Cowpea is a highly valuable leguminous crop that provides a good source of protein to humans and livestock. Legume seeds, or pulses, are sometimes called grain legume.
There are different species of cowpea, depending on their structure and appearance. These are classified into erect and climbing types. Cowpea leaves are alternate and trifoliate and its flowers may have white, reddish, violet or pale purple colour. The seeds of cowpea are borne in pods, which have different lengths, and shapes and this can be used for distinguishing the species of cowpea. The colour of pod may be green to green with purple, or brown at the immature stage and straw to straw with dull black splashes, to purple or brown at maturity. The seed ranges in colour from white and brown to nearly black. It is divided into the ‘body’ and the hilium or ‘eye’. Generally, cowpea matures between 75-105 days. Those adapted to areas of short rainy season mature in 60 days and are ready for consumption in 90 days.
Cowpea is generally grown in areas of low rainfall, and most species will not flower during periods of high rainfall, which causes the shedding of flowers under high rainfall, leading to crop failure. Enough amount of rainfall ranges between 1500 -2000mm. This is needed for the proper growth and development of cowpea plant. It also needs low relative humidity for growth which is affected by high relative humidity because it favours pest development and disease infection.
Most species of cowpea grow at a temperature range of 20-30’C. Some other species depend on the neutral day length, that is equal day and night periods. Some other species growth depends on the presence of short day length. Cowpea normally grows well at a height of 1000m above the sea level and also grows on different range of soils, from clay to loam soils. Sandy soil is not good for growth and formulation of nodules in cowpea. The pH of the soil, that is, degree of acidity or alkalinity in soil, should be from acidic to basic, cowpea can withstand a pH sandy – loam to clay –loam soil between p H 6 -7.
Cowpea grown on acid soils are prone to the attack of diseases: fusarium wilts and root knots. Cowpea are sensitive to water logging, however, cowpea can withstand severe drought condition than any other legume.
Generally, cowpea is grown under water condition supplied from rainfall, but can be grown using surface or sprinkler irrigation or drip irrigation whereby water is sprayed as a sheet over the farm land, at a properly controlled level. Excess or lack of water can limit cowpea growth and yield. Poor water condition in the soil can reduce the formation of nodules and development during appearance of the cotyledons but may not greatly affect yield. Cowpeas have a higher capacity to absorb water from deeper soil profile. Under adequate water condition, cowpea, especially the climbing types, tend to produce too many leaves.
Generally, legumes require high amount of phosphorus. Simple or compound fertiliser containing phosphorus tends to greatly increase cowpea yields. Cowpea needs 30-40kg of super phosphate per hectare and 25-30kg/ha of potassium applied as muriate of potash drilled or broadcast 10 days before seeding.
Cowpea requires a low quantity of nitrogen for nodule formation, which starts at 6 weeks after planting. Nitrogen is synthesised in the root nodules of cowpea by symbiotic fixation due to the presence of bacteria, Rhizobium. The legume roots contain large molecules made up of carbohydrates with protein called lectin. Note that high nitrogen causes delay in pod formation in cowpea. Other nutrients such as sulphur, S and calcium, Ca, are required in little amount and need only be supplied where the soil is deficient (e.g. in heavily leached or eroded soils). Lime may be necessary in soils with low organic matter (low buffering capacity) which have low resistance to sudden change in pH. Lime can be applied at 2 tonnes per hectare. Thus, a common operation in cowpea production which normally starts from flowering stage, is the application of NPK granulated fertilisers.
Land clearing is done depending on the scale of operation and the fertility of the soil about to be used for cowpea cultivation. Where most of the cowpea is planted on small plots, hand clearing is practiced. This operation is, however, carried out with the use of modern farm machinery under mechanised system in a few commercial farms.
Seed bed preparation
Cowpea can be planted on ridges, on flat land or on sides of mounds. A mound is a heap of soil used for cultivation of crops in a farm. Cowpea can be grown with zero, high or low tillage. Zero -tillage is the agricultural practice where tillage of the soil is not done at all. This greatly helps, to reduce the risk of crop failure due to drought, and is an economical process in terms of labour and energy during land preparation. Medium tillage where ploughing and harrowing are carried out only once in a farm and high tillage (with rotation) can also be practiced, but, in most cowpea growing areas which are found in less developed countries of the world, zero or minimum tillage is adopted. Tillage is the process of tilling soil to loosen the soil to facilitate cultivation of crops.
Cowpea is propagated by seeds. Seeds should be selected on the basis of vigor, high yield, pest and disease tolerance, plant height, early maturity and other desirable qualities.
Time of sowing
In semi-arid areas of Northern Nigeria, the best planting period of cowpea is between July-August, while in the high rainforest areas of Nigeria, including Rivers State with the highest rainfall in Nigeria, September is the best planting time due to the low amount of rainfall required for maximum growth, development and maturity. Generally, International Institute of Tropical Agriculture (IITA) recommends that the ideal time to plant medium maturing types is about 65-70 days to the end of the rain, and the early maturing type, about 45-50 days before the end of the rains.
The seed rate for cowpea 20-25kg per hectare at 2 per stand.
This varies with the growth habit of cowpea plant. In soil, which is well drained, cowpea could be planted either in flat land ridges or on sides of mounds. The spacing of the climbing type is 25 x 50cm or 75 x 20cm, yielding 50,000 to 80,000 plants per hectare. For erect low branching type, 16 x 34cm or 17 x 40cm could be adopted. Cowpea planted for forage use, for livestock feeds should have closer spacing and should be planted in March or April, for high vegetative (Leafy) growth.
Seed germination test could be carried out if there is doubt about the viability of the seed for cultivation. This may help to adjust seedling rate. The seeds are then dressed with Demosan, Arasan or Aldrex T, as prescribed in the containers.
Cowpea in intercropping systems
Cowpea is locally grown in mixed cropping system in Africa. In intercropping system, short duration erect culture is suitable as an intercrop with maize, sorghum, millet, cassava, cotton, pigeon pea, sugar cane and yam. Under this intercropping system, cowpea matures in 60 days with maximum yields which is not reduced by shading. It is carefully done to avoid a situation where it competes with the major staple crop for moisture in the latter part of the wet season. Intercropping system is where a second crop is planted after the major staple crop, under crop rotation system, and the second crop with short duration grows and matures, and becomes harvested before the major crop. The second crop so harvested in this way is known as intercrop or catch crop.
Cowpea is a good cover crop, which controls the growth of weed in a farm. Two to three weedings should be done in cowpea plot in the first 12 months after planting; i.e. before the plant develops sufficient canopy to cover the ground. In small holdings, farmers generally control weeds either by hand weeding or hoeing. Herbicides are rarely applied; although several pre-emergence herbicides such as Alachlor, Metalachloor at 2-3kg per hectare and Metachlor and Metribuzin (applied at 1.5+ 0.25kg/ha) are used for weed control in cowpea.
Cowpea is harvested based on use or need. Cowpea needed for seed is normally harvested when 85-90% of the pods are dry. Delay in harvesting often leads to shattering, yield loss and poor seed quality. After harvesting, the crop should be piled up, threshed and allowed to dry for 2-3 days. Cowpea as green pods must be harvested 10-12 days after flowering for best protein yield and freshness. If harvested at 20 days, the pod become fibrous and crude protein and total yield are affected. Since flowering and pod formation continue over several weeks, green pods are picked at 3-4 days interval to maintain freshness of pod and high yield. In most cowpea growing areas such as Elele, and Bori in Rivers State and other parts of the world, where cowpea is produced in small holdings, hand picking is the popular method of harvesting but in commercial production combine harvesters could be used.
Cowpea yield depends on the climatic and soils factors, variety planted, cropping patterns, and date of planting, plant population and weed control. Yields in pure stands may vary between 0.2-0.3 tonnes in Rivers State of Nigeria, and other areas in the tropics. But under optimum management, high yield outputs on average of range 1.5-0.3 tonnes/ha can be obtained. Cowpea is a good example of an arable crop that is planted and harvested for consumption within the period of maturity.
This is done after harvesting and threshing processes have been carried out. The threshed seeds are stored in sack, treated with fumigant such as phostoxin at the rate of 1 tab per 100kg sack, in sealed drums, plastic and in polythene as well as in Rhumbus and Silos. The function of the fumigant is to preserve the seeds and maintain their viability for over a long period.
Uses of cowpea
(i) Source of food: Food legumes have high protein and serve as a source of natural protein supplement to staple foods. Cowpea can be consumed as dry pulse, green pods and green seeds. It is generally consumed together with staples such as cereals and tubers. Cereals are grasses grown for food. Cereals include maize, wheat, rice, rye, barley, millet, sorghum, and guinea corn. Cereals contain high-energy source. Cowpea contains 24% protein, 62% soluble carbohydrate and little amounts of other nutrients, and the leaves also serve as vegetables in certain areas.
(ii) Source of livestock feed: Cowpea is used as source of protein when ground with maize and bones and with other additives to livestock and it is sometimes grown primarily as forage crops. Cowpea is also processed into cowpea meal and flour. This meal can be used to replace some percentage of meat in ground beef patties, saving cost.
(a) Mosaic disease
Causative agent: Virus.
Transmissive agent (Vector): Aphids.
(i) Appearance of stunted growth.
(ii) Regular leaf falls from branches.
(iii) Dwarfish pod size.
(i) Planting disease resistant variety.
(ii) Uprooting and burning of infected plants.
(b) Bacterial blight.
Causative agent: Bacterium.
Transmissive agent wind, rain, planting unhealthy plants stock.
Appearance of dark-brown spots in the leaves.
(i) Planting resistant varieties.
(ii) Sowing viable and healthy seeds.
(c) Nematode disease.
Causative agent: Nematode.
Transmissive agent: Soil-borne nematodes which attack roots.
(i) Irregular twisting of leaves.
(ii) Rolling of leaves.
(iii) Reduced, size of stems and roots.
(iv) Rottening of swollen roots.
(i) By spraying with nematicides.
(ii) By breeding resistant varites.
(d) Damping of disease
Pythium spp and Phytophthora spp
Causative agent: Fungus.
Transmissive agent: Soil-borne spores through hyphae.
(i) Rottening of seedling in the soil.
(i) By uprooting and burning of infected plants.
(ii) By practising crop rotation.
(iii) By planting resistant varieties.
(iv) By spraying with fungicides.