Groundnuts (Arachis hypogaea) are a species in the legume family thought to have first been domesticated in the valleys of Paraguay. It is an annual herbaceous plant growing up to 30cm ‐ 50cm tall. The flowers are yellow with reddish veins. After pollination the flower stalk elongates and bends until it touches the ground. The stalk continues to grow, pushing the ovary underground where the mature fruit develops into a legume pod containing the groundnuts. The plant was later spread worldwide by European traders and for a long time has been an important constituent of the African peoples’ diet.
Groundnuts require a rainfall of 500 – 700 mm over the growing season. The mean annual rainfall should be at least 700 mm. There are limited areas in South Africa where groundnuts are successfully produced under dryland conditions; therefore, the runner type varieties are excluded because of their long growing season and high water demands. These long‐season varieties are only suitable for irrigation production in South Africa. Groundnut varieties that are suitable for dryland production have a short growing season, upright growth habit and are of a Spanish type. Groundnuts are tolerant of hail but moisture stress leads to a large percentage of empty pods.
For optimum growth the mean daily temperature is 22 ‐ 28⁰C and for optimum germination soil temperature should be between 20 and 30°C. Yield reduction occurs above 33⁰C and below 18⁰C. Groundnuts should not be planted in warm dry soil and should be irrigated with cold water.
Figure 1: Groundnut Plant
Groundnuts must be grown in a wide rotation to help prevent the buildup of diseases such as Cercospora Leaf Spot. Under dryland conditions they must not be cropped more often than once in 4 years on the same area of land. Under irrigation where lands are producing 2 crops a year it is thought the groundnuts can be grown on the same land every second year, provided the crop is regularly sprayed with fungicide and lands are kept free of volunteer plants and crop residues between each planting.
In the Middle and Highveld areas of South Africa the harvesting of winter wheat and planting of long‐season groundnut varieties will clash and will only be possible to plant the groundnut crop after a winter fallow. This means that long‐season crops like maize and cotton could be included in a rotation such as maize or cotton, fallow, groundnuts, and thereafter wheat.
Figure 2: A thriving field of groundnuts
Groundnut varieties are divided into two groups according to the time taken to reach maturity. The table set out below shows the time taken to reach maturity by the different varieties:
Table 1: Varieties of groundnuts grown in South Africa
|Maturity Group||Variety||Maturity (Days)|
Akwa is short‐season bunch‐type variety which matures in 150 days depending upon temperature. It has a good resistance against pod‐nematodes, black hull and sclerotinia with a moderate resistance to botrytis stem rot. Under drought conditions (low yield conditions), Akwa can still produce acceptable yields and in high potential conditions Akwa can obtain a yield of up to 3.75t/ha. They are recommended for dry and warm areas ie: North‐West, Northern Cape and the Western Free State.
This variety is also a short‐season crop very similar to Akwa maturing in 150 days. Although it is similar it does not have a good resistance to pod nematode, black hull and sclerotinia but has a moderate resistance to botrytis stem rot. This variety performs best under low yielding conditions and is also recommended for dry warm areas as it preforms better under adverse conditions.
This is a short‐season bunch‐type variety which has the shortest growth period and matures in 140 days depending upon temperature. It has a good resistance to black hull and botrytis stem rot, poor resistance to pod nematode and moderate to sclerotinia. Under adverse conditions the Harts variety does not preform very well and should not be planted in areas which are susceptible to drought or an unstable climate.
This variety has a short growing season of 150 days. It has a moderate resistance to pod nematode, black hull and botrytis stem rot with good resistance to sclerotinia. Jasper does not do well in areas or conditions of low yield potentials but does well when the conditions are good.
This variety is a long‐season variety which matures in 180 days. It is not suitable to grow under dryland conditions and can only be grown successfully under irrigation. This variety has good resistance to pod nematodes and black hull but moderate to botrytis stem rot and a poor resistance to sclerotinia.
This variety is very similar to Billy in that it is a long‐season crop which has the same maturity days of 180 days and is only successful under irrigation. It does not have a good resistance (poor) to pod nematode, botrytis stem rot and sclerotinia but has a moderate resistance to black hull.
Selmani is a long‐season variety that matures in 175 days and is only recommended to grow under irrigation. It has poor resistance to sclerotinia but moderate to pod nematodes, black hull and botrytis stem rot.
Figure 3: A crop of harvested groundnuts
SEED DRESSING AND INOCULANT
Plant populations may be drastically reduced if a seed dressing is not used. Seed dressings must be non‐mercurial. Mercurial seed dressings affect nodulation and are believed to cause a build‐up of crown rot. Thiram and Mancozeb are the recommended non‐mercurial seed dressings.
Research has shown that inoculation can increase yield by up to 160 kg of kernels per hectare. Seed must be inoculated just prior to planting and kept in the shade until planted. The inoculant is simply a solution containing the bacteria which go to form the nodules on the roots of the groundnut plants as they grow. These bacteria are not always present in the soil and it is a wise precaution to add them to the seed before planting. The inoculant is a liquid which can be diluted with skim milk and added to the seed at the rate of 1 bottle of inoculant for 1 pocket of seed. Apply the inoculant and allow the seed to dry before sowing.
Groundnuts have the remarkable ability to adapt themselves to almost any soil type. Although they grow best on the well‐drained medium‐textured soils, high yields are nevertheless attainable on the sandier and heavier‐textured soils.
This crop should preferably be grown in rotation with other crops which have been well fertilised as evidence suggests that groundnuts respond better when fertiliser is applied to the previous crop. Nevertheless the nitrogen, phosphorus and potassium requirements of a particular land are most reliably assessed by soil analysis, and farmers are strongly advised to have samples from their lands analysed regularly. Failing this, a rough assessment of the probable requirements can be made from what is known of the past history and fertiliser treatment of the land.
|Table 2: The Average Fertiliser Nutrient Requirements of Groundnuts|
|Fertiliser||Good Soil||Medium Soil||Poor Soil|
|Nutrient||(Kilograms per hectare of fertiliser nutrient required)|
|Nitrogen||Nil||Nil||20 ‐ 30|
|Phosphorus||Nil||20 ‐ 40||40 ‐ 60|
Groundnuts have a high calcium requirement, particularly during pod development. Low availability at this time will result in a decreased shelling percentage and high incidence of ‘pops’ or empty pods. Gypsum should be applied at the rate of 200 – 300 kg/ha to ensure an adequate available calcium status.
METHOD AND TIME OF APPLICATION
The whole of the nitrogen, phosphorus and potassium requirement is applied in the form of straights or compound fertilisers. These should be broadcast and disced in as deeply as possible after ploughing.
Under dryland conditions the gypsum should be applied along the top of the rows at early flowering. On early‐planted irrigated crops 2 dressings of gypsum each of 200 – 300 kg/ha should be given 8 and 12 weeks respectively after germination.
For maximum yields the calcium‐chloride pH value of the soil must be above 5.3. It is, therefore, essential to correct the pH of the land by adequate applications of lime where necessary, but this should only be done on the basis of a soil analysis.
The lime may be applied at any suitable stage in the rotation and may be applied immediately before planting. When applied immediately before planting it must be broadcast and disced in deeply, preferably after ploughing.
5. LAND PREPARATION
The land must be well ploughed ‐ deep enough to make beds easily, i.e. 200 – 250 mm. The tilth needs to be fine in order to produce a fine uniform seedbed so that the soil and the seed establish good contact. Groundnut seed is very susceptible to rotting if there is too much air around the seed. Fertiliser should be broadcast over the soil and worked in before planting.
Best results will be achieved on deep well‐drained soils in good condition. Production should not be attempted on soils of low fertility or soils known to be worked out. Good yields have been achieved on sandy soils and red clay loams.
Lifting is a problem in the heavier soils because some pods break away from the plants and remain in the ground and excessive quantities of soil often stick to the lifted pods. Built‐up beds together with irrigation prior to lifting will help reduce the number of pods left in the soil and reduce the quantity of soil adhering to the pods which have been lifted.
If maximum yields are to be realised, crops must be planted early. Depending on variety and temperature, a delay in planting will cause a marked drop in yield. Optimum planting dates are set out in the table below:
Table 1: Varieties and planting dates
|Type of||Variety||Altitude: Metres||Planting|
|Dryland||Earlies||Below 900||1 – 30 Nov|
|900 – 1500||1 – 20 Nov|
|Above 1500||20 Oct – 14 Nov|
|Irrigated||Earlies||Below 900||1 Oct – 15 Dec|
|900 – 1500||1 – 30 Nov|
|Above 1500||15 Oct – 30 Nov|
|Lates||Below 900||1 Oct – 14 Dec|
|900 – 1500||15 Oct – 1 Dec|
|Above 1500||15 Oct – 15 Nov|
The dates in the previous table are suggested as being optimal but will depend upon the other crops being grown and the rainfall pattern. All the above dates depend upon many climatic factors as well as cultivar choice. These are merely guidelines for the optimum production of groundnuts.
Early planting for maximum yield is dependent upon good management at reaping time. Both early and late varieties must be lifted when they reach maturity to prevent sprouting and loss of quality. A farmer growing the crop for the first time should plant slightly on the late side to acquaint himself with the lifting and harvesting problems encountered after the main rains have ceased. Farmers who have knowledge of the crop and sufficient machinery and labour should be able to lift 6 to 8 hectares per day preventing yield and quality losses.
There are a number of ways in which groundnuts can be planted in order to achieve the best results. The in‐row or intra‐row spacing of groundnuts should be 50 – 75 mm for all available varieties. If they are being grown on Sandveld and all operations are being carried out by hand, the seed should be planted in rows which are 900 mm apart for dryland and 300 – 720 mm for irrigated land, depending on the cultivar used. There are a number of other factors which influence the row spacings such as moisture, disease and weed control. The narrower the row spacing, the higher demands in management. This is in terms of spraying the crop for diseases as there is a dense leaf canopy which does not allow all the leaves to obtain an effective spray.
Figure 1: Plant spacing on dryland groundnuts
Table 2: Row widths and cultivar choice
|Norden and Selmani||–||600 – 720 mm|
|Harts and Kwarts||900 mm||300 mm|
|Sellie and other Cultivars||900 mm||450 mm|
2. WEED CONTROL
Weed control is most important if high yields are to be achieved. The following points are stressed for each crop.
Groundnuts are normally planted in closely spaced rows so weeds must be largely controlled by the use of herbicides. The herbicides recommended break down completely within 3 or 4 months and will not affect following year’s winter crops.
Cultivation: the action ofcultivating land, or the state of being cultivated.
Herbicides will not, however, provide complete weed control and a certain amount of hand and machine cultivation will be
necessary. On suitable layouts and particularly sandy soils, a rotary hoe or rotary tined cultivator (millipede) will remove shallow rooted weeds resistant to the herbicide applied. This cultivation must be done 2 – 4 days after irrigation while the weeds are very small. Rotary hoe cultivation may continue until flowering or when the crop is 150 mm high. Hand cultivation is also possible until flowering but beyond this stage weeds must be hand‐pulled to prevent damage to the pegs.
Table 3: Herbicides
|Common Name||Sold As||Method of Use|
|Trifluralin||Trifluralin||Incorporate immediately 50 to|
|100 mm deep prior to planting.|
|On ridged crops, the above herbicides must be incorporated after ridging.|
|Apply at crack stage or earlier if|
|weeds are seen. Crop damage|
|Bendioxide||Bentazone 480 SL||will occur if applied later than|
|the above‐mentioned stage of|
|Terbutryne||Igran 50 WP (wettable powder)|
|Igran 500 FW|
|Afalon||Apply prior to crop and weed|
|Paraquat||Gramoxone||Post‐emergence. Will kill all|
|plants growth which it contacts.|
This grass occurs only on the heavier soils. Good herbicidal control can
be achieved by the use of trifluralin which requires soil incorporation. These occur on all soil types and are controlled by trifluralin, alachlor and metolachlor.
Crow Foot Grass
Figure 2 and 3: Itch grass (left) and Crow foot grass (right)
|Source: invasive||Source: wikipedia|
|Pigweed||Easily controlled by bentazone, terbutryne, linuron and prometryn.|
|May be controlled by trifluralin, alachlor and metolachlor.|
|Apple of Peru||Control with alachlor, metolachlor, terbutryne, prometryne and|
|Mexican Clover||A serious problem on some lighter soil areas, particularly in a wet|
|season.Trifluralin, alachlor and metolachlor may give some control.|
|Starbur and Wild Hibiscus||Control with alachlor, metolachlor, terbutryne, prometryne and|
|The most effective herbicides are terbutryne, prometryne and linuron.|
|Mariglold Gallant Soldier||Alachlor and metolachlor will give some control.|
|Wandering Jew||This weed can be particularly troublesome in the wet seasons. None of|
|the herbicides recommended for groundnuts are reliably effective|
|although metolachlor or alachlor will give some control.|
|Satisfactory control can be achieved by using alachlor or metolachlor at|
|Nutgrass||the full rate recommended for the particular soil type. However, the|
|soil must be disced immediately prior to planting followed by herbicide|
|application immediately after planting. Rainfall or irrigation within 1 ‐ 2|
|days of application is also needed.|
Figure 4 and 5: Broad Leaf Weeds, Buckwheat (left) and Pigweed (right)
Source: ecolibrary Source: permies
Figure 6 and 7: Apple of Peru (left) and Mexican Clover (right)
Source: floreznursery.blogspot Source: ricgra_dig1_lr
Figure 8 and 9: Wild Hibiscus (left) and Black Jack (right)
Source: nams. Source: sbwildflowers.wordpress
Figure 10 and 11: Wandering Jew (left) and Nutgrass (right)
Source: ehive. Source: patchofweedsinparadise.wordpress
Late weeds prior to harvest should be pulled by hand only.
Increased production and the introduction of irrigation into the cropping programme have significantly altered the disease pattern in recent years. At present the most serious groundnut diseases include the following:
Large dark stem lesions covered with grey sporing fungal growth ultimately leading to the death of growth above the infection. Top growth appears normal during the early stages of attack.
Growers should, therefore, examine stems beneath this top canopy from January onwards for first signs of attack. Botrytis (Grey Mould) will not develop until plants have completely covered the ground and so have established a micro‐climate favourable for infection.
Figure 12 and 13: Grey Mould or Botrytis Stem Rot
Source: peanut.ncsu Source: peanut.ncsu.
Despite the many recent changes in the name for this pathogen it is the one originally called Ascochyta sp. Diffuse olive brown lesions with wide pale margins develop on the upper leafsurface which eventually darken in colour and penetrate through the leaf. With age tissue tears and leaves drop. Symptoms which may appear from November onwards, are most severe during the January/February period and result in severe defoliation.
Figure 14 and 15: Leaf Blotch or Web Blotch
Source: ncsupdicblog.blogspot Source: insectimages
This disease causes similar leaf spotting on the upper surface. The spots are dark brown with a yellowish halo and up to 50 mm in diameter. The diseases can be distinguished by examining the lower leaf surface where one variety causes a definite black spot. Spore initials are visible to the naked eye. In the other variety lesions on the underside of the leaf are brown in colour. In the main groundnut areas on the highveld this disease appears only late in the growing season, if at all, whereas in the lowveld these pathogens cause extensive defoliation.
Figure 16 and 17: Leaf Spot
Source: peanut.ncsu Source: insectimages
In most instances pod rot is associated with some other factor restricting optimum crop development, e.g. drought conditions, insect damage (Hilda), soil pH and so forth. Various pathogens can then invade the pods and cause discoloration and, very often, attack the
seed. To control pod rot it is advisable to maintain optimum growing conditions.
Figure 18: The beginning stages of black pod rot.
ROOT AND CROWN ROT
This pathogen attacks a wide range of hosts but under local conditions has not severely damaged any particular one, including groundnuts. Damage is largely restricted to individual or small groups of plants which show growth of thick white fungal strands over roots and crowns adjacent to the soil surface. Associated are tan coloured sclerotia (small bodies about the size of a cabbage seed). Top growth withers. To control Root and Crown Rot, it is advisable to maintain good growing conditions and plough deep to bury sclerotia.
Another pathogen which attacks many different hosts. Infection of stems causes foliage to wilt. Large black sclerotia associated with white mycelial growth are produced in the diseased stem. To control this disease, it is advisable to plough deep and rotate crops with non‐susceptible hosts.
Figure 19 and 20: Sclerotinia Wilt
Source: ncsupdicblog.blogspot. Source; peanut.ncsu
Undersides of the leaves show small orange to dark brown pustules. Infection occurs early in the season in lowveld crops and can cause severe defoliation. In Highveld plantings infection comes in so late that the disease has not been of any consequence so far. To control this disease no specific rust control has so far been found necessary in Highveld crops.
Figure 21 and 22: Rust on Ground nuts can be detrimental.
Source: agropedia.iitk.ac. Source: agropedia.iitk.ac.
Chlorosis: stunting and some deformity of young leaves. Very often such an area will besurrounded by a fringe of normal dark green leaves which developed before infection occurred. This is only serious when plant stands are poor. The control of this disease is advised by establishing stands of high density and destroying over‐wintering plants.
The following is a summary of the more important groundnut pests with their control measures. Insect control is very important where maximum yields are expected. The correct selection of insecticides is very necessary since several crops have been rejected in world markets owing to excessive residues of pesticides.
The importance of termite damage was not fully appreciated until investigations into the cause of mouldy kernels showed that substantial pod damage could be attributed to these insects. Plants may be attacked at any stage of growth by Macrotermes spp. which constructs the familiar large mounds in the veld. Young plants may be surrounded by the coarse‐grained earthworks of the termites, and severed and killed. Damage to mature pods may be considerable, especially if the plants are under stress from drought conditions. Small fungus‐ growing termites, e.g. Microtermes spp. are also responsible for damage to mature pods and are on the whole more important than Macrotermes spp.
Termites can be controlled by the fumigation of nest sites after boring into the roof of the mound or through air shafts. Fumigants that may be used are:
Ethylene dibromide: 0.3 litres per mound. Place wads of cotton wool below the bored holes or in the air shafts to absorb the liquid fumigant.
Methyl bromide: 1 canister (454 grams) per mound. Applied by jiffy applicator with the tube led into the mound. After fumigation has been completed, treated nests should be excavated and refilled to prevent re‐infestation of the site. All small packages (680 grams) of methyl bromide have been withdrawn from the market.
Figure 23 and 24: Termite Damage to Groundnuts
Source: on‐the‐farm Source: on‐the‐farm
Cutworms are the caterpillars of night‐flying moths. The grey greasy caterpillars are often found lying curled in the soil near the damaged plants which are generally eaten off just below ground level. The moths will lay eggs on or near a wide range of crops at any time of the year. Lands on which a green‐manure crop such as Sunnhemp or Dhal has been grown and irrigated and/or manured, will attract moths to lay eggs before the crop is planted. If cutworms are present when crops are at an early stage of growth, the young shoots will be eaten but the crop may recover. Should cutworms be present 5 ‐ 6 weeks after planting they will cause considerable damage by consuming young peas and penetrating the more developed pods while these are still succulent. Mature pods are not attacked by cutworms. There may be no sign of damage above ground so examination of developing pods during the susceptible period is recommended.
Where cutworms are expected, they may be controlled by pre‐plant baiting with:
120 g trichlorfon 50% WP
20 kg maize meal/25 litres water 500g sugar or 1 kg molasses
20 litres of water
Mix the items above and then add water to obtain a crumbly texture. These quantities are sufficient to treat one hectare and should be applied in the late afternoon.
Figure 25 and 26: A cutworm (left) and the damage inflicted by a cutworm (right)
Source: entomology.unl Source: highdesertgarden
This is a common black aphid which is often found infesting beans and other legumes. It is important as the vector of the virus disease known as rosette of groundnuts. The symptoms of the disease in its severest form consist of stunting of growth and chlorosis of the leaves. Attack at a later stage of growth produces only the occasional chlorotic leaf. The disease may cause substantial reduction of yield particularly in a poor stand of groundnuts. The risk of rosette disease may be minimised by establishing a good stand. Diseased plants should be removed to prevent the spread of the disease within the crop.
Aphids can be controlled by sprays of:
500 ml of Demeton‐s methyl (Demeton EC) per hectare in a convenient volume of water, i.e.
200 litres or more/ha; or 800 ml/100 litres of water of dimethoate (Deus 400 EC) per hectare.
Figure 27 and 28: Aphids
Source: retirementdreams Source: discoverlife
Close examination of a wilting groundnut plant may reveal large numbers of ants around the base of the stem. If this plant is pulled out of the ground, small hopping insects may be found in the disturbed soil. Adults are about 5 mm in length. Batches of small elongated white eggs will also be found firmly fixed and flat against the underground parts of the stem and pods and often within chambers excavated by the associated ants. One generation is completed in about 6 weeks in Summer. Breeding continues slowly through the Winter on alternate hosts or overwintering volunteer plants.
Hilda patruelis would seem to be important only when encouraged by ants which probably protect them from predators and, in return, feed on the honeydew secretions of the hopper. The presence of these hoppers feeding on the sap of the plant and ants which expose the roots to desiccation through their workings, results in the wilting of the plant.
Hilda patruelis can be controlled by spot‐spraying infected plants and plants within a radius of 3 metres around them with an appropriate insecticide. Alternatively band 3 metres wide may be sprayed around the land. In cases of severe attack on over‐all application of insecticide may be necessary. Demeton‐s‐methyl, monocrotophos and parathion have been used in the past but monocrotophos and parathion have since been banned.
If demeton‐s‐methyl is to be used, it may be applied at the rate of 500 ml EC in 100 litres of water per hectare.
SEMI‐LOOPER CATERPILLARS, PLUSIA SPECIES
These caterpillars are smooth, usually bright green with longitudinal stripes of dark green and white down the length of the body. Where large numbers of these caterpillars feed together on the same crop they become very dark green and have been confused with army‐worm caterpillar. However, the semi‐looper caterpillars are easily distinguished by their mode of
progression. When these caterpillars move they draw the posterior part of the abdomen up to the thoracic region thereby producing the characteristic loop in the middle of the body, whereas army‐worm caterpillars move in a normal crawling manner like most caterpillars.
Semi‐looper caterpillars can be controlled by spraying with:
1 kg carbaryl 85% WP per hectare in a convenient volume of water with 5% molasses (by volume)
Figure 29 and 30: Cabbage Looper Larvae (left) and Moth (right)
Source: waggabirds.wordpress Source: extension.umn
LOOPER CATERPILLARS: (ARCHAEA FINITE GUEN)
These are yellowish green caterpillars which may be recognised by the presence of a thin red line running along each side of the body above the legs. Their maximum length is approximately 60 mm.
The main points to be borne in mind are:
Soil moisture must be brought to field capacity to a depth of 900 to 1 200 mm before or just after planting. On sandy soils this will require approximately 75 mm of water and for heavy soils about 125 mm. Furrows opened for hand planting retain their shape better where lands are pre‐irrigated thereby permitting an even depth of planting. There is evidence that groundnuts respond well to stress after planting. Irrigation equipment can be used for other crops during this period.
The timing of the initial irrigation will depend on soil type and the herbicide treatment adopted. It is advisable to pre‐irrigate sandy soil to prevent fertiliser leaching. When herbicides need to be applied as well as irrigation, 1 or 2 days after planting, it is advised to apply the herbicide, whent the soil starts to dry and crack. It is important not to irrigate at the cracking stage as this has been observed to hamper the emergence of the seedlings.
Irrigation applications from the time of flowering to lifting must be related to readings from a Class A Evaporation pan.
It is very important for the crop to be fully irrigated throughout the growing period up to the lifting stage. Once pegging commences sandy soils should be irrigated at intervals of approximately 8 days, intervals on heavy soils should be approximately 16 days. Groundnuts require 9 mm of water a day or 63 mm of water a week. Calcium uptake by pods in sandy soils can be adversely affected if the top 50 to 100 mm of soil is allowed to dry out.
Crops which are irrigated by overhead systems should be planted on the flat or on raised beds which will assist at lifting. Rows should run across the slope to reduce runoff.
Water management of flood‐irrigated crops appears to be easier where the crop is planted in beds between well‐constructed border strips. Border strips must be large enough to force water along the bed and permit tractor access even during wet spells. Germination on flood schemes may be assisted by the use of water planting techniques, e.g. rigid plastic piping, thereby overcoming the problem of wash in the beds.
Harvesting is a broad term used to describe the operations necessary to remove groundnut pods from the soil and prepare them for market. No part of groundnut production calls for more knowledge, skill, care and attention than harvesting. Losses are probably greater at harvesting than at any other time.
Harvesting should commence when 70 – 75% of the pods contain mature kernels. A delay in lifting beyond the 75% kernel‐maturity stage will result in pod loss and lower quality as a result of excessive discolouration and mould. Mature kernels have thin seed coats which are not easily rubbed off by hand and have attained the colour of the variety being grown.
Discolouration: theprocess of changing to a different, less attractive colour.
By comparison immature kernels have thick fleshy seed coats which rub off easily and are lighter in colour than mature kernels.
Actual timing of lifting will depend upon the size of the crop and the time taken to plant it. To ensure that lifting is complete before the crop reaches 75% maturity, an earlier start must be made when lifting large areas of crops than when lifting small areas. Time of maturity cannot be set accurately from the time of planting owing to the fact that maturity is also influenced by other factors such as altitude, soil and weather conditions during plant growth. Growers should examine plants in the field regularly to determine the percentage maturity and the stage when harvesting should commence. The crop must be lifted when it is 90% defoliated irrespective of kernel development.
Present harvesting methods can be divided into conventional and mechanical depending on the methods used. The equipment used will influence the further subdivision of mechanical harvesting.
The operations involved with conventional and mechanical harvesting are discussed below:
Loosening consists of cutting the tap root 100 – 150 mm below the soil surface and loosening the soil by means of a groundnut lifting blade. This is done to enable the plants to be lifted from the soil without breaking the pods from the pegs and so leaving them in the soil.
Lifting consists of pulling the plants by hand shaking them to get rid of excess soil and placing them in a windrow. At this stage the moisture content is between 40 and 50%.
Plants are left in the windrow for 2 to 3 days until the moisture content has dropped to ± 20%.
Prevailing: existing at aparticular time; current.
Cocks are constructed so that the groundnuts dry to ± 10% without damage to the kernels. Various forms of cock construction are being used by farmers. The main points to bear in mind in their construction are:
- Plant material must be kept off the ground to prevent moisture uptake, and subsequent moulding and insect damage.
- Construction must allow for air movement through the cock to facilitate rapid drying.
- The top of the cock must be constructed so that water cannot penetrate the cock during rainy spells.
Groundnuts are normally left in the cocks for 3 to 6 weeks before picking commences and depends on the prevailing weather conditions.
Figure 1: Tripod commonly used for drying groundnuts
Figure 2: An A frame used for drying groundnuts in Africa
Picking should commence when the kernels rattle inside the pods and the pods can easily be removed without causing long shreds. The moisture content of the seed should be 10% when shelling. Picking can be done by machine or by hand. Hand picking has the advantage that mouldy and damaged pods can be discarded during picking, thereby improving quality. Output should vary from 1 to 2 bags of pods per labourer per day. Various devices will increase this output to 3 or 4, even 10 bags per labourer per day.
For crops of more than 10 hectares, hand picking can be extremely laborious and investment in a machine picker would be worthwhile. Two South African machines are in limited supply, the most common to date being the Slattery which has an output of approximately 10 bags per hour. This machine produces a good clean sample on light sandveld but additional hand‐ cleaning is usually required for nuts grown in the heavier soils. The Nigel 5 picker has a claimed output in excess of 20 bags of groundnuts per hour.
Lifting with the digger‐shaker machine commences the same as for hand lifting when the crop is judged to be about 50% mature. The digger‐shaker should be adjusted correctly to avoid loss of pods. Attention to the following points is important:
Set the blades to cut all roots across their full width, i.e. position the back end points of the blades close enough together to ensure that all plants are cut. The blades should cut the tap root just below the cluster of nuts on each plant. Adjust the depth wheels and tractor linkage setting to achieve the correct depth with the blades horizontal.
Adjust the height of the leading pick‐up bars low enough to allow the crop to flow without being so low as to dig the ground below blade level.
Set the PTO drive speed carefully depending on forward speed of the machine. If the PTO runs too fast, plants will be ripped and give rise to high pod losses. Too slow a PTO speed will result in bulldozing ahead of the pick‐up possibly causing damage to the rattler bars.
Dig only as much as can be handled by the combine which is up to approximately 4 hectares a day.
The moisture content of the nuts must not be too high when placed in the drying bins. The crop should accordingly be wilted in the windrow for 2 or 3 days. Avoid over‐wilting in hot dry weather.
Re‐shaking of windrows may be necessary if soil separation was poor during the initial lifting. This operation should only be performed if absolutely necessary and be done as soon after lifting as possible, as delay will result in pod loss. To re‐shake the crop, remove the cutting blades from the digger‐shaker and use the machine as a ‘pick‐up’.
Nuts should be in the region of containing 20 to 25% moisture after 2 or 3 days wilting. This is the optimum moisture content for the least damage during combining. Careful adjustment to the stationary fingers, the air blast and cleaning screens, should be made according to the instruction book.
To avoid picking up clods, the pick‐up head should run so that the fingers are just above ground level. Again the speed of the PTO drive to the pick‐up mechanism should be regulated in relation to forward speed so that the crop flows evenly into the combine. A constant check must be kept on the picked sample and the pneumatic elevator. A check must also be made behind the combine where shelled nuts and other problems can be identified.
Adjustment to the combine may be necessary as harvesting progresses during the day and the crop dries out. Earlier combine models still in use are the bagger types but a bulk type of machine has more recently been used. Wherever harvesting is to be done by combine the whole crop from land preparation to drying and dispatch should be specially planned to avoid delays which can be expensive.
The combine is designed to pick a relatively green crop. Although it has been used to pick a dry cocked crop this usually results in a high percentage of cracked pods and shelled kernels and therefore should be avoided if possible.
An increasing numbers of farmers are now drying groundnuts artificially. Some of the benefits are:
Reduced field losses from rodents, termites, mould damage, shattering, etc., while the crop is in the cock and handled dry;
Earlier cash income;
Larger quantity and a much better quality hay since the tops will still be green and whole after picking. This high quality roughage is excellent stock feed;
Better use can be made of many existing drying installations and;
Since the crop is taken off the land early, a quicker turn around for the winter irrigated crop is possible.
Drying systems can be simple and inexpensive. The simplest form is the in‐bag tunnel system using ambient air. A 300‐bag tunnel can, given ideal weather conditions, be brought from 15% moisture content to 8 or 9% in 2 days, drying only during the day and using a 285 m3 per min (10 000cm) fan.
Drawbacks to this system are:
It demands a fairly large labour force to stack and un‐stack;
A power source is necessary for the fan and, wet weather conditions will lengthen drying time considerably.
The addition of solar heat to improve this system has been tried and basic data involving the use of the Solar Sock are given below.
The solar sock should consist of at least 30 metres of black PVA ducting within a similar length of clear ducting;
Black ducting should have a minimum wall thickness of 600 gauge (150 micron);
Wall thickness of the clear ducting is 300 gauges (75 micron). The clear duct is approximately 75 mm larger in diameter than the black PVA ducting; and
To avoid undue friction losses in the duct, air velocity should not exceed 500 metres/minute (1 600 ft/min.)
The table on the next page shows the duct diameter recommended for various air flows. Table 1: Duct diameters recommended for various air flows
|Air Flow m³/minute||Diameter of Black Duct in mm||Air Flow cfm||Diameter of Black Duct in inches|
Temperature rises of over 15°C above ambient air are frequently achieved during the midday period.
These could be excessive for groundnuts and possibly for seed maize.
Provision for shade over the sock would be wise in the event of excessive solar temperatures and for when fans are not operating. A simple frame to support a hessian cover should be adequate.
These recommendations are based on preliminary observations and should be used as a guide only.
Some farmers also use heated air to dry the crop and standard heat exchangers but dangers exist in drying groundnuts too quickly. Until further local experience is available, it would be advisable when considering artificial drying to use the figures below as a guide:
Maximum drying air temperature of 35°C: Skin slipping and reduced shelling quality are dangers when temperatures rise above this point;
About 7 mᶟ per minute per cubic metre of pods is the minimum air flowrequired. Drying rate should not exceed 0.5% per hour;
Static pressure in the plenum chamber will vary with the depth of nuts and moisture content. About 13 mm water gauge is correct for pods, 1.5 m deep at 15% moisture content; and
Heated air should not be used to reduce moisture below 9%. Use cold air or stack and allow it to dry naturally to 7.5%.
Farmers generally do not shell their groundnuts. They are sold in the pods. There are several shellers available ranging from the small hand‐operated machine producing about 22 kilogrammes shelled nuts per hour to the engine driven and PTO driven machines capable of 250 to 13000 kilogrammes per hour. The settings which are important on a sheller are screen hole size which should be slightly larger than the larger kernels in a representative sample, and the clearance between screen and
beater, which should be about the width of two unshelled pods.
For the best shelling quality the pre‐grading of pods into three sizes is desirable. The sheller must be reset for each size.
Groundnuts can be marketed in various ways. The most important aspect in marketing groundnuts is the quality delivered. The topics below are important quality aspects that a farmer must be aware of when marketing groundnuts.
Where groundnuts have been down‐graded or rejected, it has primarily been due to the following reasons:
EXCESSIVE EXTRANEOUS MATTER (OVER 8%)
The return to the farm and subsequent reprocessing could have been avoided had trial grading and mechanical adjustments been made before several hundred bags were filled.
Since groundnuts are not shelled before delivery this can only be the result of exposing the pods over long periods to the hot sun between lifting and picking.
Incorrect operation of picking equipment will break shells and create loose‐shelled kernels, both of which will result in penalties when the consignment is being graded.
MARKEDLY DISCOLOURED KERNELS (INCLUDING PURPLE STAINING)
Kernels are excluded from the Sound Mature Kernels (SMK) category if they are markedly discoloured or have any discoloration which affects the underlying flesh. These defects are found in most samples and where the groundnuts have been allowed to remain in the soil beyond the proper lifting time in quantities which can influence the grade. Even the SMK in such a sample which presently is accepted without penalty, have rather a dull dark appearance making them unattractive to buyers of edible kernels.
DISEASED KERNELS (MOULD)
This problem is one for which there is no ready‐made remedy. An excessively mouldy crop must be shelled. After which it will be for the producer to decide whether it will be practical to remove the excess mould and obtain a higher grade or accept the low grade laid down in the rules for such a sample.
BROKEN SHELLS IN UNSHELLED DELIVERIES
Producers have argued that to penalise broken and open shells is unreasonable since the entire product will be shelled in due course. Control is necessary for the following reasons:
Kernels contained in broken shells are open to contamination by dirt and consequently constitute a much higher Aflatoxin risk than kernels contained in complete shells. In pre‐ cleaning operations a sample high in broken shells produces loose shelled kernels which are deliberately rejected from the edible kernel production stream because of the Aflatoxin risk and;
The insect infestation risk which is low in sound unshelled groundnuts becomes higher when stocks contain a significant proportion of broken shells.