Cowpea (Vigna unguiculata) forage

History

Cowpea ( Vigna unguiculata L. Walp.), an annual legume, is also commonly referred to as southern pea, blackeye pea, crowder pea, lubia, niebe, coupe or frijole. Cowpea originated in Africa and is widely grown in Africa, Latin America, Southeast Asia and in the southern United States. It is chiefly used as a grain crop, for animal fodder, or as a vegetable. The history of cowpea dates to ancient West African cereal farming, 5 to 6 thousand years ago, where it was closely associated with the cultivation of sorghum and pearl millet.

Wordwide cowpea production has increased dramatically in the last 25 years. United States production of dry cowpea has declined from 3/4 million acres to a few thousand over the same period. The blackeyed cowpea type is grown primarily in California and is marketed as California blackeyed peas.

Uses

Cowpea seed is a nutritious component in the human diet, as well as a nutritious livestock feed. Nutrient content of cowpea seed is summarized in Table 1.

Table 1. Nutrient content of mature cowpea seed (average of eight varieties).

Protein 24.8%
Fat 1.9%
Fiber 6.3%
Carbohydrate 63.6%
Thiamine 0.00074%
Riboflavin 0.00042%
Niacin 0.00281%

The protein in cowpea seed is rich in the amino acids, lysine and tryptophan, compared to cereal grains; however, it is deficient in methionine and cystine when compared to animal proteins. Therefore, cowpea seed is valued as a nutritional supplement to cereals and an extender of animal proteins.

Cowpea can be used at all stages of growth as a vegetable crop. The tender green leaves are an important food source in Africa and are prepared as a pot herb, like spinach. Immature snapped pods are used in the same way as snapbeans, often being mixed with other foods. Green cowpea seeds are boiled as a fresh vegetable, or may be canned or frozen. Dry mature seeds are also suitable for boiling and canning.

In many areas of the world, the cowpea is the only available high quality legume hay for livestock feed. Digestibility and yield of certain cultivars have been shown to be comparable to alfalfa. Cowpea may be used green or as dry fodder. It also is used as a green manure crop, a nitrogen fixing crop, or for erosion control. Similar to other grain legumes, cowpea contains trypsin inhibitors which limit protein utilization.

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Growth Habits

Cowpea is a warm-season, annual, herbaceous legume. Plant types are often categorized as erect, semi-erect, prostrate (trailing), or climbing. There is much variability within the species. Growth habit ranges from indeterminate to fairly determinate with the non-vining types tending to be more determinate. Cowpea generally is strongly taprooted. Root depth has been measured at 95 in. 8 weeks after seeding.

Cowpea seed ranges in size from the very small wild types up to nearly 14 in. long and the number of seeds per pounds range from 1600 to 4300. Seed shape is a major characteristic correlated with seed development in the pod. Seeds develop a kidney shape if not restricted within the pod. When seed growth is restricted by the pod the seed becomes progressively more globular.

The seed coat can be either smooth or wrinkled and of various colors including white, cream, green, buff, red, brown, and black. Seed may also be speckled. mottled, or blotchy. Many are also referred to as “eyed” (blackeye, pinkeye purple hull, etc.) where the white colored hilum is surrounded by another color.

Emergence is epigeal (similar to common bean, and lupin) where the cotyledons emerge from the ground during germination. This type of emergence makes cowpea more susceptible to seedling injury, since the plant does not regenerate buds below the cotyledonary node.

The trifoliolate leaves develop alternately. Leaves are smooth, dull to shiny, and rarely pubescent. Commonly, the terminal leaflet is longer and larger than the lateral leaflets. There is a wide range in leaf size and shape.

Cowpea generally is day neutral. Flowers are borne in multiple racemes on 8 to 20 in. flower stalks (peduncles) that arise from the leaf axil. Two or three pods per peduncle are common and often four or more pods are carried on a single peduncle. The presence of these long peduncles is a distinguishing feature of cowpea and this characteristic also facilitates harvest. The open display of flowers above the foliage and the presence of floral nectaries contribute to the attraction of insects. Cowpea primarily is self pollinating.

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Cowpea pods are smooth, 6 to 10 in. long, cylindrical and generally somewhat curved. As the seeds approach the green-mature stage for use as a vegetable, pod color may be distinctive, most commonly green. yellow or purple. As the seeds dry, pod color of the green and yellow types becomes tan or brown.

Environment Requirements

A. Climate:

Cowpea is a warm-season crop well adapted to many areas of the humid tropics and temperate zones. It tolerates heat and dry conditions, but is intolerant of frost. Germination is rapid at temperatures above 65°F; colder temperatures slow germination.

Cowpeas are grown under both irrigated and non-irrigated regimes. The crop responds positively to irrigation but will also produce well under dryland conditions. Cowpea is more drought resistant than common bean. Drought resistance is one reason that cowpea is such an important crop in many underdeveloped parts of the world. If irrigation is used, more vegetative growth and some delay in maturity may result. Application rates should insure that the crop is not overwatered, especially in more northern latitudes, as this will suppress growth by lowering soil temperatures. The most critical moisture requiring period is just prior to and during bloom.

B. Soil:

Cowpea performs well on a wide variety of soils and soil conditions, but performs best on well-drained sandy loams or sandy soils where soil pH is in the range of 5.5 to 6.5.

Cultural Practices

A. Seedbed Preparation:

Soils should be cultivated deeply enough to insure that no barrier to penetration of the soil by the taproot (such as a hardpan) exists. Cowpea may be adversely affected by soil crusting under certain soil and environmental conditions.

B. Seeding Date:

Cowpea should not be planted until soil temperatures are consistently above 65°F and soil moisture is adequate for germination and growth. Seeds will decay in cool, wet soils. In the Minnesota-Wisconsin area, optimum seeding dates usually correspond to those for fieldbean (May 15-30).

C. Method and Rate of Seeding:

Traditionally, cowpea in the United States has been seeded in rows spaced 30 to 36 in. apart with seeds spaced 2 to 4 in. in the row. Recently, higher plant populations achieved by using narrow rows 12 to 20 in. have been used in commercial plantings. For forage purposes, the crop may be seeded in rows or broadcast (solid-seeded). Seed should be planted 1 to 1 1/2 in. deep and good seed-soil contact is important. The amount of seed to sow per acre depends on seed weight, germination percentage, and plant spacing. Recommended field seeding rates range from 18 to 22 lb/acre for viney, indeterminate types to 40 to 50 lbs for large-seeded determinate types. Optimum plant spacing depends on vine type. Highly determinate types may be planted 2 to 3 in. apart. Viney indeterminate types require more space, and a final stand with 8 to 9 in. between plants in 30 in. rows is considered to be a minimally acceptable population.

D. Fertility and Lime Requirements:

Cowpea, like all legumes, forms a symbiotic relationship with a specific soil bacterium ( Rhizobium spp.). Rhizobium makes atmospheric nitrogen available to the plant by a process called nitrogen fixation. Fixation occurs in root nodules of the plant and the bacteria utilize sugars produced by the plant. Although cowpea Rhizobium is normally widespread, seed inoculation with Rhizobium specific to cowpea would be beneficial in areas where it is not present. Always use Rhizobium of the cowpea type.

Excess nitrogen (N) promotes lush vegetative growth, delays maturity, may reduce seed yield and may suppress nitrogen fixation. The plant will perform well under low N conditions due to a high capacity for N fixation. A starter N rate of around 27 lb/acre is sometimes required for early plant development on low-N soils.

A soil test is the best way to determine soil nutrient levels. In general, at least 27 lb P/acre and 40 lb K/acre are. recommended on soils of medium fertility but individual soils will vary in fertilizer requirements. Band fertilizer 3 to 4 in. deep and 2 to 3 in. away from the seed, or broadcast and disc in all fertilizer, including nitrogen, before planting.

E. Variety Selection:

The International Institute for Tropical Agriculture (IITA) in lbadan, Nigeria is the center for world-wide collection and testing of cowpea germplasm. The Institute has developed high yielding, short season, multiple disease-resistant varieties that are ready for harvest in 60 days. Several of the U.S. State Agricultural Experiment Stations conduct cowpea variety development programs. Cowpea researchers at the University of Minnesota have released two extra-early-maturing varieties, MN 13 and MN 150 (Table 3). Crude protein and digestibility of the whole plant are reported to be similar to alfalfa with yields ranging from 1.3 to 1.8 ton/acre after 60 days of growth.

Source: Field Crops