Encycl. 3(1): 233 (1789).
2n = 36, 40, 54, 60, 66, 80, 140
Dioscorea rotundata Poir. (1813).
Guinea yam (En). Igname de Guinée (Fr). Inhame da Guiné (Po).
Origin and geographic distribution
The Guinea yam group comprises Dioscorea cayenensis (yellow Guinea yam) and Dioscorea rotundata (white Guinea yam). Neither occurs in the wild. Guinea yam originated in West Africa, and occurs from Senegal to Ethiopia and Uganda. It probably arose in cultivation resulting from hybridization in the section Enantiophyllum of Dioscorea. Dioscorea praehensilis Benth., Dioscorea abyssinica Hochst. ex Kunth and Dioscorea burkilliana J.Miège are possible parents. Domestication still continues, e.g. in Benin where local farmers collect wild yam plants, and cross them with cultivated ones. Guinea yam is also planted in Central and East Africa, the Caribbean, Brazil and the Philippines. It has been introduced only very recently into Papua New Guinea and Oceania.
Guinea yam is grown for its starchy underground tuber, which is the staple food in a belt from Côte d'Ivoire to Cameroon. This region accounts for over 95% of the world's cultivation of Guinea yam. The tuber is used almost exclusively for human consumption; only the peels are fed to animals. Unpeeled tubers may be boiled, roasted or baked; peeled tubers may be boiled or fried. A traditional method of preparation in West Africa is to pound the boiled peeled tuber to produce a thick dough (pounded yam). Pounded yam is consumed by rolling it into small balls which are then dipped into a sauce and swallowed (usually without chewing). Occasionally, the peeled tuber is cut into small chips, dried and then milled to produce yam flour. Chips may also be fermented before further processing. At meal time, the flour is stirred in boiling water and kneaded to produce a paste which is consumed in the same way as pounded yam. Industrial processes have resulted in the production of yam flakes which can be reconstituted in hot water to give a product similar to pounded yam or mashed yam. In West Africa, Guinea yam has a high sociocultural value attached to its production and use. It is a prime object for traditional religious observances, social gift exchange, and cultural festivity. The start of its harvest even signifies the start of a new year. In these contexts, there is a strong preference for large tubers.
Production and international trade
World production of all yams in 2000 was 38 million t grown on 3.9 million ha. Nearly 90% of this production came from the West African yam belt. Separate figures for Guinea yam are not available. However, given the fact that it is the predominant cultivated yam in West Africa, and that it is not extensively cultivated elsewhere, the production of Guinea yam in 2000 can be estimated at about 30 million t. Nigeria is by far the largest producer, followed in order by Ghana, Côte d'Ivoire and Benin. Virtually all production is by smallholders.
Small quantities of Guinea yam are exported from West Africa to Europe, and from the Caribbean to North America and Europe. However, the quantities are insignificant compared to total production.
On a fresh weight basis, 100 g of tuber contains 58–80 g water, 15–23 g carbohydrate, 1–2 g crude protein, 0.05–0.12 g lipids, 0.35–0.79 g crude cellulose, and 0.68-2.56 g ash. Carbohydrates, predominantly starch (50–80%), comprise most of the dry matter. The starch granules of white Guinea yam are oval and measure 5–45 µm in diameter, whereas those of yellow Guinea yam are generally smaller and triangular. Starch granule size generally increases from the top of the tuber downwards, and from the subcutaneous region towards the centre. The protein content, though generally low, is highest close to the skin. Peeling therefore has to be done carefully in order to conserve as much of the protein as possible. The protein fraction is high in aspartic and glutamic acids, and low in tryptophan and cystine. Some cultivars have significant amounts of vitamin C and thiamine.
Freshly cut tubers cause skin irritation due to the presence of raphides, which are destroyed when the tubers are cooked. In some cultivars, the cut tuber is subject to oxidative browning or discoloration. Cooking immediately after peeling or cutting reduces the degree of discoloration.
Adulterations and substitutes
Cassava products are often added as adulterants to pounded yam or yam flour because of the relative cheapness of cassava.
Dioecious, exceptionally monoecious, glabrous herb with annual twining stem arising from tuber; tuber usually solitary, cylindrical to irregularly shaped, up to 10(–25) kg in weight, flesh whitish or yellowish, with or without prickly superficial roots, giving rise to 1 or few annually renewed tubercules; stem up to 12 m long, twining to the right, glabrous, spiny or not. Leaves usually alternate in basal part of stem and opposite in upper part, basal leaves often strongly reduced, simple; stipules absent; petiole 5–12 cm long; blade broadly ovate to suborbicular, 5.5–12 cm × 5–10 cm, broadly cordate at base, acuminate at apex, entire, 5–7-veined. Inflorescence an axillary unisexual spike, male 1-3 together and 4–6 cm long, female 1–2 together and 10–12 cm long. Flowers unisexual, regular, with 6-lobed perianth; male flowers small (1–2 mm in diameter), sessile or shortly pedicelled, with 6 stamens; female flowers with inferior, 3-celled ovary, styles 3, short. Fruit a capsule wider than long, 2–2.5 cm × 3–3.5 cm, opening by 3 valves, up to 6-seeded. Seeds 1–1.5 cm × 1–1.5 cm, with large circular wing.
Other botanical information
It is still not clear whether Dioscorea cayenensis and Dioscorea rotundata represent different taxa, or are the same. In the botanical literature, the latter is often considered a synonym of the former. However, two distinct groups are recognized in agronomical practice: yellow Guinea yam and white Guinea yam, broadly corresponding to Dioscorea cayenensis and Dioscorea rotundata, respectively. The tuber flesh is yellow in Dioscorea cayenensis and white in Dioscorea rotundata, but intermediate cultivars are common. Molecular marker-based taxonomy supports the view of 2 different taxa. Both are only known from cultivation, and it would be appropriate to treat them as cultivar-groups.
Numerous (possibly 2500) named and unnamed cultivars exist, characterized by leaf shape, tuber shape, size and colour, stem colour, degree of spinescence of stems and roots and ecological requirements.
Growth and development
Four growth phases have been recognized in white Guinea yam grown from tubers. The first phase lasts for 6 weeks from emergence. It includes root proliferation and extensive vine elongation, but very little leaf expansion. Growth during this phase is dependent on food stored in the parent tuber. The second phase lasts from 6–10 weeks after emergence and is marked by strong leaf expansion and transition to full autotrophy. Tuber initiation occurs towards the end of this phase. The third phase lasts from 10–18 weeks after emergence and includes tuber bulking. The leaf area and vine length do not increase much during this phase, and the quantity of living roots decreases. The fourth phase lasts from 18 weeks after emergence till leaf, vine and root senescence at the end of the season, 6–7 months after emergence. The resulting tuber (whether harvested or left in the ground) remains dormant for 2–3 months before it begins to sprout.
Flowering is irregular, but commences late in the second phase or early in the third phase in cultivars that do flower. Fruit and seed production are uncommon. The seeds have a dormancy of 3–4 months after the senescence of aerial parts of the plant. Seedlings are less vigorous than plantlets raised from tuber ‘sets’, but generally go through the same four growth phases.
Yellow Guinea yam requires a growing period of 10–12 months. The tuber has only a short dormancy period.
Guinea yam requires a temperature of 25–30°C for normal growth; therefore it is strictly tropical. Vine growth is severely limited below 20°C, while soil temperature above 35°C retards sprouting of planted sets. White Guinea yam is better adapted to savanna regions with their longer dry season, while yellow Guinea yam is cultivated in the West African forest zone, where the dry season is comparatively short and the growing season lasts about 11 months. In white Guinea yam, water supply must be adequate for the 6–7 months of the plant's growth phases. Evenly distributed rainfall of 1500 mm/year is optimal, but small crops can be obtained with as little as 600 mm/year. At such low rainfall, however, production of seed tubers is low. Drought tolerance is best during sprouting and the first growth phase. Drought stress during later phases results in leaf abscission and subsequent reduction in tuber yield.
Guinea yam performs poorly unless the soil is very fertile. Mycorrhizal associations promote phosphorus uptake in soils that are phosphorus-deficient. Nitrogen and potassium deficiencies are frequently encountered. Optimum pH is 5.5–6.5. Aluminium toxicity is a problem at pH less than 5.5. The soil must be well drained to at least 90 cm depth. The soil should be free of coarse gravel or stones, and devoid of a hard pan, otherwise tuber shape is distorted. Low light intensity decreases the tuber/vine ratio, and results in low yields.
Propagation and planting
Propagation is normally by tubers. Up to 20% of the yield of Guinea yam is set aside as planting material. The best planting material is a small intact tuber (or ‘set’) 100–500 g in fresh weight. For sets derived by cutting up large tubers, the head set is preferable to middle or tail sets. Intact tubers or head sets sprout from buds present in the primary nodal complex. Middle or tail sets (or heads with the nodal complex removed) sprout de novo through meristematic activity beneath the skin. Generally, sets that sprout readily from cultivars that are resistant to post-planting rotting or degradation are preferred.
Freshly-harvested tubers of white Guinea yam are dormant for 2–3 months before they sprout. The longer a tuber has been stored after harvest, the more rapidly budless sets derived from it will sprout. However, even for long-stored tubers, the minimum time for sprout formation in budless sets taken from them is 3 weeks. Under field conditions, time from planting to emergence is 1–3 months. Emergence in a given field is scattered over a long period of time; thus the uniformity of emergence is low.
The weight of the parent sets profoundly affects the growth and performance of plants raised from tubers. The larger the set, the more vigorously it sprouts, the greater the resultant leaf area, the earlier the tuber development, and the larger the tuber produced. However, large sets result in a low multiplication ratio. The planting of very small sets (called ‘minisets’, 15–50 g each) to produce tubers for subsequent commercial planting is extensively practised.
Planting is on mounds 30–60 cm high, or on ridges. Spacing is 1 m × 1 m. Sets are inserted 10–15 cm deep. Intercropping is the most frequent practice, but sole-cropping is also common.
Propagation by seed is possible, but uneconomic. Seed production is irregular; some cultivars do not set seed. There is a dormancy period of 3–4 months prior to germination. Seedlings are weak and require careful attention in the nursery. The tuber yield of plants propagated by seed is very small. Propagation by tissue culture is also possible, but as with seed propagation, the resulting tuber is extremely small. Propagation by seed and tissue culture is, therefore, essentially a research tool, and for rapid multiplication of disease-free cultivars.
Mulching is often necessary to protect sets from excessive heat between planting and emergence. At 1–2 months after emergence, stakes 1–2 m long are installed. Plants may be staked individually, or several plants may be trained on to one large stake. Pollarded shrubs (live stakes), left after clearing, may also serve as stakes. Staking is a very labour-intensive operation, and is most critical in high-rainfall regions and in forest regions where plants are shaded and where staking materials are abundant. In savanna regions, adequate yields are obtained without staking.
The critical period for weed control is the first 1–3 months after emergence. Weeding 2–3 times during the season with hand tools is the general practice. Herbicides such as diuron and ametryne are sometimes used.
The crop responds well to nitrogen and potassium fertilizers; it responds less well to phosphorus applications, possibly because mycorrhizal associations make sufficient soil phosphorus available. Compound NPK fertilizers of various formulations are in use. Split application of the fertilizer is recommended, with the first application 1 month after emergence (in the first growth phase) and the second application 3 months after emergence, during tuber bulking (the third growth phase). Most traditional farmers avoid use of chemical fertilizers in the belief that they reduce the storability of the resulting tubers.
Diseases and pests
Storage and tuber rots, caused by Penicillium, Fusarium and Botrydiplodia species, are responsible for high post-harvest losses. Guinea yam is relatively devastating in greater yam (Dioscorea alata L.).
The yam beetle (Heteroligus sp.) is a major pest. Adults migrate into yam plots in mid-season, burrow into the base of the plant, and begin feeding on the enlarging tubers, causing hemispherical holes which render harvested tubers unmarketable and predisposed to rotting. At end of the season, the adult beetles mate and migrate to swampy areas to lay their eggs. Larval and pupal stages are spent in these locations, until the resulting adult beetle emerges early in the rainy season, ready to migrate to yam plots. Yam beetle is controlled with insecticides or by planting very late.
Yam nematode (Scutellonema bradys) and root-knot nematode (Meloidogyne sp.) occur in some locations. They cause damage to the meristem of the tuber, and result in a warty appearance of the tuber. The main control measure is crop rotation.
A virus complex is reportedly endemic in the yam belt, and may be responsible for the generally low yields. Thermotherapy and meristem culture have been used to produce virus-free materials for distribution.
White Guinea yam is harvested 6–8 months after emergence; yellow Guinea yam after about 12 months. The signal for harvesting is the senescence of the shoot. Harvesting is invariably done with hand tools.
Double-harvesting of the same plants is sometimes practised in white Guinea yam. This involves a first harvest 4–5 months after emergence, during which the tuber is carefully removed with minimum damage to the roots and the rest of the plant. This is followed 2–3 months later by a second harvest of the same plants, after shoot senescence. The first harvest supplies new yams for food early in the season, although the tubers have a high water content. The second harvest yields excellent planting material since the tubers possess numerous buds and are less prone to rotting.
Average yield in tropical Africa is about 10 t/ha of fresh tubers. Typical yields in 1999 were 12.7 t/ha for Ghana, 11.1 t/ha for Benin, 10.8 t/ha for Côte d'Ivoire, and 9.6 t/ha for Nigeria. In experiments, yields of 60 t/ha have been obtained.
Handling after harvest
Traditionally, the harvested tubers are collected from the field, and then tied up in a yam barn, which is essentially a framework of vertical wooden poles. The barn is usually constructed in the open, either on the farm or behind the home. When the next rainy season sets in, the tubers are usually moved indoors and stored on a platform.
It is essential that the tubers are well aerated during storage, and frequently inspected for rotting or sprouting, and kept as cool as possible. However, temperatures below 15°C cause the tuber to darken and deteriorate. This, in addition to the cost of refrigeration, makes refrigerated cold storage unfeasible.
Virtually all harvested tubers are stored and marketed whole. Only a very small quantity is used to make yam flour (from dry yam chips), or yam flakes.
Field collections of Guinea yam occur in various parts of tropical Africa and elsewhere in the world. Significant collections are maintained in Côte d'Ivoire (University), IITA (International Institute of Tropical Agriculture) in Ibadan, Nigeria, the National Root Crops Research Institute at Umudike, Nigeria, at various agricultural research stations in the western part of Cameroon, and at Mayagüez in Puerto Rico. A great deal of genetic diversity exists and is maintained on farmer's fields. Significant in vitro collections exist at the IITA in Nigeria, as well as at IRD (Institut de Recherche pour le Développement) in Montpellier, France. Much of this has been characterized and indexed for the yam mosaic virus.
The main breeding objectives include: 1) yield improvement; 2) production of more rounded (as opposed to cylindrical) tubers that are easier to harvest and handle; 3) improved flavour, texture and protein content of the tuber; 4) production of disease and pest tolerant cultivars; and 5) the improvement of plant architecture so as to eliminate the need for staking. Most of the crop improvement effort to date has concentrated on selecting from the very broad genetic diversity that already exists in farmer's fields. This activity is carried out at the centres where collections are kept. Some DNA fingerprinting work has been carried out at IITA.
The main constraints to production are very high labour requirements, low yields, storage problems and the large quantity of planting material needed. These factors give rise to poor production economics and a high unit cost of tubers reaching the table. Genetic engineering may help by introducing desirable characteristics into the crop. Declining soil fertility and poor production economics will progressively give the competitive edge to more resilient crops, especially cassava. However, the socio-cultural significance of Guinea yam in tropical Africa will ensure its production and value.
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Sources of illustration
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Correct citation of this article:
Onwueme, I.C. & Hamon, P., 2002. Dioscorea cayenensis Lam.. Record from Protabase. Oyen, L.P.A. & Lemmens, R.H.M.J. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, the Netherlands.
Dioscorea cayenensis – planted
1, tuber; 2, part of stem with male inflorescences
Redrawn and adapted by W. Wessel-Brand
tuber, probably Dioscorea cayenensis
traditional storage in West Africa