Prota 1: Cereals and pulses/Cιrιales et lιgumes secs
Kew Bull. 35(3): 474 (1980).
Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
2n = 22
Glycine subterranea L. (1763), Voandzeia subterranea (L.) DC. (1825).
Bambara groundnut, bambarra groundnut, earth pea, jugo bean (En). Voandzou, pois de terre, pois bambara (Fr). Mancara de Bijagσ, jinguba de Cabambe (Po). Njugu mawe (Sw).
Origin and geographic distribution
The centre of origin of bambara groundnut is probably north-eastern Nigeria and northern Cameroon. It is found in the wild from central Nigeria eastwards to southern Sudan, and is now cultivated throughout tropical Africa, and to a lesser extent in tropical parts of the Americas, Asia and Australia. Its use as a pulse in West Africa was recorded by Arabic travellers in the 14th Century. Its importance declined after the introduction of groundnut from the New World tropics.
Bambara groundnut is grown primarily for its seeds, which are used in many types of foods, some of which are an important part of the diet and play a role in traditional ceremonies (e.g. funeral rites) and gift exchanges. Mature, dry seeds are boiled and eaten as a pulse. Dried seeds, either whole or split, are also mixed with maize or plantains and then boiled. The seeds may be ground into flour, sometimes after roasting, to prepare a porridge. They are also added to maize flour to enrich traditional preparations. Sometimes seeds are soaked in water and ground into a paste which is used to prepare fried or steamed dishes. Immature seeds are often boiled with salt and eaten as a snack. They are eaten during the hungry gap during the growing season, when stores are empty and crops are not yet ready for harvest. Vegetable milk and fermented products similar to tempeh (from Glycine max L.) and dawadawa (Parkia biglobosa (Jacq.) R.Br. ex G.Don) can be made from the seeds.
The seeds are fed to pigs and poultry, and the leafy shoots are used as fodder. In Senegal leaf preparations are applied to abscesses and infected wounds, leaf sap is applied to the eyes to treat epilepsy, and the roots are sometimes taken as an aphrodisiac. Pounded seeds mixed with water are administered to treat cataracts. The Igbo in Nigeria use the plant to treat venereal diseases.
Production and international trade
Reliable production figures for bambara groundnut are difficult to obtain, because the crop is mainly grown for home consumption and sale at local markets. In the early 1980s the estimated annual world production was 330,000 t, 4550% of which was produced in West Africa. The major producers are Burkina Faso, Chad, Cτte dIvoire, Ghana, Mali, Niger and Nigeria, but the crop is also widely grown in eastern and southern Africa and in Madagascar. The main exporting countries are Burkina Faso, Chad, Mali, Niger and Senegal; they supply markets in Benin, Ghana, Nigeria and Togo.
Raw immature bambara groundnut seeds contain per 100 g edible portion: water 57.3 g, energy 636 kJ (152 kcal), protein 7.8 g, fat 3.1 g, carbohydrate 30.0 g, fibre 3.0 g, ash 1.8 g, Ca 14 mg, P 258 mg and Fe 1.2 mg. The composition of mature dry seeds per 100 g edible portion is: water 10.3 g, energy 1537 kJ (367 kcal), protein 18.8 g, fat 6.2 g, carbohydrate 61.3 g, fibre 4.8 g, ash 3.4 g, Ca 62 mg, P 276 mg, Fe 12.2 mg, β-carotene 10 μg, thiamin 0.47 mg, riboflavin 0.14 mg, niacin 1.8 mg and ascorbic acid traces (Leung, Busson & Jardin, 1968). The content of essential amino acids per 100 g food is: tryptophan 192 mg, lysine 1141 mg, methionine 312 mg, phenylalanine 991 mg, threonine 617 mg, valine 937 mg, leucine 1385 mg and isoleucine 776 mg (FAO, 1970). As in other pulses, the sulphur-containing amino acids cystine and methionine are limiting. The main fatty acids in the seed oil are palmitic acid 1824%, stearic acid 512%, oleic acid 1824%, linoleic acid 3440%, linolenic acid 23% and behenic acid 37%. A content of 21% linolenic acid and no oleic acid, however, has also been recorded. The ratio of saturated to unsaturated fatty acids is approximately 1:2. The oil content is too low for the seed to be used as a source of oil. Trypsin inhibition occurs. The seeds contain tannins, mainly in the seed coat. In comparative studies in Botswana and Ghana, tannin levels were found to be lowest in cream-coloured seeds, intermediate in red seeds and highest in black seeds. Cooking and other forms of processing (e.g. soaking, milling, hulling, germination, fermentation) reduce the concentration of antinutritional factors. Ripe seeds are very hard and usually have to be cooked longer than those of other legumes. Cream-coloured seeds are often preferred to red and black seeds, because they are less bitter (sweeter) and take less time to cook. Large seeds are preferred over smaller ones, e.g. for use as snack; smaller seeds are ground into flour for use in various recipes. Dried leaves for fodder contain crude protein 15.9%, crude fibre 31.7%, ash 7.5% and fat 1.8%.
Annual herb with creeping stems branching just above ground level; root system consisting of a tap root with lateral roots lower down, with rounded and sometimes lobed nodules. Leaves alternate, 3-foliolate, glabrous; stipules c. 3 mm long, spurred, striate; petiole erect, grooved, up to 30 cm long, thickened at base, rachis (0.1)12.5 cm long; stipels ovate-oblong, up to 3 mm long; petiolules 13 mm long; leaflets elliptical to oblanceolate, 310 cm Χ 15 cm. Inflorescence an axillary false raceme, close to the ground, (1)2(3)-flowered; peduncle 0.52 cm long. Flowers bisexual, papilionaceous, shortly pedicelled; calyx with tube c. 1 mm long and 5 lobes c. 1 mm long; corolla whitish-yellow, standard obovate, 47 mm long, wings and keel slightly shorter; stamens 10, 9 with filaments fused for more than half their length and 1 free; ovary superior, 1-celled, style bent. Fruit an almost globose indehiscent pod c. 2.5 cm in diameter, usually 1-seeded. Seed 8.515 mm Χ 6.510 mm Χ 5.59 mm, variously coloured from white to cream, red, black or brown, sometimes mottled, blotched or striped; eye around the hilum sometimes present, colour and shape variable. Seedling with hypogeal germination.
Other botanical information
Vigna comprises about 80 species and occurs throughout the tropics. However, it is likely that the American species will be placed in a separate genus in the near future. There are considerable morphological difference between wild and domesticated types of bambara groundnut. Wild bambara groundnut produces long runners, the pods are thin and do not wrinkle upon drying, and the seeds are small (911 mm long) and uniform in size. Domesticated types are more compact, with longer, less slender and more erect petioles, fleshy pods which wrinkle on drying, and larger seeds (1115 mm long). Morphological and isozyme data indicate a gradation from wild to domesticated bambara groundnut through weedy populations. Wild and domesticated types are sometimes distinguished as var. spontanea (Harms) Hepper (wild) and var. subterranea (cultivated). No cultivars of bambara groundnut have been named, but genotypes are distinguished on the basis of seed attributes (colour, size, hardness) and plant form (bushy or spreading). Sometimes names are based on the location where the seed was collected.
Growth and development
The optimum temperature for germination of bambara groundnut is 3035°C; below 15°C and above 40°C, germination is very poor. Emergence takes 521 days. Vegetative development may continue after reproductive development has started. Flowering starts 3055 days after sowing and may continue until the plant dies. Self-pollination is the rule. After fertilization, the peduncle grows and pods form on or below the ground. Pods reach their maximum size in about 30 days. The seeds expand and reach maturity during the following 10 days, when the parenchymatous layer surrounding the embryo has disappeared and brown patches appear on the outside of the pod. Seeds are mature 36 months after germination. Bambara groundnut is able to fix atmospheric nitrogen by nodulating with bacteria of the Bradyrhizobium group.
Bambara groundnut is cultivated in the tropics at altitudes up to 2000 m. A frost-free period of at least 3 months is necessary. Average day temperatures of 2028°C and full sun are preferred. The crop tolerates drought and is cultivated successfully in areas with an average annual rainfall of 600750 mm, though optimum yields are obtained when rainfall is higher (9001200 mm/year). It is also grown in humid conditions, e.g. in northern Sierra Leone, where the annual rainfall exceeds 2000 mm. There are considerable differences between genotypes in their response to temperature and photoperiod. In many genotypes, flowering is photoperiod-insensitive, while the onset of podding is retarded by long photoperiods. In some genotypes both flowering and the onset of podding are delayed by long photoperiods. Podding may also be delayed by drought.
The plant grows on any well-drained soil, but light sandy loams with a pH of 5.06.5 are most suitable. Soils rich in phosphorus and potassium are suitable, but calcareous soils are not. Nitrogen-rich soils promote vegetative growth at the expense of seed yield. Sandy soils enhance pod penetration into the soil, but nematode incidence is generally higher on sandy than on loamy soils.
Propagation and planting
Bambara groundnut is propagated by seed. The seeds are orthodox and can be stored below 0°C. The 1000-seed weight is 500750 g; sowing rates range from 25160 kg/ha, depending on cropping system and climate. Seed to be sown is usually retained from the previous harvest or bought at local markets. Planting material is usually selected after harvesting on the basis of seed characteristics and not on plant characteristics. Often large seeds are selected for planting. Seeds are stored in bags, bottles, gourds or calabashes sometimes sealed with mud. They should be shelled just before sowing to retain maximum viability, but otherwise are rarely pretreated.
Bambara groundnut is not usually sown immediately after the first rains, because staple food and cash crops tend to receive priority. Sowing dates vary considerably within locations. In Zambia and Botswana, for example, sowing takes place from November to February. Late sowing, however, may result in large yield reductions. Sometimes phased planting occurs, e.g. in Sukumaland, Tanzania. Land is cleared, and may be ploughed and ridged before sowing. In Botswana, fields are sometimes ploughed after the seed has been broadcast. The crop performs best on deeply ploughed fields with a fine seedbed, eventually allowing the plant to bury its developing fruits. Ridging is advisable if the soil is shallow or prone to waterlogging. Bambara groundnut may be sown on mounds, e.g. in Ghana. When sowing a new field, inoculation with soil from an old bambara groundnut field is recommended to promote nodulation with rhizobial bacteria.
Bambara groundnut is sown in rows or broadcast; densities range from 2,500 plants/ha (intercropping in Botswana) to 250,000 plants/ha (sole cropping in Nigeria). Rows can be 2040 cm apart (Nigeria) to 50100(400) cm apart (Botswana). Spacings can be 2040 cm within rows (Nigeria) to 10100 cm (Botswana). Dry matter production of bambara groundnut is low, so high plant densities are recommended. However, high densities are only possible where rainfall and soil fertility are adequate. Furthermore, close spacing makes earthing up difficult. Seed is often dibbled, dropping 14 seeds in the hole and covering with soil. Sometimes a planter is used, or the seed is sown immediately behind a plough. Under rainfed conditions in sandy soils a sowing depth of at least 6 cm is advisable, but farmers often sow less deep. Thinning may be practised, often in combination with weeding. When establishment problems occur, gaps are sometimes filled in with seeds or plants thinned out elsewhere. Bambara groundnut may be grown in intercropping with cereals, other pulses, root and tuber crops, or vegetables. It is often grown together with maize, sorghum, pearl millet, groundnut and cowpea. Bambara groundnut is mainly grown by smallholders, often women, usually on small fields (less than 0.5 ha).
Weeding of bambara groundnut takes place 13 times, often with a hoe. Earthing up to cover the young pods is common, and may be done by hand, with a hoe or with ox-drawn equipment. Earthing up improves yields, but is labour intensive; it is often combined with weeding. Nitrogen needs may be met by symbiotic nitrogen fixation. Nitrogen-fixation rates of up to 100 kg/ha have been reported, but sufficient phosphorus availability is essential for nodulation. The use of animal manure or chemical fertilizers is not common. Research in Botswana has shown that under the prevailing conditions nitrogen fertilization is not advisable, whereas phosphorus application is only beneficial when it is done close to the seedlings within 2 weeks of sowing and when the soil during this period is moist.
Bambara groundnut is used in rotations, e.g. with maize, sorghum, pearl millet, cassava and yam. Farmers in Swaziland and in parts of South Africa prefer to sow bambara groundnut immediately after fallow, to maximize yields.
Diseases and pests
Although bambara groundnut is considered to be generally less affected by diseases and pests than groundnut or cowpea, several diseases and pests can cause serious damage to the crop. The most important fungal diseases are Cercospora leaf spot (Cercospora spp.), powdery mildew (Erysiphe polygoni) and Fusarium wilt (Fusarium oxysporum). Symptoms of Cercospora leaf spot are reddish-brown circular spots on the leaves, as well as lesions on the stems, petioles, peduncles and pods. The lesions may coalesce to give the appearance of blight. In cases of severe attack, defoliation occurs and plants may die prematurely. Crop rotation and burning of crop debris of the previous season are recommended to reduce damage, but the best solution is to use more resistant types. Symptoms of powdery mildew are a whitish powder on both sides of the leaves, especially on the upper surface. Infected leaves dry out and die. Treatment with a chlorothalonil-based fungicide has sometimes been effective. Fusarium wilt causes vascular discolouration, yellowing, necrosis and wilting and plants become stunted and eventually die. Crop rotation may help, but planting more resistant types is the best control. Other fungal pathogens affecting bambara groundnut include Macrophomina phaseolina (charcoal rot), Phomopsis sp.(blight), Phyllosticta spp. (Phyllosticta leaf spot) and Sclerotium rolfsii (southern blight and pod rot). Virus diseases include cowpea mottle virus (CPMoV), cowpea aphid-borne mosaic virus (CABMV) and peanut mottle virus (PeMoV). Genotypes resistant to cowpea mottle virus have been identified. Root-knot nematodes (Meloidogyne incognita, Meloidogyne javanica) can seriously affect yields.
Pests of germinating seeds include rodents, termites, ants and cutworms (Agrotis). The standing crop may be attacked by insect pests such as aphids, groundnut jassid (Empoasca facialis), groundnut hopper (Hilda patruelis), brown leaf beetle (Ootheca mutabilis), and bean leaf webber (Hedylepta indicata, synonym: Lamprosema indicata). A serious pest in Swaziland is the American bollworm (Helicoverpa armigera). Control measures of insect pests include the use of insecticides, e.g. malathion against aphids. Leaves may also be eaten by mammals, such as duikers. Maturing seeds may be attacked by rodents, ants, wild pigs, monkeys and bush babies (Galago spp.).
Important storage pests are the bruchid beetles Callosobruchus maculatus and Callosobruchus subinnotatus, and the maize weevil Sitophilus zeamais. Infestation often begins in seeds ripening in the field and is later carried into the stores. Seeds stored in the pod shell suffer less from deterioration and infestation by insects than shelled seeds. Stored seeds are sometimes protected by applying ash, chemical products (malathion, carbamyl) or plant products, such as ground tobacco leaves, ground peppers or the leaves of basil (Ocimum basilicum L.). The parasitic plants Alectra vogelii Benth. and Striga gesnerioides (Willd.) Vatke may reduce yields considerably.
Bambara groundnut is harvested 90180 days after sowing, depending on genotype, ecological conditions and farmers objectives. As the seeds may be consumed either unripe or ripe, different harvesting methods exist. Unripe seeds may be harvested in several rounds from the same plants. Mature seeds are harvested when the leaves turn yellow and fall, and when the pods have become hard. In the latter case, harvesting is usually done by uprooting the plants by hand or with a hoe. The leaves are left in the field or fed to animals.
Yield fluctuations between years are large in bambara groundnut and mainly depend on rainfall. The highest recorded seed yield under field conditions is 4 t/ha. Average yields are 300800 kg/ha, but yields of less than 100 kg/ha are not uncommon. Bambara groundnut still gives some yield under conditions (poor soils, drought) which are submarginal for groundnut.
Handling after harvest
The pods of bambara groundnut are sun-dried to a moisture content of 12% and stored in bags or drums in granaries or in the house. They may be shelled first with mortar and pestle, flails or modified groundnut shellers. The shelling percentage ranges from 7077% by pod weight. Bambara groundnut is a typical dual-purpose crop: usually part of the harvest is sold and the rest is kept for own consumption. Canning of bambara groundnut seeds has been done in Ghana and Zimbabwe.
The largest germplasm collection of bambara groundnut (2000 accessions from sub-Saharan Africa) is held by IITA (International Institute of Tropical Agriculture), Ibadan, Nigeria. Most of the accessions (1400) in this collection have been characterized, evaluated and documented. Other large collections are found at the IRD (Institut de Recherche pour le Dιveloppement), Montpellier, France (about 1200 cultivated and 60 wild accessions from Cameroon, of which 50 were morphologically characterized), the University of Zambia, Lusaka, Zambia (460 accessions), the Grain Crops Institute, Potchefstroom, South Africa (200 accessions) and the Plant Genetic Resources Centre, Accra, Ghana (170 accessions). In many African countries smaller collections are maintained.
In studies of genetic diversity in cultivated bambara groundnut with RAPD and AFLP markers, considerable genetic variation was found, with accessions clustering mainly according to their geographical origin. Sometimes, e.g. in Swaziland, farmers sow a mixture of landraces as a buffer to biotic and abiotic stresses, thus helping to maintain the diversity of the crop.
Bambara groundnut breeding has mainly been confined to selection between and within populations for yield, disease resistance (Fusarium wilt and Cercospora leaf spot) and drought tolerance. From the IITA germplasm collection genotypes have been identified with a longer and denser root system, which may be useful in breeding for drought tolerance. Breeding of genotypes with a shorter growth period also seems useful for drier regions. Selection of the most effective combinations of genotypes and rhizobial strains seems promising to improve nitrogen fixation and increase crop yields.
Artificial hybrids between cultivated genotypes and between cultivated and wild accessions have been made in the United Kingdom and Swaziland, but success rates are generally low. A genetic linkage map of bambara groundnut using AFLP markers is being developed in the United Kingdom as well. Micropropagation of bambara groundnut is possible using stem nodal cuttings or embryo axes.
Bambara groundnut is a suitable crop for semi-arid regions, because it tolerates drought and poor soil conditions and appears to be less affected by diseases and pests than cowpea or groundnut. Farmers also value its multiple uses and good taste. Although bambara groundnut will remain an important secondary food crop in Africa, the area under cultivation will probably decline, because of high labour requirements, especially for earthing up and harvesting, the absence of an export market outside Africa, and the competition from groundnut and cowpea. The prospects of bambara groundnut as a food crop can be improved by developing high-yielding cultivars with improved disease resistance and lower anti-nutritional factors. The development of new food product composites with cereals may also lead to increased use of the crop.
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Sources of illustration
Linnemann, A.R., 1989. Vigna subterranea (L.) Verdc. In: van der Maesen, L.J.G. & Sadikin Somaatmadja (Editors). Plant Resources of South-East Asia No 1. Pulses. Pudoc, Wageningen, Netherlands. pp. 7475.
Correct citation of this article:
Brink, M., Ramolemana, G.M. & Sibuga, K.P., 2006. Vigna subterranea (L.) Verdc. In: Brink, M. & Belay, G. (Editors). PROTA 1: Cereals and pulses/Cιrιales et lιgumes secs. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, habit of flowering plant; 2, flower; 3, fruits; 4, seed.
flowers and young fruits
plant with ripe and unripe fruits
plant with subterranean fruits
intercropping with maize