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Cowpea

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Summary

Common Name
Common names include cowpea (Duke, 1981; Miller, 1988; Yost and Evans, 1988), blackeye bean (Miller, 1988), blackeye pea (Miller et al., 1989), blackeyed pea (Duke, 1981), crowder pea (Duke, 1981), and southern pea (Duke, 1981).

Scientific Name
Vigna unguiculata (L.) Walpers ssp. unguiculata (Duke, 1981).

Cultivar
There are great differences among cultivars in environmental tolerances, and some 7,000 cv accessions were housed at the International Institute of Tropical Agriculture, as of 1975 (Duke, 1981). Forty-six commercial varieties were listed in the 1990-91 (41st) Edition of the Southern Seedsmen's Association (S.S.A.) Directory and Buyers' Guide (Bugg., pers. comm.).

The cowpea variety Ife BPC had yields equal to those of two other varieties tested (Ife Brown and AFB 1757) but contributed a net positive of 30.3 kg N/ha to the soil (from N contained in vegetative structures) whereas other varieties showed a net deficit. Although there was evidence that uninoculated cowpea was heavily nodulated, inoculated Ife BPC had better N content in vegetative structures (Awonaike et al., 1990).

Seed Description
Seeds 2-12 mm long, globular to reniform, smooth or wrinkled, red, black, brown, green buff, or white as dominant color; full colored, spotted, marbled, speckled, eyed, or blotched; (5-30 g/100 seeds) (Duke, 1981).

Seedling Description
Seeds germinate epigeally (on the soil surface). First leaves above cotyledons are simple and opposite; subsequent trifoliate leaves are alternate (Duke, 1981). Seedlings have large leaves and quickly provide a canopy (Miller et al., 1989).

Mature Plant Description
Duke (1981) reports that cowpea is an annual herb, erect or suberect and spreading, reaching 80 cm or more in height. Foliage is glabrous. Taproot is stout with laterals near surface; nodules are large.

Temperature
Cowpea tolerates annual mean temperatures of from 12.5-27.8 degrees C, with the mean of 54 cases being 22.1 (Duke, 1981); the species is cold sensitive and killed by frost (Duke, 1981; Miller, 1989). Cowpea is more sensitive to frost than is soybean (McLeod, 1982). The most productive heat-adapted legume (Miller et al., 1988, 1989), cowpea is adapted to almost the same hot, moist climatic conditions as corn although it requires more heat (McLeod, 1982).

Geographic Range
The species has been cultured from ancient times in Africa and Asia and is now widespread throughout the tropics and subtropics. It originated near Ethiopia and subsequently was developed mainly in the farms of the African Savannah. It is assigned to the African and Hindustani Centers of Diversity and is adapted to the Warm Temperate Thorn to Moist through Tropical Thorn to Wet Forest Life Zones (Duke, 1981). Cowpea cannot be grown for grain as far north as soybean because cowpea is more sensitive to frost (McLeod, 1982). The species was at one time used extensively as a green manure in northern Queensland, Australia; it has also been used in the southern United States, Nigeria, Rhodesia, the Philippines, Papua New Guinea, and elsewhere (Yost and Evans, 1988).

Water
Cowpea tolerates from 2.8 to 41.0 dm of precipitation (11-161.42 inches), with the mean of 54 cases being 14.2 (55.91 inches); cowpea is usually grown under rainfed rather than irrigated conditions (Duke, 1981). Cowpea is regarded as fairly drought resistant (Peaceful Valley, 1988), slightly more so than soybean (McLeod, 1982), with a taproot that can explore deep soil profiles for moisture (Miller, 1988). However, most of the moisture used is obtained from the surface foot of soil (McLeod, 1982). A cover crop can be obtained in 60-90 days with very little irrigation (Miller, 1988). Cowpea is less tolerant of waterlogging than is soybean (Duke, 1981).

Nutrients
Cowpea or varieties thereof are said to be tolerant of aluminum, high pH, low pH, laterite, and poor soil. Although cowpea does well on poor soil, it often responds favorably to added P, and it responds slightly to K up to an application rate of 45 kg/ha (Duke, 1981). Cowpea has a low lime requirement (McLeod, 1982) but appears particularly intolerant of excess amounts of boron (Agbenin, Lombin, and Owonubi, 1990).

There was a non-significant increase in cowpea grain yield up to a nitrogen application rate of 30 kg/ha (Agbenin, Lombin, and Owonubi, 1990).

Soil pH
Cowpea tolerates soil pH of from 4.3-7.9, with the mean of 46 cases being 6.2; cowpea or varieties thereof are said to be tolerant of low pH and high pH but in general are less tolerant of alkaline conditions (Duke, 1981). It is said to thrive in low-fertility soil if pH is between 5.5 and 6.5 (McLeod, 1982; Peaceful Valley, 1988).

Maas and Poss (1989) found used cowpea (cv 'California Blackeye No. 5') conducted pot studies in sand culture with saline solutions of various strengths (2:1 molar ratio of NaCl to CaCl) used for irrigation. Based data for the osmotic potentials that induced 50% reduction in pod and seed yields, cowpea was more susceptible to salinity increases in the vegetative (-0.45 MPA) than in the flowering (-0.76 MPa) or pod-filling (-0.88 MPa) stages. Seed number, but not individual seed size, was affected.

Soil Type
Duke (1981) stated that cowpea does well on many types of soil from highly-acid to neutral but is less tolerant of alkaline conditions. Although adapted to a wide range of soil types, including low-fertility soils, best growth occurs on sandy loams, and for optimum growth, good drainage, aeration and water supply are important (McLeod, 1982; Peaceful Valley, 1988). Miller et al. (1989) stated that best growth is made on deep well-drained acid to neutral soils. Cowpea is sensitive to excess amounts of boron (Agbenin, Lombin, and Owonubi, 1990). This species is thought to be better than clover or alfalfa on thin soils or soils low in lime and those too poor and acid for clover (McLeod, 1982). For grain production, a poor soil is actually better because a poor soil produces little vegetation and ample seed, whereas a rich soil leads to abundant vegetative growth and little seed (McLeod, 1982).

Shade Tolerance
Cowpea is stated as tolerant of moderate shade (Duke, 1981; McLeod, 1982; Peaceful Valley, 1988) but subject to mildew in heavy shade (McLeod, 1982).

In Costa Rica, Bazill (1987) reported that tropical legumes showing good shade tolerance in the understory of Pinus caribaea var hondurensis were Centrosema brasilianum (L.) Benth., C. macrocarpum, C. plumieri (Pers.) Benth., and C. pubescens Benth.; Desmodium gyroides DC, D. heterocarpon (L.) DC, D. heterophyllum DC, D. intortum Mill. Urb., D. ovalifolium Wall. ex Merr, and D. uncinatumj (esp. D. heterocarpon); Flemingia congesta Roxb. Galactia striata (Jacq.) Urb., Canavaglia ensiformis (L.) DC, Lablab purpureus (L.) Sweet, and Vigna unguiculata (L. Walp.). On the other hand, performing poorly were Indigofera hirsuta L., Cajanus cajan (L.) Millsp., and Stizolobium sp. No quantitative data were given in support of these conclusions.

Salinity Tolerance
Based on yields of pods and seeds, cowpea (cv 'California Buckeye No. 5') was most sensitive to salinity during the vegetative stage and became less so through the flowering and pod-filling stages. Seed yields were depressed 50% at osmotic potentials of -0.45, -0.76, and -0.88 MPa for vegetative, flowering, and pod-filling stages respectively (Mass and Poss, 1989).

Maas and Poss (1989) found used cowpea (cv 'California Blackeye No. 5') conducted pot studies in sand culture with saline solutions of various strengths (2:1 molar ratio of NaCl to CaCl) used for irrigation. Based data for the osmotic potentials that induced 50% reduction in pod and seed yields, cowpea was more susceptible to salinity increases in the vegetative (-0.45 MPA) than in the flowering (-0.76 MPa) or pod-filling (-0.88 MPa) stages. Seed number, but not individual seed size, was affected.

Herbicide Sensitivity
No specific information available on tolerance to herbicides. According to Duke's (1981) review, in cowpea culture, amines of 2,4-D and MCPA are used as preemergence sprays; trifluralin before sowing also gives good weed control.

Life Cycle
Cowpea is chiefly a summer annual (McLeod, 1982; Miller, 1988; Miller et al., 1989) that grows rapidly during short 60 to 90 day summer growing periods (Miller et al., 1989). Some varieties are indeterminate, and where not frost killed may behave as weak perennials (Duke, 1981).

Seeding Rate
Recommended seeding rates include: 10-40 kg/ha if sown in rows or 90 kg/ha if broadcast (Duke, 1981); 90 lb/acre if broadcast (McLeod, 1982; Peaceful Valley, 1988); 30 lb/acre (Miller, 1988); and 30 to 50 lbs/acre (Miller et al. 1989).

A recommended spacing for a grain crop is 3 feet between rows, with 2-3 inches between plants, at a rate of 30-40 lb/acre (McLeod, 1982).

Seeding Depth
Seeding depth should be 1/2-1 inch, or deeper in sandy soil (McLeod, 1982).

Seeding Method
Sow after the last threat of frost (Duke, 1981). Before sowing, the field should be pre-irrigated; the seeds should be planted into moist soil (Miller et al., 1989).

Seed can be drilled in rows 6-8 inches apart or broadcast at a rate of 90 lb/acre (McLeod, 1982).

Seeding Dates
Sow after the last threat of frost (Duke, 1981), or when soil is thoroughly warmed and the weather settled (McLeod, 1982). Mid or late summer sowings are often used for green manure (McLeod, 1982).

Inoculation
The appropriate rhizobial inoculant type is "EL" (Nitragin Co.) (Burton and Martinez. 1980; Nitragin Company, Inc., No Date; Duke, 1981). EL type rhizobia can persist for several years in the soil, so the first crop of cowpeas in an area may require inoculation, but subsequent crops may not (see McLeod, 1982).

The cowpea variety Ife BPC had yields equal to those of two other varieties tested ((Ife Brown and AFB 1757) but contributed a net positive of 30.3 kg N/ha to the soil (from N contained in vegetative structures) whereas other varieties showed a net deficit. Although there was evidence that uninoculated cowpea was heavily nodulated, inoculated Ife BPC had greater N content in vegetative structures (Awonaike et al., 1990).

Awonaike et al. (1990) in Nigerian field trials, determined that inoculation with any of several strains of Bradyrhizobium did not increase the biomass or N fixation by three cowpea varieties. This lack of effect was ascribed to the occurrence of the resident Bradyrhizobia bacteria in the soil.

Seed Cost
Seed is relatively inexpensive (Miller et al., 1989). Quotes included $1.25/lb or $41.00/50# bag raw seed (Peaceful Valley, 1988).

Seed Availability
Seed is abundant (Miller et al., 1989).

Days to Flowering
Duke (1981) wrote that cowpea is a short-day plant, and listed flowering as occurring in early summer, but flowering occurs at greatly varying times for different varieties (Bugg, pers. comm.). In southern Georgia, Bugg (unpublished data) sowed several cultivars of cowpea on June 7 and 8 in a pecan orchard, and weekly inspection indicated that the varieties 'Calhoun,' 'Chinese Red,' and 'Red Ripper' all were vegetative on August 11 but attained flower by August 18; cv 'Iron Clay' flowered later. In Massachusetts, cv 'Mississippi Silver' sown on May 7 reached peak flowering on August 7 (Bugg and Ellis, 1990). In southern Georgia, "combine" or "reseeding" cowpea sown on July 3-4 began flowering in early August (Bugg and Dutcher, 1989).

Days to Maturity
Duke (1981) mentioned that early-maturing varieties produce mature seed in 90 days; late varieties may require 240 days. Pods ripen unevenly, with green and ripe pods occurring at the same time; harvest of seed usually occurs when 1/2 to 2/3 of seed are ripe.

Seed Production
In the U.S., seed production averages 1,000-1,500 kg/ha, but in California, up to 3,000 kg/ha; seed may be harvested when 1/2 to 2/3 of seed are ripe, using a special harvester, a self-rake reaper, or an ordinary mowing machine (Duke, 1981). In poor soil, plants produce little vegetation and ample seed, whereas a rich soil produces abundant vegetation and little seed (McLeod, 1982).

Seed Storage
Cowpea seed can remain viable in storage for several years, according to Duke (1981), but McLeod (1982) wrote that it remains viable for a relatively short time.

Growth Habit
Cowpea is an annual herb, erect or suberect and stems usually procumbent (Duke, 1981). In a solid stand, plants form a viny canopy 2' high (Miller, 1988).

Maximum Height
Mean heights (+/- S.E.M.) obtained in replicated (r=5) monocultural plots in a mature pecan orchard in Tifton, Georgia, September 1, 1988, were for the following varieties, in cm (inches in parentheses): 'Calhoun': 48.40 +/- 4.57 (19.10 +/- 1.80); 'Chinese Red': 52.85 +/- 3.09 (20.81 +/- 1.22); 'Iron-Clay': 60.70 +/- 2.08(23.90 +/- 0.82); and 'Red Ripper': 47.95 +/- 0.80 (18.88 +/- 0.31) (P=0.0051, Fisher's Protected Least Significant Difference = 6.797 cm or 2.676 inches) (Bugg, unpublished data). These measurements show varietal differences in height but relate well to the 2 feet (60.96 cm) given by several authors (Miller, 1988; Peaceful Valley, 1988; Miller et al., 1989).

Root System
According to Duke (1981), the taproot is stout with laterals near the surface; nodules are large. The root system is more extensive than those of soybean or moth bean (McLeod, 1982), and it can explore deep soil profiles for moisture (Miller, 1988).

Establishment
Seed after frost danger is past, plant in rows or broadcast; germination is epigeal (Duke, 1981), i.e., occurs on the soil surface. The species is said to tolerate aluminum, drought, high and low pH, heat, laterite, nematodes, poor soil, shade, slope, virus, weeds, and wilt (Duke, 1981). For a 60-day green manure crop in the California Central Valley, seed into moisture and apply one irrigation thereafter (Mark Van Horn, pers. comm.).

Maintenance
For a 60-day green manure crop in the California Central Valley, seed into moisture and apply one irrigation thereafter (Mark Van Horn, pers. comm.). Cowpea tolerates moderate shade, but not waterlogging (Duke, 1981). Cowpea should not be used as a cover crop in flood-irrigated orchards (Fred Thomas, pers. comm.).

Mowing
No specific information is available. Viny varieties like 'Chinese Red' or 'Red Ripper' are probably not so easily mowed as erect cultivars like "combine" or "reseeding" cowpea (Bugg, pers. comm.). Ordinary mowers may be used to harvest cowpea (Duke, 1981).

Incorporation
As green manure, cowpea is sometimes rolled down before being turned under (McLeod, 1982).

Harvesting
When harvested for seed, cut crop when 1/2-2/3 of seed are ripe. Self-rake reapers or special harvesters may be used. For hay, cut when most pods are fully formed and the first have ripened. Ordinary mowers may be used with cowpea (Duke, 1981)

Equipment
Seeded cowpeas may be incorporated using disk harrow or cultipacker (Bugg, pers. comm.). Self-rake reapers or special harvesters may be used. For hay, cut when most pods are fully formed and the first have ripened (Duke, 1981). Ordinary mowers may be used with cowpea (Duke, 1981).

Uses
Seeds can be eaten fresh or dried for storage; leaves can be eaten as a vegetable, or used for forage or silage; and plants can be incorporated as green manure. Indeterminate cvs are best for subsistence farming, whereas erect, determinate forms are more suitable for commercial farming in monocultural systems (Duke, 1981; see McLeod, 1982).

Cowpea is a heat-adapted legume that will make maximum growth during short summer periods before late-summer or fall-planted crops (Miller, 1988). It is also suitable for summer legume cover in orchards and vineyards or undersown with corn (Peaceful Valley, 1988). To prepare poor soil before sowing a crimson clover crop, turn under two successive cowpea crops (McLeod, 1982).

Cultivars with spreading habit and luxuriant growth have been widely used as green manures. Cowpea is a promising multipurpose legume in cropping systems (Yost and Evans, 1988).

Cowpea is useful for suppressing weeds; a thick stand will smother bermudagrass (Peaceful Valley, 1988).

Cowpea can follow crops such as early sweet corn and early processing tomatoes, and precede fall-planted crops such as vegetable brassicas and cereal grains (Miller et al., 1989).

Mixtures
Duke (1981) mentioned that cowpea can be grown in polycultures with cassava, corn, sorghum, sudangrass, johnsongrass, millets, peanuts, or soybeans.

De Queiroz and Galwey (1987) tested five sorghum (S1006, 2219 B, CSH 5, CSH 6, and Ethiopian landrace E 35-1) and 2 cowpea varieties (the semi-erect C 152 and the spreading, later-maturing GFC 4) planted as sole crops and in sorghum- cowpea bicultures during the dry season. There was no significant effect of cowpea variety on sorghum yield, nor of sorghum variety on cowpea. This will probably not be the case for rainy-season plantings, in which sorghum tends to shade the intercrop more completely. For dry-season plantings, the authors suggest that a single cowpea variety might be used to evaluate sorghum performance in biculture.

Biomass
In Davis, Yolo County, California, Klein (1990, pers. com) obtained the following above-ground biomass yields (dry matter in kg/ha, mean +/- SEM): 'California Blackeye #46,' 4,461.4+/-375.2; 'California Blackeye #3,' 4,777.6+/-324.1; 'Chinese Red,' 3,828.8+185.4; 'Spotted Purple Hull,' 3,912.0+/-420.2.

N Contribution
Duke (1981) stated that cowpea above-ground N content ranges globally from 73-354 kg/ha, with a mean of 198 kg/ha. Peaceful Valley (1988) estimated only as much as 80 lb/acre. In Davis, California, Klein (1990) found the following nitrogen contents (in kg N/ha, mean +/- SEM): 'California Blackeye # 46, 151.4+/-13.0; 'California Blackeye #3, 152.8+/-3.8; 'Chinese Red,' 142.5+/- 13.7; 'Spotted Purple Hull,' 146.2+/-13.1 kg/ha nitrogen (Mean+/-SEM).

At plowdown, available N from cover crops can be estimated by multiplying harvested fresh weight of cover crop from a sixteen square feet (4 x 4 ft.) sample plot by 12 (to estimate lbs of N/acre) (Miller et al. 1989).

The cowpea variety Ife BPC had yields equal to those of two other varieties tested ((Ife Brown and AFB 1757) but contributed a net positive of 30.3 kg N/ha to the soil (from N contained in vegetative structures), whereas other varieties showed a net deficit. Although there was evidence that uninoculated cowpea was heavily nodulated, inoculated Ife BPC had better N content in vegetative structures (Awonaike et al., 1990).

Non-N Nutrient Contribution
No information is available in this database on this topic.

Effects on Water
Cowpea requires less water than sesbania (Bugg, pers. comm.).

Effects on Microclimate
No information is available in this database on this topic.

Effects on Soil
Cowpea was better than rice, maize, or pigeon pea for reducing soil loss from 2 percent slopes (Yost and Evans, 1988).

Effects on Livestock
Cowpea may be used as pasture for hogs or used in mixture with corn or sorghum for silage (Duke, 1981).

Effects on Workers
No information is available in this database on this topic.

Pest Effects, Insects
Stinkbugs (Pentatomidae) and leaffooted bug (Coreidae) can build up on cowpea; cowpea should not be used as cover crop in pecan orchards because these insects can disperse from the cover crop and damage the nuts (Dutcher and Todd, 1983). McLeod (1982) mentioned that a so-called squash bug attacks both the cowpeas and the producing pecans, so cowpeas cannot be used as green manure after pecan trees begin producing nuts. It is more likely that stink bugs were again the culprits (Bugg, pers. comm.).

In southern Georgia, extrafloral nectaries of cowpea attracted 26 types of wasps, including 6 types of Vespidae, 4 Pompilidae, 10 Sphecidae, 3 Tiphiidae, and one each of Chrysididae, Scoliidae, and Ichneumonidae (Bugg, unpublished data). In California, various predatory and parasitic wasps are often seen at the nectaries, as are predatory lady beetles, Collops vittatus (striped flower beetle), various bigyed bugs (Geocoris spp.), and the pest, 11-spotted cucumber beetle (Diabrotica undecimpunctata) (Bugg, pers. comm.).

In India, Natarajan and Sheshadri (1988) assessed cotton (cv MCU 5 VT) for effect of intercropping on densities of aphid (Aphis gossypii Glover), leafhopper (Amrasca devastans Distant), bollworms (Earias insulana Boisduval and E. vittella [Fabricius]), entomophagous lady beetles (Coccinellidae) (Melanochilus sexmaculatus Fabricius and Coccinella sp.), and braconid wasps (Rogas aligarhensis Qadri and Agathis fabiae Nixon) that attack bollworms. Cotton was grown in a replicated trial under four intercropping regimes: (1) cotton and cowpea (cv Co.Vu.623); (2) cotton and soybean (cv Co.1); (3) cotton and country onion; and (4) cotton alone. The second factor was insecticidal application, assigned by a split-plot approach. No effects were observed on aphid and leafhopper populations, nor on bollworms. The latter occurred at generally low densities. Parasitism of bollworms by the braconids was higher in the cotton + soybean and the cotton + cowpea treatments. The cotton + cowpea system showed elevated densities of the coccinellids.

Red imported fire ant (Solenopsis invicta) forages for honeydew produced by cowpea aphid (Aphis craccivora) on cowpea and hairy indigo (Indigofera hirsuta) but not on sesbania (Sesbania exaltata), based on a replicated study conducted in Georgia (Kaakeh and Dutcher, 1992). Ethanol and water extractions of sesbania are toxic to S. invicta.

Pest Effects, Nematodes
Duke (1981) listed 39 types of plant-parasitic nematodes that can infest cowpea. Cv 'Iron' reduced soybean nematodes in a greenhouse experiment (Rodriguez-Kabana et al., 1988). In a greenhouse pot survey of various warm-season legumes, cowpea was particularly effective in reducing rootknot nematodes (Meloidogyne arenaria and M. incognita) and soybean cyst nematode (Heterodera glycines race 4) (Rodriguez-Kabana, 1988).

Rhoades (1983) found that sting nematode was moderately abundant following cover cropping with cowpea ('California Blackeye No. 5'). In two out of three years, yield of cabbage was not thereafter reduced below that obtained using the nematicide fenamiphos. Rootknot nematode, Meloidogyne incognita, failed to develop high densities on any of the cover crops, but was particularly abundant on vegetables following sesbania and cowpea.

McKenry (pers. comm.) noted that 'Blackeye 5' was resistant to Meloidogyne incognita and that 'Magnolia' was resistant to M. incognita and M. javonica.

All cultivars host Protylenchus scribneri, but this is not a problem for trees and vines (UC 4045, 1985).

Pest Effects, Diseases
Cowpea is subject to mildew in heavy shade (McLeod, 1982).

Pest Effects, Weeds
Cowpea or varieties thereof are said to tolerate weeds (Duke, 1981). The more vigorous varieties of cowpea can compete with grass weeds; for example, where bermudagrass does not produce seed, it can be controlled by first plowing it under, then growing a thickly sown and strong growing crop such as cowpeas (McLeod, 1982; Peaceful Valley, 1988). Likewise, the dense canopy will shade out any in-row weeds when cowpea is sown agronomically (Miller et al., 1989).

In a study by Bugg and Dutcher (1989), "combine" or "reseeding" cowpea sown in early July in a mature pecan orchard (which included bahia and bermuda grasses in the understory) apparently conferred more rapid vegetational cover and better reduction of weed cover than did other warm-season legumes (lespedeza, American jointvetch, sesbania, or alyceclover). No inferential statistics on legume or weed cover were presented in this replicated study, but means were plotted through time.

Pest Effects, Vertebrates
Mark Van Horn (pers. comm.) reported that red-seeded cowpea varieties appear less palatable to ground squirrels do the pale California blackeye cultivars.

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