Yam

(Dioscorea species)

Yams are starchy staples in the form of large tubers produced by annual and perennial vines grown in Africa, the Americas, the Caribbean, South Pacific and Asia. There are hundreds of wild and domesticated Dioscorea species. White Guinea yam, D. rotundata, is the most important species especially in the dominant yam production zone in West and Central Africa. It is indigenous to West Africa, as is the Yellow yam, D. cayenensis. Water yam, D. alata, the second most cultivated species, originated from Asia and is the most widely distributed species in the world.

Importance
Yams are primary agricultural commodities and major staple crops in Africa, where yam 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 yam is generally very high in this sub-region and yam cultivation is very profitable despite high production costs.

Production
Worldwide yam production in 2007 amounted to 52 million tons, of which Africa produced 96%. Most of the world's production comes from West Africa representing 94%, with Nigeria alone producing 71%, equalling more than 37 million tons. African countries imported more than 2,000 tons in 2002, and exported 15,500, of which Nigeria exported 12%.

Yam production is declining in some traditional producing areas due to declining soil fertility, increasing pest pressures and the high cost of labor. Smallholders therefore need access to innovations to reduce labor and improve productivity.

Yams are grown by planting pieces of tuber, or small whole tubers (‘seed yams') saved from the previous season. Small-scale farmers, the majority of producers, often intercrop yams with cereals and vegetables.

The major pests that affect yams include insects such as leaf and tuber beetles, mealy bugs, and scales; parasitic nematodes; fungi causing anthracnose, leaf spot, leaf blight, and tuber rot; and viruses, especially the yam mosaic virus (YMV).

Harvesting
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.

Consumption
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.

IITA's research and impact
IITA’s objectives are to develop new varieties of yams with desired agronomic and quality traits (e.g., high and stable yields and reduced labor requirements) and improve yam-based cropping systems.

IITA scientists have made progress in finding and analyzing the genetics of yam resistance to anthracnose and viral diseases. The sources of resistance have been used to develop improved varieties, which have been delivered to research partners.

We are increasing our efforts in developing and applying molecular markers for genetic improvement. We use state-of-the art techniques in tissue culture to eliminate pathogens, principally viruses, from samples planned for international distribution. Ongoing research includes improving protocols for rapid field propagation using vine cuttings.

In food science and technology we conduct research to improve the understanding of the relationships between physical and chemical properties of yams and the range and quality of processed yam products. Our current work also includes screening new variety samples and yam food products for their retention of micronutrients such as carotenoids, iron, and zinc, ascorbic acid, tannins, and phytic acid.

Our social scientists assess the financial viability and adoption potential of the technologies being developed. They are also working towards a better understanding of markets and demand for yams and yam products.