Yam (Dioscorea species)
White Guineaya m, D. rotundata, is the most important species especially in the dominant y am production zone in West and Central Africa. It is indigenous to West Africa, as is the Yellow ya m, D. cayenensis.
Water ya m, D. alata, the second most cultivated species, originated from Asia and is the most widely distributed species in the world.
World Producers Of Yam
Nigeria is by far the world’s largest producer of ya ms, accounting for over 70–76 percent of the world production. According to the Food and Agricultural Organization report, in 1985, Nigeria produced 18.3 million tonnes of ya m from 1.5 million hectares, representing 73.8 percent of total ya m production in Africa.
According to 2008 figures, yam production in Nigeria has nearly doubled since 1985, with Nigeria producing 35.017 million metric tonnes with value equivalent of US$5.654 billion. In perspective, the world’s second and third largest producers of y ams, Côte d’Ivoire and Ghana, only produced 6.9 and 4.8 tonnes of ya ms in 2008 respectively.
According to the International Institute of Tropical Agriculture, Nigeria accounted for about 70 percent of the world production amounting to 17 million tonnes from land area 2,837,000 hectares under ya m cultivation.
Ya m, a tropical crop in the genus Dioscorea, has as many as 600 species out of which six are economically important staple species. These are: Dioscorea rotundata (white guinea ya m), Dioscorea alata (yellow ya m), Dioscorea bulbifera (aerial ya m), Dioscorea esculant (Chinese y am) and Dioscorea dumetorum (trifoliate ya m).
Out of these, Dioscorea rotundata (white ya m) and Dioscorea alata (water ya m) are the most common species in Nigeria. Ya ms are grown in the coastal region in rain forests, wood savanna and southern savanna habitats.
Y am is in the class of roots and tubers that is a staple of the Nigerian and West African diet, which provides some 200 calories of energy per capita daily. In Nigeria, in many yam-producing areas, it is said that “ya m is food and food is yam.
” However, the production of yam in Nigeria is substantially short and cannot meet the growing demand at its present level of use. It also has an important social status in gatherings and religious functions, which is assessed by the size of ya m holdings one possesses.
Importance Of Yam
Ya ms are primary agricultural commodities and major staple crops in Africa, where cultivation began 11,000 years ago. In West Africa they are major sources of income and have high cultural value. They are used in fertility and marriage ceremonies, and a festival is held annually to celebrate its harvest.
Consumer demand for ya m is generally very high in this sub-region and ya m cultivation is very profitable despite high production costs.
Y am is grown on free draining, sandy and fertile soil, after clearing the first fallow. Land is prepared in the form of mound or ridge or heap of 1 metre (3 ft 3 in) height.
The ya ms recommended for such soil conditions in Nigeria are white y am or white guinea ya m (Discorea rotundata) and water ya m or yellow y am (Discorea alata).
Planting is done by seed ya m or cut setts from ware tubers. One day before planting, the tubers have to be subjected to treatment with wood ash or a fungicide (thiabendazole) to prevent damage to the soils.
The setts are planted at an interval of 15–20 centimetres (5.9–7.9 in) with the cut face facing up. Mulching is essential during October–November with dry grass or plant debris weighed down with balls of mud.
Dosage of fertilizer application, as essential, is decided after chemical analysis of the soil samples. Manual weeding by hoeing is done three or four times depending on the rate of weed growth.
Two Stakes, each of 2 metres (6 ft 7 in) height are used for staking the plants to vine over it; one for two plants with the other used for bracing with the adjacent stakes. Sorghum stovers are also used for this purpose in the savannah land.
Pest and disease control is addressed by cultural control and chemical methods; the pests which affect the plant are nematodes such as root knot Meloidogyne spp. and ya m nematode (Scutellonema bradys), and insects such as y am shoot beetle, y am tuber beetle and crickets.
Weeding of the field is essential and maintaining a 2–3 metres (6 ft 7 in–9 ft 10 in) weed free border around the field is to be ensured. Disease resistant [cultivars] are normally recommended for use. Harvesting is done before the vines become dry and soil becomes dry and hard.
Generally, a yield of 10–15 tonnes per ha for white y am and 16–25 tonnes for water y am are obtained by following prescribed management practices. The harvested ya ms are stored by tying them with ropes.
They have a shelf life of about 5 months. Warehouses where they are stored should be made rodent proof with a metal base and wire netting. Rotten buds and sprouted buds should be removed.
In terms of area harvested, 4.6 million hectares were planted worldwide in 2007, with about 4.3 million in Central and West Africa.
In West and Central Africa tubers are planted between February and April, depending on whether in humid forest or on the savanna, and are harvested 180 to 270 days later.
Care is needed during harvesting to minimize damage to tubers that lead to rot and a decrease in market value. Harvested tubers normally stay dormant (do not develop sprouts) for 30 to 120 days depending on environmental conditions, the date of harvest, and the species. This means that only one crop cycle is possible per year, possibly restricting supply.
Yam tubers consist of about 21% dietary fiber and are rich in carbohydrates, vitamin C and essential minerals. Worldwide annual consumption of yams is 18 million tons, with 15 million in West Africa. Annual consumption in West Africa is 61 kilograms per capita. Yams are boiled, roasted, baked or fried. In Africa they are also mashed into a sticky paste or dough after boiling the yam.