GUIDELINES TO SUCCESSFUL Cowpea PRODUCTION
Cowpea (Vigna unguiculata (L.) Walp) is one of the most ancient human food sources and has probably been used as a crop plant since long.
In South Africa a large producer of Cowpeas is small-scale farmers under dryland farming conditions. There are no records with regard to the size of area under production and the quantities produced. Cowpea belongs to the Leguminosae/Fabaceae.
The Genus is Vigna and the Species unguiculata
Other members of the genus are:
Vigna subterranean – bambara groundnut
V. radiate – mung bean
V. mungo – black gram
V. ungularis – adzuki bean
V. umbellate – rice bean
V. aconitifolia – moth bean
Several subspecies are also identified under the species unguiculata:
Dekindtiana, mensensis, unguiculata, cylindtica and sesquipedalis
Dekindtiana and Mensensis are wild types whereas the last three are cultivated types.
Cylindrical and Sesquipedalis are of Asian origin whereas the Unguiculata is of African origin.
USES
Cowpeas are indigenous to Africa. It is a protein-rich bean crop which leaves nitrogen in the Soil and therefore has a beneficial effect on the follow-up crop. Cowpeas can be planted as an intercrop or in rotation and can tolerate drought.
For the home
The leaves and growth points can be picked and used as a vegetable dish. The leaves can be dried and used as a meat substitute. About 1 kg cowpea is a full meal. The green seeds are sometimes roasted like peanuts and these are then used as a substitute for coffee. Ground dried seeds mixed with onions and spices can be fried in oil. The seeds can also be cooked. It can be used as green manure. It can be planted for hay production.
As a cash crop
Silage can be made by mixing the green leaves with sorghum or maize.
Cowpeas can be planted for hay production and the hay can be sold.
General utilisation
The cowpea is used both as a vegetable and grain. The semi-spreading types are suitable for use as a vegetable. The use of cowpea seeds as a seed vegetable provides an inexpensive source of protein in the diet. The dried pulse may be cooked together with other vegetables to make a thick soup, or ground into a meal or paste, before preparation in a variety of ways. Similarly, fresh, immature pods may be boiled as a vegetable. Fresh leaves and growth points are often picked and eaten in the same way as spinach. Dried leaves are preserved and eaten as a meat substitute. Cowpea is also grown as a dual-purpose crop the green pods are used as a vegetable and the remaining parts as livestock fodder. It is very palatable, highly nutritious and relatively free of metabolites or other toxins.
CULTIVARS
Of all the old and the well-known cultivars that have been planted, it is the only Renoster (also known Rhino), Glenda and Bechuana White which seed is available in sufficient quantities. Renoster is an erect cultivar with a semi determinate growth habit. It has a growing period of 46 days to flowering. Glenda is a semi vining type, with a semi-determinate growth habit and a growing period is of 56 days to flowering. Bechuana White is a vining type with an indeterminate growth habit and a growing period of 63 days to until the first flower appears.
Choice of varieties
The choice of variety is based on maturity period, yield potential, drought tolerance, responsiveness to day length, and pest and disease resistance. The colour and size of the seeds are very important to consumers and farmers. These, however, vary with regions. Some regions have a strong preference for brown, large seeded varieties, while others prefer white seeds.
GROWTH HABIT
TRAILING OR PROSTRATE
Development of these type occurred in Africa from the true Unguiulata type following several years of selection. The main use of crop in African context is as a pulse crop (dry edible seeds) and to a minor extend as a leafy vegetable. The crop is generally grown and allowed to creep on the ground without any support. This is possible because the inflorescence grows well above the leaf canopy and therefore pods can manoeuvre very well without coming into contact with the soil. Since the main use of crop was for dry seeds and these could be obtained successfully without any support, the prostrate plants were allowed to grow just like that. Cowpea was later introduced to Asia (specifically India) by European and Asian traders where it was subject to a different line of pressure.
TWINING TYPES
Other line of selection in India is for succulent off fleshy pods. Long pods (50 – 100 cm) therefore developed over the years. It became important to keep the long pods off the ground and so poles were used to accomplish this objective. Unconsciously types suitable for poles were selected and this led to the development of twining types.
ERECT TYPES
Development of this type occurred in India. The crop was mainly used as fodder. It was easier to select an erect type for fodder than a trailing type. This led to an unconscious selection of erect types. Later developments through plant breeding for types suitable for machine harvest further contributed to erect types.
BIOLIGICAL NITROGEN FIXATION (BNF)
Cowpea has the ability to fix nitrogen. Biological nitrogen fixation is a process where certain living organisms reduce atmospheric nitrogen from the molecular form Nitrogen to Ammonium for use by plants.
The reduction processes in legumes involves the roots of legumes and an associated suitable Bradyrhizobium spp (Bacteria).
CULTURAL PRACTICES FOR COWPEA
SITE SELECTION
Proper site selection is very important. Select a well-drained sandy loam soil for rainfed cowpea, or inland depressions and along the shores of a lake for dry season cowpea using residual moisture. Cowpea does not tolerate excessively wet conditions or waterlogging and should not be grown on poorly drained soil. The crop can be produced successfully with minimum inputs, provided cultural practices are neglected to the point where production is not economical. Cultural practices should also be related to crop characteristics to ensure that the correct practices are followed.
CLIMATIC REQUIREMENTS
Temperature
Cowpeas grow best during summer. The base temperature for germination is 8,5 °C and for leaf growth 20 °C. Cowpea is a heat-loving and drought-tolerant crop. The optimum temperature for growth and development is around 30 °C. Varieties differ in their response to day length, some being insensitive and flowering within 30 days after sowing when grown at a temperature around 30 °C. The time of flowering of photosensitive varieties is dependent on time and location of sowing and may be more than 100 days. Even in early flowering varieties, the flowering period can be extended by warm and moist conditions, leading to asynchronous maturity. The optimum sowing times are December to January. Early-sown crops tend to have elongated internodes, are less erect, more vegetative and have a lower yield than those sown at the optimum time.
The presence of nodular bacteria specific to cowpea (Bradyrhizobium spp.), make it suitable for cultivation in the hot, marginal cropping areas of Southern Africa, as well as in the cooler, higher rainfall areas. However, cowpeas are much less tolerant to cold soils. Cowpeas grow best during summer.
Water
Cowpea is a higher drought-tolerant crop than many other crops. It can grow under rainfall ranging from 400 to 700 mm per annum. Cowpeas also have a great tolerance to waterlogging. Well-distributed rainfall is important for normal growth and development of cowpeas. The frequency and unreliability of rainfall pose problems to cowpea growth in South Africa. In some areas, the frequency of rain is too high, resulting in flooding, while in some other areas it is so unreliable that moisture conservation remains vitally important for crop production. Cowpeas utilise soil moisture efficiently and are more drought-tolerant than groundnuts, soya-beans and sunflowers. Cowpeas can be produced satisfactorily with an annual rainfall between 400 and 750 mm. In some areas of Mpumalanga, where annual rainfall is high, cowpeas could be planted at a time to coincide with the peak period of rainfall during the vegetative phase or flowering stage so that pod-drying could take place during dry weather. Adequate rainfall is important during the flowering stage. Cowpeas react to serious moisture stress by limiting growth (especially leaf growth) and reducing leaf area by changing leaf orientation and closing the stomata. Flower and pod abscission during severe moisture stress also serves as a growth-restricting mechanism.
SOIL REQUIREMENTS
Cowpeas are grown on a wide range of soils but the crop shows a preference for sandy soils, which tend to be less restrictive on root growth. It is more tolerant to infertile and acid soils than many other crops. Cowpeas are grown on a wide range of soils but prefer sandy soils which are less restrictive to root growth. This adaptation to lighter soils is coupled with drought tolerance through reduced leaf growth, less water loss through stomata, and leaf movement to reduce light and heat load under stress. Cowpeas are much less tolerant to cold soils than common beans and show a poor tolerance to waterlogging. Cowpeas thrive in well-drained soil and less on heavy soils. It requires a soil pH of between 5,6 and 6,0.
PROPAGATION AND SOIL PREPARATION
Cowpea is directly grown from seed. The land must not be waterlogged but well drained. During land preparation, the existing fallow weeds, trees and shrubs in the site are cut down manually, or slashed with a tractor and fallen trees should be removed. This should be followed by ploughing and harrowing, using a disc plough and harrow. Some 4 to 6 days between each operation should be allowed to enhance good soil tilth for good seed germination. The land may be ridged or left as flat seedbeds after harrowing.
PLANTING DATE
Seeds to be used for planting must be sorted to make sure that these are free from insect damage (without damage holes or wrinkles) or any inert materials. The earliest possible planting date in the cooler, high lying production areas is the end of October, but planting in November holds a few temperature risks and will also produce better results. Planting slow growing vining types after mid-December in the cool, high lying areas could be risky in terms of economic production. Planting in the hot, more arid marginal areas should be done from the start of the rainy season to as late as the end of December. Plantings in January and even in the latter half of December could be risky in terms of economic production and also for other reasons. The attraction of cowpeas for insects and the build-up of insect populations as the growing season progresses, with the resultant of aphid infestations or viral diseases (in the case of late plantings), are important considerations when determining the most auspicious planting date. Specific problems on the farm such as the occurrence of early or late weeds and incorporation with the farming activities should be considered when choosing the most favourable planting date, so as to keep production inputs low.
PLANT POPULATION AND SPACING
The effective of environmental potential (soil, rainfall and temperature) determines the most favourable plant population and spacing of the crop. In the case of a 120 cm deep soil in an area with rainfall of 600 mm/a, the following guidelines could be used. Cultivars that grow relatively erect are the usually fast growers with a determinate or semi-determinate growth habit, that produces relatively less plant material and can be planted at 75 000 plants/ha (100 x 13 cm), because they are usually slow growers with an indeterminate growth habit and produce much more plant material. This also means that they have higher water requirements, spread over a longer period.
The row widths at which other crops are planted on the farm, and especially mechanical weed control or incidental pest control from 5 weeks after planting, will determine the most favourable row width. Relatively erect types can be planted in row widths of 75 – 115 cm, vining types can, be planted in row widths of 100 to 220 cm. Planting cowpeas between maize rows is common practice, but shading by the maize has an adverse effect on cowpea production.
SOIL REQUIREMENTS AND FERTILIZATION
Cowpeas are fairly sensitive to water-logging and perform best on sandy soils. Waterlogged conditions cause loss of Rhizobium nodules on the roots and the ability of the plants to recover depends on resumed growth and renewed Rhizobium activity under unfavourable conditions have been restored to normal. Cowpeas also have a deeper root system than many other legumes, penetrate the soil deeper than 120 cm and utilize available soil moisture stored deeper than 100 cm. soils in marginal areas with a depth of less than 90 cm should be regarded as risky for cowpea production. Soils with a neutral pH, adequate P and Ca and available B, Co, Cu, Mo, and Zn, ensure optimum Rhizobium activity and therefore necessary K fertilization serves no purpose unless soil K is critically low. Cowpeas react moderately to P fertilization, but excessive applications, or cultivation on soil with high P status could lead to excess P uptake that could promote flower abscission and have an adverse effect on seed set in the pods. Fertilizers should not be band placed near the seed. Nutrient uptake by the roots is improved by mycorrhiza associations and cowpeas are therefore regarded as efficient utilisers of available nutrients.
Cowpea does not require too much nitrogen fertilizer because it fixes its own nitrogen from the air using the nodules in its roots. However, in areas where soils are poor in nitrogen, there is a need to apply a small quantity of about 15 kg of nitrogen as a starter dose for a good crop. If too much nitrogen fertilizer is used, the plant will grow luxuriantly with poor grain yield. Cowpea requires more phosphorus than nitrogen in the form of single super phosphate or SUPA. About 30 kg of P/ha in the form of SUPA is recommended for cowpea production to help the crop to nodulate well and fix its own nitrogen from the air. N fertilization should only be considered to bridge poor seedling growth resulting from temporary N deficiencies. In which case row applications of not more than 25 kg N/ha could be considered.
Fertiliser application in cowpea production depends on anticipated yield and soil fertility. As a legume, cowpea fixes its own nitrogen, and does not need nitrogen fertiliser. Seed should be inoculated with the appropriate Rhizobium species for optimum nitrogen fixation however, nodules will generally form on cowpeas. Application of a phosphate fertiliser is usually beneficial. Cowpea can grow in a pH range of 5,6 to 6,5.
WEEDS
Annual grasses and some broadleaf weeds can be controlled by a pre-sowing application of herbicide. Row crop cultivation may be necessary with cowpeas, depending on the weed pressure, soil conditions, and rainfall. Preplant tillage can assist greatly in reducing early weed pressure, and the use of cover crops. Striga gesnerioides and Alectra spp. are the principal parasitic weeds attacking cowpeas, particularly in the semiarid regions. The following three are the most common Striga species that are a pest to cowpea: S. hermonthica, S. asiatica and S. gesnerioides.
The pest status is complex because the forms of parasitic weeds that are found on one species cannot germinate on another host plant. Careful observations and records are therefore necessary to clarify which crops are parasitised by which species.
Control
Control of Striga is difficult and time consuming. At present, chemical control is not recommended, as the chemicals are expensive, handling them is very difficult and no research results are available to support chemical treatment. Farmers are advised to improve soil fertility where this weed is a problem. Soil fertility has an effect on Striga infestation; more fertile soils are less infested with Striga. Use of manure and/or small quantities of fertiliser may reduce the infestation, when combined with weeding of plants before seed setting. Hand weeding of the infested areas before Striga sets seeds is the most important control method at present. Striga should be weeded out as soon as any flowering is observed, as the development of seeds takes only a few weeks. It may be necessary to weed the area twice in a season.
PESTS CONTROL
Cowpea is very attractive to insects. Insect pests have remained the most important setback to cowpea production, because each phase attracts a number of insect pests. The main pests during the growing season are pod sucking bugs (Riptortus spp., Nezara viridula and Acantomia sp.), aphis (Aphis fabae, Aphis craccivora), blister beetle (Mylabris spp.) and pod borer (Maruca vitrata).
Control by one or two applications of insecticide is invariably necessary. For commercial production this will lead to downgrading of grain.
Aphid control
In most cases, it is not recommended to control aphids. Control should only be considered where large infestations are threatening the crop or when viral infections have been observed.
DISEASE CONTROL
The most important disease of cowpea is stem rot caused by Phytophthora vignae. This disease frequently occurs in the wetter coastal and subcoastal areas, and on heavier soils which may become waterlogged.
Bacterial blight (Xanthomonas vignicola) causes severe damage to cowpeas, while the most frequent virus disease encountered is aphid-borne mosaic virus (CabMV). Fusarium wilt, bacterial canker, Cercospora leaf spot, rust and powdery mildew.
Cowpea is susceptible to nematodes and should therefore not be planted consecutively on the same land.
Birds
Birds, especially of the parrot family, could be a problem, as they can pull up emerging seedlings and feed on developing green pods.
HARVESTING
Harvest maturity
Cowpeas vary in growth habit from erect or semi-erect types with short (120 days) duration in semi-erect to trailing plants which are normally grown primarily for forage. At maturity, leaves will dry down but may not drop off completely. They need to be harvested when seed moisture content is 14 to 20 %, depending on the consumer’s requirement. In cowpeas grown for vegetable purposes, the leaves are picked 4 weeks after planting, and this continues until the plants start to flower.
HARVESTING STAGE
The most favourable harvesting stage for seed types (bean production) is when 90 % of the pods have dried. Fodder types (runners) reach their highest crude protein content with the onset of the flowering stage, but if the crop is harvested at that stage, leaf losses will result. The most favourable harvesting time for fodder types is when the first pods start colouring, but are not yet ready.
Harvesting methods
Cowpea can be harvested using a harvester or by hand. The upright cultivars are easy to harvest by machine. Cowpea grown as a dried seed product can be direct combined, using a platform head or a row crop head. Adjustments to combine settings and possibly sieve sizes should be made for the cowpea seed. Because the pods are relatively long, some will touch the ground or be close to it, making it important to run the grain table close to the ground. In the case of cowpeas grown for vegetable purposes, young leaves are mainly picked by hand; older leaves accumulate dust or get spattered with mud from raindrops if not harvested. Harvesting of cowpea in most cases should coincide with the onset of dry season when the dry pods can remain about a week awaiting harvesting without spoilage. However, to avoid field weathering or shattering, dry pods should not be left in the field longer than 2 weeks after full pod maturity. Harvesting can be carried out manually (hand harvesting) or by using a combine harvester in the case of large-scale production.
Fodder types are harvested with implements designed on the farm and cut at ground level. They can either be allowed to wilt or be windrowed immediately. The dry fodder is either baled or stacked. Weather damage to the hay leads to poor quality, but if a green colour is retained, the hay is of good quality. Stems and runners dry much more slowly than the leaves, and leaf losses must be kept to a minimum if good quality hay is to be produced.
Cowpea (Vigna unguiculata (L.) Walp) is one of the most ancient human food sources and has probably been used as a crop plant since long.
In South Africa a large producer of Cowpeas is small-scale farmers under dryland farming conditions. There are no records with regard to the size of area under production and the quantities produced. Cowpea belongs to the Leguminosae/Fabaceae.
The Genus is Vigna and the Species unguiculata
Other members of the genus are:
Vigna subterranean – bambara groundnut
V. radiate – mung bean
V. mungo – black gram
V. ungularis – adzuki bean
V. umbellate – rice bean
V. aconitifolia – moth bean
Several subspecies are also identified under the species unguiculata:
Dekindtiana, mensensis, unguiculata, cylindtica and sesquipedalis
Dekindtiana and Mensensis are wild types whereas the last three are cultivated types.
Cylindrical and Sesquipedalis are of Asian origin whereas the Unguiculata is of African origin.
USES
Cowpeas are indigenous to Africa. It is a protein-rich bean crop which leaves nitrogen in the Soil and therefore has a beneficial effect on the follow-up crop. Cowpeas can be planted as an intercrop or in rotation and can tolerate drought.
For the home
The leaves and growth points can be picked and used as a vegetable dish. The leaves can be dried and used as a meat substitute. About 1 kg cowpea is a full meal. The green seeds are sometimes roasted like peanuts and these are then used as a substitute for coffee. Ground dried seeds mixed with onions and spices can be fried in oil. The seeds can also be cooked. It can be used as green manure. It can be planted for hay production.
As a cash crop
Silage can be made by mixing the green leaves with sorghum or maize.
Cowpeas can be planted for hay production and the hay can be sold.
General utilisation
The cowpea is used both as a vegetable and grain. The semi-spreading types are suitable for use as a vegetable. The use of cowpea seeds as a seed vegetable provides an inexpensive source of protein in the diet. The dried pulse may be cooked together with other vegetables to make a thick soup, or ground into a meal or paste, before preparation in a variety of ways. Similarly, fresh, immature pods may be boiled as a vegetable. Fresh leaves and growth points are often picked and eaten in the same way as spinach. Dried leaves are preserved and eaten as a meat substitute. Cowpea is also grown as a dual-purpose crop the green pods are used as a vegetable and the remaining parts as livestock fodder. It is very palatable, highly nutritious and relatively free of metabolites or other toxins.
CULTIVARS
Of all the old and the well-known cultivars that have been planted, it is the only Renoster (also known Rhino), Glenda and Bechuana White which seed is available in sufficient quantities. Renoster is an erect cultivar with a semi determinate growth habit. It has a growing period of 46 days to flowering. Glenda is a semi vining type, with a semi-determinate growth habit and a growing period is of 56 days to flowering. Bechuana White is a vining type with an indeterminate growth habit and a growing period of 63 days to until the first flower appears.
Choice of varieties
The choice of variety is based on maturity period, yield potential, drought tolerance, responsiveness to day length, and pest and disease resistance. The colour and size of the seeds are very important to consumers and farmers. These, however, vary with regions. Some regions have a strong preference for brown, large seeded varieties, while others prefer white seeds.
GROWTH HABIT
TRAILING OR PROSTRATE
Development of these type occurred in Africa from the true Unguiulata type following several years of selection. The main use of crop in African context is as a pulse crop (dry edible seeds) and to a minor extend as a leafy vegetable. The crop is generally grown and allowed to creep on the ground without any support. This is possible because the inflorescence grows well above the leaf canopy and therefore pods can manoeuvre very well without coming into contact with the soil. Since the main use of crop was for dry seeds and these could be obtained successfully without any support, the prostrate plants were allowed to grow just like that. Cowpea was later introduced to Asia (specifically India) by European and Asian traders where it was subject to a different line of pressure.
TWINING TYPES
Other line of selection in India is for succulent off fleshy pods. Long pods (50 – 100 cm) therefore developed over the years. It became important to keep the long pods off the ground and so poles were used to accomplish this objective. Unconsciously types suitable for poles were selected and this led to the development of twining types.
ERECT TYPES
Development of this type occurred in India. The crop was mainly used as fodder. It was easier to select an erect type for fodder than a trailing type. This led to an unconscious selection of erect types. Later developments through plant breeding for types suitable for machine harvest further contributed to erect types.
BIOLIGICAL NITROGEN FIXATION (BNF)
Cowpea has the ability to fix nitrogen. Biological nitrogen fixation is a process where certain living organisms reduce atmospheric nitrogen from the molecular form Nitrogen to Ammonium for use by plants.
The reduction processes in legumes involves the roots of legumes and an associated suitable Bradyrhizobium spp (Bacteria).
CULTURAL PRACTICES FOR COWPEA
SITE SELECTION
Proper site selection is very important. Select a well-drained sandy loam soil for rainfed cowpea, or inland depressions and along the shores of a lake for dry season cowpea using residual moisture. Cowpea does not tolerate excessively wet conditions or waterlogging and should not be grown on poorly drained soil. The crop can be produced successfully with minimum inputs, provided cultural practices are neglected to the point where production is not economical. Cultural practices should also be related to crop characteristics to ensure that the correct practices are followed.
CLIMATIC REQUIREMENTS
Temperature
Cowpeas grow best during summer. The base temperature for germination is 8,5 °C and for leaf growth 20 °C. Cowpea is a heat-loving and drought-tolerant crop. The optimum temperature for growth and development is around 30 °C. Varieties differ in their response to day length, some being insensitive and flowering within 30 days after sowing when grown at a temperature around 30 °C. The time of flowering of photosensitive varieties is dependent on time and location of sowing and may be more than 100 days. Even in early flowering varieties, the flowering period can be extended by warm and moist conditions, leading to asynchronous maturity. The optimum sowing times are December to January. Early-sown crops tend to have elongated internodes, are less erect, more vegetative and have a lower yield than those sown at the optimum time.
The presence of nodular bacteria specific to cowpea (Bradyrhizobium spp.), make it suitable for cultivation in the hot, marginal cropping areas of Southern Africa, as well as in the cooler, higher rainfall areas. However, cowpeas are much less tolerant to cold soils. Cowpeas grow best during summer.
Water
Cowpea is a higher drought-tolerant crop than many other crops. It can grow under rainfall ranging from 400 to 700 mm per annum. Cowpeas also have a great tolerance to waterlogging. Well-distributed rainfall is important for normal growth and development of cowpeas. The frequency and unreliability of rainfall pose problems to cowpea growth in South Africa. In some areas, the frequency of rain is too high, resulting in flooding, while in some other areas it is so unreliable that moisture conservation remains vitally important for crop production. Cowpeas utilise soil moisture efficiently and are more drought-tolerant than groundnuts, soya-beans and sunflowers. Cowpeas can be produced satisfactorily with an annual rainfall between 400 and 750 mm. In some areas of Mpumalanga, where annual rainfall is high, cowpeas could be planted at a time to coincide with the peak period of rainfall during the vegetative phase or flowering stage so that pod-drying could take place during dry weather. Adequate rainfall is important during the flowering stage. Cowpeas react to serious moisture stress by limiting growth (especially leaf growth) and reducing leaf area by changing leaf orientation and closing the stomata. Flower and pod abscission during severe moisture stress also serves as a growth-restricting mechanism.
SOIL REQUIREMENTS
Cowpeas are grown on a wide range of soils but the crop shows a preference for sandy soils, which tend to be less restrictive on root growth. It is more tolerant to infertile and acid soils than many other crops. Cowpeas are grown on a wide range of soils but prefer sandy soils which are less restrictive to root growth. This adaptation to lighter soils is coupled with drought tolerance through reduced leaf growth, less water loss through stomata, and leaf movement to reduce light and heat load under stress. Cowpeas are much less tolerant to cold soils than common beans and show a poor tolerance to waterlogging. Cowpeas thrive in well-drained soil and less on heavy soils. It requires a soil pH of between 5,6 and 6,0.
PROPAGATION AND SOIL PREPARATION
Cowpea is directly grown from seed. The land must not be waterlogged but well drained. During land preparation, the existing fallow weeds, trees and shrubs in the site are cut down manually, or slashed with a tractor and fallen trees should be removed. This should be followed by ploughing and harrowing, using a disc plough and harrow. Some 4 to 6 days between each operation should be allowed to enhance good soil tilth for good seed germination. The land may be ridged or left as flat seedbeds after harrowing.
PLANTING DATE
Seeds to be used for planting must be sorted to make sure that these are free from insect damage (without damage holes or wrinkles) or any inert materials. The earliest possible planting date in the cooler, high lying production areas is the end of October, but planting in November holds a few temperature risks and will also produce better results. Planting slow growing vining types after mid-December in the cool, high lying areas could be risky in terms of economic production. Planting in the hot, more arid marginal areas should be done from the start of the rainy season to as late as the end of December. Plantings in January and even in the latter half of December could be risky in terms of economic production and also for other reasons. The attraction of cowpeas for insects and the build-up of insect populations as the growing season progresses, with the resultant of aphid infestations or viral diseases (in the case of late plantings), are important considerations when determining the most auspicious planting date. Specific problems on the farm such as the occurrence of early or late weeds and incorporation with the farming activities should be considered when choosing the most favourable planting date, so as to keep production inputs low.
PLANT POPULATION AND SPACING
The effective of environmental potential (soil, rainfall and temperature) determines the most favourable plant population and spacing of the crop. In the case of a 120 cm deep soil in an area with rainfall of 600 mm/a, the following guidelines could be used. Cultivars that grow relatively erect are the usually fast growers with a determinate or semi-determinate growth habit, that produces relatively less plant material and can be planted at 75 000 plants/ha (100 x 13 cm), because they are usually slow growers with an indeterminate growth habit and produce much more plant material. This also means that they have higher water requirements, spread over a longer period.
The row widths at which other crops are planted on the farm, and especially mechanical weed control or incidental pest control from 5 weeks after planting, will determine the most favourable row width. Relatively erect types can be planted in row widths of 75 – 115 cm, vining types can, be planted in row widths of 100 to 220 cm. Planting cowpeas between maize rows is common practice, but shading by the maize has an adverse effect on cowpea production.
SOIL REQUIREMENTS AND FERTILIZATION
Cowpeas are fairly sensitive to water-logging and perform best on sandy soils. Waterlogged conditions cause loss of Rhizobium nodules on the roots and the ability of the plants to recover depends on resumed growth and renewed Rhizobium activity under unfavourable conditions have been restored to normal. Cowpeas also have a deeper root system than many other legumes, penetrate the soil deeper than 120 cm and utilize available soil moisture stored deeper than 100 cm. soils in marginal areas with a depth of less than 90 cm should be regarded as risky for cowpea production. Soils with a neutral pH, adequate P and Ca and available B, Co, Cu, Mo, and Zn, ensure optimum Rhizobium activity and therefore necessary K fertilization serves no purpose unless soil K is critically low. Cowpeas react moderately to P fertilization, but excessive applications, or cultivation on soil with high P status could lead to excess P uptake that could promote flower abscission and have an adverse effect on seed set in the pods. Fertilizers should not be band placed near the seed. Nutrient uptake by the roots is improved by mycorrhiza associations and cowpeas are therefore regarded as efficient utilisers of available nutrients.
Cowpea does not require too much nitrogen fertilizer because it fixes its own nitrogen from the air using the nodules in its roots. However, in areas where soils are poor in nitrogen, there is a need to apply a small quantity of about 15 kg of nitrogen as a starter dose for a good crop. If too much nitrogen fertilizer is used, the plant will grow luxuriantly with poor grain yield. Cowpea requires more phosphorus than nitrogen in the form of single super phosphate or SUPA. About 30 kg of P/ha in the form of SUPA is recommended for cowpea production to help the crop to nodulate well and fix its own nitrogen from the air. N fertilization should only be considered to bridge poor seedling growth resulting from temporary N deficiencies. In which case row applications of not more than 25 kg N/ha could be considered.
Fertiliser application in cowpea production depends on anticipated yield and soil fertility. As a legume, cowpea fixes its own nitrogen, and does not need nitrogen fertiliser. Seed should be inoculated with the appropriate Rhizobium species for optimum nitrogen fixation however, nodules will generally form on cowpeas. Application of a phosphate fertiliser is usually beneficial. Cowpea can grow in a pH range of 5,6 to 6,5.
WEEDS
Annual grasses and some broadleaf weeds can be controlled by a pre-sowing application of herbicide. Row crop cultivation may be necessary with cowpeas, depending on the weed pressure, soil conditions, and rainfall. Preplant tillage can assist greatly in reducing early weed pressure, and the use of cover crops. Striga gesnerioides and Alectra spp. are the principal parasitic weeds attacking cowpeas, particularly in the semiarid regions. The following three are the most common Striga species that are a pest to cowpea: S. hermonthica, S. asiatica and S. gesnerioides.
The pest status is complex because the forms of parasitic weeds that are found on one species cannot germinate on another host plant. Careful observations and records are therefore necessary to clarify which crops are parasitised by which species.
Control
Control of Striga is difficult and time consuming. At present, chemical control is not recommended, as the chemicals are expensive, handling them is very difficult and no research results are available to support chemical treatment. Farmers are advised to improve soil fertility where this weed is a problem. Soil fertility has an effect on Striga infestation; more fertile soils are less infested with Striga. Use of manure and/or small quantities of fertiliser may reduce the infestation, when combined with weeding of plants before seed setting. Hand weeding of the infested areas before Striga sets seeds is the most important control method at present. Striga should be weeded out as soon as any flowering is observed, as the development of seeds takes only a few weeks. It may be necessary to weed the area twice in a season.
PESTS CONTROL
Cowpea is very attractive to insects. Insect pests have remained the most important setback to cowpea production, because each phase attracts a number of insect pests. The main pests during the growing season are pod sucking bugs (Riptortus spp., Nezara viridula and Acantomia sp.), aphis (Aphis fabae, Aphis craccivora), blister beetle (Mylabris spp.) and pod borer (Maruca vitrata).
Control by one or two applications of insecticide is invariably necessary. For commercial production this will lead to downgrading of grain.
Aphid control
In most cases, it is not recommended to control aphids. Control should only be considered where large infestations are threatening the crop or when viral infections have been observed.
DISEASE CONTROL
The most important disease of cowpea is stem rot caused by Phytophthora vignae. This disease frequently occurs in the wetter coastal and subcoastal areas, and on heavier soils which may become waterlogged.
Bacterial blight (Xanthomonas vignicola) causes severe damage to cowpeas, while the most frequent virus disease encountered is aphid-borne mosaic virus (CabMV). Fusarium wilt, bacterial canker, Cercospora leaf spot, rust and powdery mildew.
Cowpea is susceptible to nematodes and should therefore not be planted consecutively on the same land.
Birds
Birds, especially of the parrot family, could be a problem, as they can pull up emerging seedlings and feed on developing green pods.
HARVESTING
Harvest maturity
Cowpeas vary in growth habit from erect or semi-erect types with short (120 days) duration in semi-erect to trailing plants which are normally grown primarily for forage. At maturity, leaves will dry down but may not drop off completely. They need to be harvested when seed moisture content is 14 to 20 %, depending on the consumer’s requirement. In cowpeas grown for vegetable purposes, the leaves are picked 4 weeks after planting, and this continues until the plants start to flower.
HARVESTING STAGE
The most favourable harvesting stage for seed types (bean production) is when 90 % of the pods have dried. Fodder types (runners) reach their highest crude protein content with the onset of the flowering stage, but if the crop is harvested at that stage, leaf losses will result. The most favourable harvesting time for fodder types is when the first pods start colouring, but are not yet ready.
Harvesting methods
Cowpea can be harvested using a harvester or by hand. The upright cultivars are easy to harvest by machine. Cowpea grown as a dried seed product can be direct combined, using a platform head or a row crop head. Adjustments to combine settings and possibly sieve sizes should be made for the cowpea seed. Because the pods are relatively long, some will touch the ground or be close to it, making it important to run the grain table close to the ground. In the case of cowpeas grown for vegetable purposes, young leaves are mainly picked by hand; older leaves accumulate dust or get spattered with mud from raindrops if not harvested. Harvesting of cowpea in most cases should coincide with the onset of dry season when the dry pods can remain about a week awaiting harvesting without spoilage. However, to avoid field weathering or shattering, dry pods should not be left in the field longer than 2 weeks after full pod maturity. Harvesting can be carried out manually (hand harvesting) or by using a combine harvester in the case of large-scale production.
Fodder types are harvested with implements designed on the farm and cut at ground level. They can either be allowed to wilt or be windrowed immediately. The dry fodder is either baled or stacked. Weather damage to the hay leads to poor quality, but if a green colour is retained, the hay is of good quality. Stems and runners dry much more slowly than the leaves, and leaf losses must be kept to a minimum if good quality hay is to be produced.
