AFRICAN YAM BEAN, Wild yam bean.
Sphenostylis stenocarpa (Hochst. Ex A. Rich.) Harms syn. Sphenostylis ornata A. Chev.
Leguminosae, sub-family Papilionoideae.
Akitereku (W. Afr.); Diegemtenguere (Mali); Girigiri (W. Afr.); Haricot igname (Fr.); Kotonosu (W. Afr.): Kulege (W. Afr.); Norouko (Sud.); Okpu dudu (W. Afr.); Pempo (W. Afr.); Pomme deterre du Mossi (Fr.); Roya (Sud.); Sese (W. Afr.); Yam pea.
A vigorous, herbaceous, climbing vine, reaching 1.5-2 m in height, with trifoliate leaves, the leaflets being up to 14 cm in length and 5 cm broad. The conspicuous flowers are mauvish-pink, purple or greenish-white in colour, about 2.5 cm in length and borne on stout axilliary peduncles. The glabrous seed pods are linear, flat, with both margins raised, 25-30 cm long and 1-1.5 cm broad, containing 20-30 seeds which may be ellipsoid, rounded or truncated, and show considerable variation in size and colour; the largest are usually about I cm long and 0.7 cm wide. Seed colour may vary from creamy-white or brownish-yellow to dark brown, sometimes with black marbling, and there appear to be a number of 'types' according to seed colour. The plant produces small spindle-shaped tubers, about 5-7.5 cm long. There is some evidence that yields of seeds and tubers are inversely related.
Origin and distribution
The African yam bean originated in Ethiopia. Both wild and cultivated types now occur in tropical Africa as far south as Zimbabwe, throughout West Africa from Guinea to southern Nigeria, being especially common in the latter and in Togo and the Ivory Coast, and in East Africa from northern Ethiopia (Eritrea) to Mozambique, including Tanzania and Zanzibar.
Small-scale cultivation is practiced throughout tropical Africa: the plant is especially suited to lowland conditions, though it can be grown up to 1800 m. Climates ranging from savannah to rainforest are tolerated provided there is a combination of adequate rainfall (100 cm or more during the growing season) and reasonably good drainage. It is often planted along with yams and beans, using the same stakes as the yam for support, though sometimes left to trail on the ground. It is sometimes stated that plants perform better when interplanted than when grown alone.
Material-both seeds and tubers can be used for propagation; planting is usually at the start of the rainy season.
Method-planting is done by hand, often two seeds to a hole (or
Field spacing-varies considerably, often according to the crop with which they have been interplanted. One spacing quoted is 45 cm apart alternated with yam in 120 cm rows.
Pests and diseases
Fungal diseases reported are powdery mildew (due to Oidium sp.), which is parasitised by Cincinnobolus cesati; leaf spot (caused by Phoma sp.) and stem rust (caused by Aecidium sp.). Virus mosaics have also been reported. Pests have not been defined in detail but include Orthopterous and Lepidopterous insects. Leaf rolling caterpillars and leaf miners have been described as causing serious damage to the foliage, and thrips damage the flowers. Nematodes may attack the root system leading to reduction in yield.
The tubers are ready for harvesting 5-10 months after planting.
Harvesting and handling
The crop is dug by hand usually towards the end of the dry season.
Tubers-these are small and spindle-shaped, externally rather similar to sweet potatoes, usually about 5-7.5 cm long and weighing on average 50-150 g, although under favourable conditions they can weigh up to 300 g. The flesh is white and watery.
On a basis of 24 200 plants/ha, yield has been calculated at 1 452-2 904 kg/ha, depending upon variety. (Seed yield on the same basis ranged from 3 461-3 872 kg/ha.)
The tubers are cooked and eaten in the same manner as potatoes, which they resemble in flavour.
Secondary and waste products
The seeds also are eaten, but must be soaked in water for about 12 hours before being cooked. They are said to cause giddiness if eaten in excess, but to cure drunkenness when mixed with water.
Tubers-the tubers are rich in starch and protein. Dry matter is approximately 35 per cent, of which starch is about 80 per cent and protein about 14 per cent, ranging from 12.5 to 19 per cent for six varieties; however, some analyses have indicated that the non-protein nitrogen can be 50 per cent of the crude protein nitrogen.
Seeds-analytical figures for the seeds showed dry matter about 90.5 per cent; the dry matter composition was: protein 24-28 per cent; fat 1.5-2 per cent; total carbohydrate 74.1 per cent; fibre 5.2-5.7 per cent; ash 2.8-3.2 per cent; calcium 61 mg/100 g; phosphorus 437 mg/100 g. The amino acid content of the protein was very similar to that of soya bean, though rather higher in histidine and iso-leucine. The energy content of the seeds per 100 g dry matter was 1,640 kJ.
The plants have beautiful flowers and are grown as ornamentals in European and other countries.
The tubers of African yam bean are regarded as an important source of starch and protein in tropical Africa, and the plant is potentially important also as a food legume. It appears likely to remain a valuable constituent of African peasant agriculture.
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