YELLOW YAM, Guinea yam, Yellow Guinea yam.
Botanical name
Dioscorea cayenensis Lam. (Enantiophyllum).
Origin and distribution
West Africa appears to be the centre of origin, with initial domestication from the Ivory Coast to Cameroon, and in this African 'yam belt' these are the most important of all the yams. From West Africa they spread to Brazil and the Caribbean (presumably by way of the slave ships), and are important in Jamaica, Puerto Rico and the French West Indies, but not in the other islands. They were introduced into New Caledonia by the French.

Cultivation conditions
A warm tropical climate is required, but while D. cayenensis needs a long rainy season (about 10 months), D. rotundata cultivars can be grown with only 6-7 months of rainfall (100-150 cm evenly distributed) and thus can be grown further away from the equator where dry seasons are longer; also, because of its greater tolerance of drought, this type is adapted to the Caribbean region, though D. cayenensis is also grown there. D. cayenensis is relatively tolerant of sandy soils: D. rotundata thrives best on heavy soils even with a high clay content. Responses to organic matter (FYM or heavy mulch) are good. Nitrogen appears to be especially important (in Ghana 67 kg nitrogen applied after tuber reserves were exhausted gave a 22 per cent increase in yield; phosphorus gave a small response but potassium none), but fertiliser requirements vary from place to place.

Planting procedure
Material-usually small whole tubers, crowns or mid-section cuttings of large tubers (in 100-150 g pieces), dried for a few days before planting and preferably treated with wood ash to protect the pieces from fungal infection. Propagation of D. rotundata by stem cuttings is possible but does not yet appear to be commercially developed. Planting material must be disease-free.

Method-in Africa, yams are normally planted in land freshly-prepared by 'slash and burn', in mounds which are usually large enough for one plant, but sometimes for several. The pieces are planted at a depth of 5-15 cm, sometimes with the stem end down. The mound may be mulched with dried grass. In the Caribbean, planting in ridges 30-50 cm high is the usual practice, the seed tubers being placed 10-15 cm deep, by hand or by machine. In some areas, where soil drainage is naturally good, planting on the flat is practiced. Staking appears to be essential.

Field spacing-mounds are 1-2 m apart, and ridges also 1-2 m apart. Maximum yields are obtained by spacings of I x I m (10 000 ha).