Yam 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 “yam 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 yam holdings one possesses. There are over 600 varieties of yams and 95% of these crops are grown in Africa.
In West Africa yams are one of the most highly regarded food products and are closely integrated into the social, cultural, economic and religious aspects of life. The ritual, ceremony and superstition often surrounding yam cultivation and utilization is a strong indication of its extensive usage.
Yam is consumed as boiled yam or fufu or fried in oil and then consumed. It is often pounded into a thick paste (pounded yam) after boiling and is consumed with soup. It can also be processed into flour for use in the preparation of “pounded yam”.
Except for potassium, vitamin B6 and vitamin C, yam is a food with low nutrient density. Yam provides around 110 calories per 100 grams. Yam is high in vitamins C and B6, potassium, manganese and dietary fibre while being low in saturated fat and sodium. A product that is high in potassium and low in sodium is likely to produce a good potassium-sodium balance in the human body, and so protects against osteoporosis and heart disease.
The most cultivated yam species in Nigeria are the White yam (Dioscorea rotundata), yellow or guinea yam, (Dioscorea cayenesis) and water yam (Dioscorea alata). Other species include: Bitter yam (Dioscorea dumetorum), Aerial yam (Dioscorea bulbifera), Trifoliate yam (Dioscorea dumentorum)
Depending on the species, yam grows for six to ten months and is dormant for two to four months, these two phases usually corresponding to the wet season and the dry season. In West and Central Africa, tubers are planted between February and April, depending on whether in humid forest or on the savanna. For maximum yield, the yam requires an annual rainfall of over 1,500 mm distributed uniformly throughout the growing season. White, yellow and water yams normally produce annually a single large tuber, often weighing 5-10 kg.
Planting is done by seed yam or cut setts from ware tubers. The tubers are treated with wood ash or fungicide. Mulching is essential during October – November with dry grass or plant debris. Stakes are used for staking the plants to vine over it.
Pest and Disease
The disease-causing agents not only reduce the quantity of yam produced but also reduce the quality by making them unappealing to the consumers.
Anthracnose (a fungus disease) is regarded as the most widely spread of all the field diseases, causing dieback of the stem while yam mosaic virus disease is considered to cause the most severe losses in yams. Dry rot is considered as the most devastating of all the storage diseases of yam. Dry rot of yams alone causes a marked reduction in the quantity, marketable value and edible portions of tubers and those reductions are more severe in stored yams.
Yam Mosaic Virus Disease: This disease is caused by an aphid-transmitted potyvirus that infects several species of Dioscorea. The symptoms observed in each host can be vein banding, curling, mottling, green-spotting, flecking etc.
Pest which affects the plant are nematodes such as root-knot (Meloidogyne spp.) and yam nematode (Scutellonema bradys), and insects such as yam shoot beetle, yam tuber beetle and crickets. These can be treated with an insecticide.
Harvesting is done by hand using sticks, spades or diggers. Sticks and spades made of wood are preferred to metallic tools as they are less likely to damage the fragile tubers; however, tools need regular replacement. Harvesting is done before the vine come dry and soil become dry and hard. A yield of 10 – 15 tons per ha for water yam and 16 – 25 tons for water yam could are obtained under good management.
Yam harvesting is a labour-intensive operation that involves standing, bending, squatting, and sometimes sitting on the ground depending on the size of the mound, size of tuber or depth of tuber penetration. In rainforest areas, tubers growing into areas where there are roots of trees can pose a problem during harvesting and often receive considerable physical damage. Care is needed during harvesting to minimize damage to tubers that lead to rot and a decrease in market value.
Roots and tubers such as yam are living organisms. When stored, they continue to respire. To reduce post-harvest losses during storage, reduced temperature in storage facilities should be maintained. The best temperature to store yams is between 14-16°C (57.2-60.8°F), with high technology controlled humidity and climatic conditions, after a process of curing. Most countries that grow yam as food staple are too poor to afford high technology storage systems.
There are several traditional storage structures used for yam storage including leaving the tubers in the ground until required, the yam barn, and Underground structures (Opara, 1999). Leaving the tubers in the ground until required is the simplest storage technique practised by rural small-scale farmers. When carried out on-farm, this type of storage prevents the use of the farmland for further cropping. Harvested yams can also be put in ashes and covered with soil, with or without grass mulch until required.
The yam barn is the principal traditional yam storage structures in the major producing areas. Barns are usually located in a shaded area and constructed so as to facilitate adequate ventilation while protecting tubers from flooding and insect attack. Barns consist of a vertical wooden framework to which the tubers are individually attached.