A Breakthrough in Yam Breeding

In sub-Saharan Africa, yam is an important foodcrop, rendering the highest average foodcrop yields, about 10 tons per hectare. Yam production is growing at a two-digit rate (close to 13 percent, 1980-1992), due to a combination of rapid area expansion and yield increases. Yam yields peaked during the drought year of 1992.

Yam barn in Nigeria

War, civil strife and natural disasters such as fire, flood or drought take their toll not only on human and material wealth of a nation but also on natural resources, especially in agriculture. Before the prolonged civil war in Sudan and drought in Ethiopia, farmers in those countries were planting white yams Dioscorea rotundata. But now yam has almost completely disappeared from their countries because the planting portion of the crop happens also to be the edible portion. A farmer is expected to set aside at least one quarter of his or her annual harvest for replanting. But during emergencies they eat up everything.

It is almost impossible to re-create germplasm because yam poorly responds to conventional plant breeding techniques. Unlike in some other crops where the male and female flowers are on the same plant, yam flowers are borne on separate plants. Flowering of male and female plants is difficult to synchronize. Flower thrips, the insects that pollinate most yam flowers, usually do a poor job resulting in the abortion of yam female flowers. These factors restrict yam hybridization using conventional breeding techniques.

In essence, whenever farmers for one reason or another consume all yams they have, the crop's genetic base is eroded. This is what happened in Sudan, Ethiopia and other crisis-ridden African countries where yam culture flourished years ago.

Variability erosion in yam also easily occurs in countries that do not experience disasters but where there is no proper documentation of the germplasm cultivated by the farmers. There is always the tendency to assume that there is no need to focus research on yams (and other rootcrops) because they are well adapted to particular situations that other crops cannot withstand.

All hope is not lost, however, to get yam culture to flourish again. Scientists at the International Institute of Tropical Agriculture (IITA) in Ibadan have achieved a breakthrough in conventional yam breeding. The problem of yam breeding which seemed impossible to solve years ago has now been successfully tackled. The Institute accumulated a large germplasm collection of new yam genotypes with better flowering qualities which can be manipulated by crop breeders to generate new varieties.

"After more than two decades of research work we are now in a position to combine different genes from different genotypes and come up with completely new materials with the required acceptable attributes," says Dr. Robert Asiedu, head of the Root and Tuber Improvement Program at IITA. This is a considerable achievement for IITA which is located in the geographical center of the genetic diversity of yam and has a global mandate for the improvement of the crop.

Of the 600 known species of yams, only six are food yams. Out of these, the most, popular white yams, Dioscorea rotundata, originated in West Africa which accounts for 90 percent of world production of about 25 million tonnes. Nigeria alone produces 70 percent or 17 million tonnes of the world output.

The yam zone of West Africa is restricted peripherally to the forest and savanna areas of Nigeria, Ghana, Côte d'Ivoire, Cameroon, Benin and Togo.

In Nigeria, as well as in Ghana, the lives of communities in yam growing areas revolve around the yam cycle. In Nigeria, chiefs and traditional title holders do not touch yam until the gods have been appeased. To get married in such communities, the bridegroom's wealth is measured by the amount of yams he can produce. Till today, the tradition is that the groom must present not less than 200 big tubers of yams to the in-laws as evidence that he can provide for the wife and the future family.

The yams harvest is a traditional festivity with masquerade dancing in the villages and prayers to thank the ancestral gods for the blessings of the land and the women's fertility. In some parts of the Ibo land in Nigeria, yam is a male totem. Women are not allowed to walk in yam farms until the yam is ready for harvesting, which is the exclusive duty of the women. To the people yam is the king of all foodcrops. In Ghana, yam is prepared to welcome important visitors while in Nigeria pounded yam is a national menu.

With the development of the new genotypes of yams through seed hybridization by IITA scientists, new materials are being made available both in seed and tissue culture form to be sent to a number of African countries. Requests have been received from Rwanda, Guinea Conakry, Gambia, Guinea Bissau, Kenya, Equatorial Guinea, as well as from Cuba, Japan and Korea. Other countries that have shown interest include Sudan, Uganda, and Malawi. It is hoped that in the near future yam culture will expand to cover the rest of the African continent, particularly East and Central Africa.

(IITA feature)