Dioscorea rotundata Prior (white yam) originated in Africa. It is grown in West Africa, including countries such as Ivory Coast and Nigeria. It is widely spread and is one of the major staple foods in
many tropical countries (Omonigbo and Ikenemomeh, 2000).
1. Dioscorea rotundata is the most widely grown and preferred yam species. Their vines are cylindrical and thorny and they twin anticlockwise (from right to left). The leaves are heart shaped, while the tubers are thin, fresh and vary from cream to white color. Dioscorea rotundata is grown and cultivated for its energy rich tuber.
many tropical countries (Omonigbo and Ikenemomeh, 2000).
1. Dioscorea rotundata is the most widely grown and preferred yam species. Their vines are cylindrical and thorny and they twin anticlockwise (from right to left). The leaves are heart shaped, while the tubers are thin, fresh and vary from cream to white color. Dioscorea rotundata is grown and cultivated for its energy rich tuber.
2. It gives more calories per unit of land area than most crops and matures within 7(seven) months. On soils of average fertility, between 20 and 30 tonnes per hectare of tubers can be obtained, and up to 55 tonnes per hectare on fertile soils, it also stores very well. It is adaptable to fairly fertile soils and is suitable for intercropping with grain legumes such as cowpeas, soybeans and a variety of leafy vegetables.
3. A well drained, rich, loamy soil is most favorable for growth of Dioscorea rotundata with a warm, humid climate.
4. A large number of white yam cultivars exist with differences in their production and post-harvest characteristics. However, they have quite demanding labor and maintenance requirements, such as hilling the soil around each plant to form mounds, to ensure a pulverized soil favorable for tuber development. Storage of tubers occurs after harvest in barns or heaps covered with grass.
5. In Nigeria Dioscorea rotundata are processed into various food forms. This includes pounded yam, boiled yam, roasted or grilled yam, fried yam slices and yam balls, mashed yam, yam chips and flakes. Another method of consumption is to leave the raw yam pieces to dry in the sun.
6. These are then milled to create a brown powder known in Nigeria as elubo. The powder can be mixed with boiling water to create a thick starchy paste, which is the favorite for the Yoruba where the reconstituted food is known as “amala”.
7. Yam is an excellent source of carbohydrate, energy and some minerals (Ward low et al, 2002). Yams are also sources of steroids and alkaloids-chemicals that are extremely active physiologically in vertebrate animals. The most important yam steroid is diosegenin that is used in the production of birth control pills.
8. Alkaloids from yams have been used to fill fish and to poison darts arrows for hunting. Some yams are poisonous to humans because of their high alkaloid content, and their tubers must be boiled before eating to remove the toxins (Degras et al, 1993). It is also used as raw material for industrial products such as starch and alcohol (Bradshaw 2010).
9. The major problems facing yam production include high labor requirement, low yield per hectare compared to crops such as cassava or sweet potato, relatively large amount of planting material required and long growing season.
10. By far the most critical of these problems is labor requirement, which exceeds that of other comparable crops. For these reasons and problem of storage of harvested yams, the cost of yam production is high making the crop to slowly lose ground to cassava.
11. It has been estimated that the cost per 1,000 calories of yam is four (4) times greater than those of cassava (reference). But, despite these high cost, the nutritional value of yam is sufficiently high to justify further work into its general improvement.
12. The supply and availability of fertilizer to boost yam production in Nigeria have inadequate. In addition there were reports that yams grown with fertilizer application were more susceptible to pathological deterioration during storage than those grown without fertilizer (reference).
13. In survey of yam storage practices carried out in southeast Nigeria, it was reported that yam tubers grown with organic manure had longer shelf life than those treated with chemical fertilizer in the field.
14. It also reported that rotting incidence during storage appeared less where no fertilizer or organic manure was applied evidently due to high native fertility of the soil (Orkwor and Ugwu, 2006).
15. Moreover, there are indications that our soils are becoming marginal and many smallholder farmers may not have the fund to purchase the fertilizers for application in their farms.
Objective of the Study
The objective of this study was therefore to:Study the morphological characteristics of the Dioscorea rotundata varieties used in the study.
Determine the effect of soil fertility on growth characteristics of Dioscorea rotundata varieties grown in southeast agro-ecological zone.
Determine the effect of soil fertility on yield of Dioscorea rotundata varieties studied
Identify possible incidence of pests and diseases on the yam varieties in the field