Dioscorea. Yams should not be confused with the Yam Bean Pachyrrhizus erosus, a
leguminous climbing plant grown mainly in India and South-East Asia; the tubers
are either thinly sliced and eaten raw in salads or cooked or pickled; the young pods
are edible, but the seed is poisonous.
The global production of yams was estimated by FAO at 40 million MT in 2004,
about 75% from West Africa, especially Nigeria. Yams are also widely grown in
southeast Asia, the Pacific and the Caribbean, although production of yams is
declining overall as they are becoming replaced by cassava and sweet potatoes, both
of which are often able to produce more food with less labour input. Since they are a
climbing crop they need vertical support, but then don’t we all. Ideally yams should
be grown up stakes.
Nutritionally the yams are more useful than cassava, particularly in protein. They
contain about 28% starch and about 5 mg/100 g Vitamin C; carotene is present in
varieties with yellow flesh. Cases of kwashiorkor have been seen to increase when
yam is replaced with cassava in the diet. They were used on long sailing journeys,
including slave ships between West Africa and the New World; they contain enough
Vitamin C to help reduce scurvy.
There are about 600 species of Dioscorea, of which three are particularly
interesting:
Dioscorea alata—Greater Yam, Water Yam, Winged Yam, Ten-month Yam,
Asiatic Yam or White Yam. A native of southeast Asia, it is now grown throughout
the tropics. The highest yielding species of them all. Requires high rainfall, about
1600 mm a year is ideal. In West Africa the White Guinea Yam (D. rotundata) is
more popular as it makes better fufu. Stores well, for five or six months at least.
Stems twine to the right.
Dioscorea rotundata—White Guinea Yam, Eight-month Yam. A native of Africa,
where it is the most important yam, mainly grown in West Africa. Not grown in
Asia. Very similar to the Yellow Guinea Yam (D. cayenensis), though it has better
drought resistance and the tubers have a longer dormancy period, and so can be
stored longer. Stem up to 10 m long, twines to the right.
Dioscorea cayenensis—Yellow Guinea Yam, Twelve-month Yam, Cut-and-come-
again. A native of West Africa where it grows well in the forest zone with a short
dry season. Not grown in Asia or the Pacific. Can be harvested throughout the year.
Stores poorly. Stems twine to the right.
There are at least eight other important sub-species, listed below:-
Dioscorea esculenta—Lesser Yam, Chinese Yam, Pana Yam. Mainly grown in Asia
and the Pacific. Produces up to 20 small tubers per plant. Does not store well. Stems
twist to the right.
Dioscorea trifida—Cush-cush Yam, Aja, Yampi, Indian Yam, Mapuey. The only
food yam native to South America. Now grown in the Caribbean and South America
where it is highly prized for its flavour, though it yields less than the introduced
Dioscorea alata. Produces a group of small tubers 15-20 cm long. Stems twine to
the left.
Dioscorea dumetorum—American Bitter Yam, Cluster Yam, Forest Yam. Occurs
wild throughout Africa between 15N and S; mainly cultivated in West Africa.
Many of the wild forms contain a poison which must be removed by soaking and
boiling, though they are often used as famine food. Tubers are either single or in
clusters. Stems twine to the left.
elle
(Elle)
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