to make sure that the grain will not be chipped or damaged. The wheat straw can be baled and used as roughage feed for cattle or for bedding.
Dryland ‐ 1.5 tons to 3 tons per hectare Irrigated ‐ 3 to 6 tons per hectare
The wheat straw will yield between 0.5 tons and 1.0 tons per ha.
Figure 8: Combine Harvester Harvesting Wheat
Figure 9: A Baler Baling Wheat Straw
Before milling begins, different wheat is blended so that the flour produced will be the right strength for baking. Hard or strong wheat which have high gluten content are mixed with soft or weak wheat, which are those with low gluten content.
The mixture is thoroughly cleaned to remove weed seeds and any other foreign matter. Milling is done by passing the grain through a series of rollers of increasing fineness and closeness of setting. This cracks the grains and the inside of each grain is set free, while the darker coloured skin is broken up as little as possible. At each stage the material coming from the rollers is sorted by means of sifting machines and fans into kinds of flour of different grades and fineness.
The final products of the milling process are:
Flours and semolinas, of different grades;
Wheat germs, which are the embryo of the grains;
Middlings, pollards or wheatings, which are the fine part of the skin; and
Bran, which is the coarse part of the skin, is graded into coarse and medium bran.
When good bread wheat is milled, it should produce from 70 – 80% of white flour with the remainder being middlings and bran. An extraction rate of 85% gives a darker grey‐brown flour but because this flour contains more of the skin of the grain, the vitamin content, particularly vitamin B, is higher than that of white flour. When the rate of extraction is raised above 85% the flour is darker and used for baking brown bread, and the vitamin content is also higher.
THE GRADING OF WHEAT
Wheat is sampled and graded according to its suitability for milling, taking into account the following factors:
The moisture content of the grain should be below 12.5%;
The sample is checked for broken, shrivelled and damaged grains and for chaff and other impurities, and a deduction is made from the weight to allow for these;
The best wheat has large grains with thin skins to give the highest proportion of flour. The size of the grain is checked by weighing a standard volume and called the bushel weight, or the 1 000 grain weight.
The wheat should be in good condition, free from mould with a sweet clean smell. Any wheat sold for milling must never be dressed with any type of seed dressing as these are highly poisonous to humans.
USES OF WHEAT
Wheat can either be eaten whole or ground into flour;
Wheat is used in the production of alcoholic beverages and industrial alcohol, which has a wide range of applications;
Starch is extracted from wheat and used widely;
Straw is made into mats and baskets, and used as packing material as well as animal bedding. It is also used in paper manufacture;
Bran is an important livestock feed and wheat germ a valuable ingredient of feed concentrate; and
Wheat grain can be fed to livestock, whole or coarsely ground.