Yams (Dioscorea species) are annual or perennial tuber-bearing and climbing plants. The genus Dioscorea has over 600 species but only a few are cultivated for food or medicine. The major edible species of African origin are white Guinea yam (D. rotundata Poir.), yellow Guinea yam (D. cayenensis Lam.), and trifoliate or bitter yam (D. dumetorum Kunth). Edible species from Asia include water or greater yam (D. alata L.), and lesser yam (D. esculenta [Lour.] Burkill), Cush-cush yam (D. trifida L.) originated from the Americas. White Guinea yam and water yam are the most important food yams in terms of cultivation and utilization. Yam tubers may be eaten with sauce direct after boiling, roasting, or frying in oil. The tubers may also be mashed or pounded into dough after boiling, processed into flour, or cooked into pottage with added protein sauce and oils. In addition to their food and market values, yams play a major role in sociocultural life for a wide range of smallholder households especially in the dominant production zone of West Africa. Consumer demand for yam is generally very high in this subregion and yam cultivation is very profitable despite high production costs.
Yams are produced on 5 million hectares in about 47 countries in tropical and subtropical regions of the world. Yields are about 11 t/ha in the major producing countries of West Africa. According to FAO statistics, 48.7 million tonnes of yams were produced worldwide in 2005, and 97% of this was in sub-Saharan Africa. West and Central Africa account for about 94% of world production. Nigeria is the leading producer with 34 million tonnes followed by Côte d’Ivoire (5 million tonnes), Ghana (3.9 million), and Bénin (2.1 million tonnes). Ethiopia (174 000 t) and Sudan (137 000 t) are the major producers in East Africa. Colombia (333 000 t) leads the production in South America followed by Brazil (230 000 t), while Japan (204 000 t) is the leader in Asia. Yams are also important in the Caribbean (e.g., Haiti with 197 000 t in 2005), and the South Pacific Islands. Ghana exports the largest quantity of yams (about 12 000 t) annually. Average yam consumption per capita per day is highest in Bénin (364 kcal) followed by Côte d’Ivoire (342 kcal), Ghana (296 kcal), and Nigeria (258 kcal).
How yam is grown
An annual rainfall of about 1000 mm spread over five to six months and deep, fertile, friable, and well-drained soils are ideal for yam cultivation. Whole seed tubers or tuber portions are usually planted into mounds or ridges before or at the beginning of the rainy season. The sett sizes planted, sizes of mounds, interplant spacing and provision of stakes for the resultant plants depend on factors such as the yam species, agroecology, and tuber sizes desired at harvest. Small-scale farmers in West and Central Africa often intercrop yams with cereals and vegetables. Care is needed during harvesting to minimize damage to tubers that lead to rot and low market value.
Constraints to yam production
Yam cultivation is generally limited by high costs of planting material and of labor, decreasing soil fertility, inadequate yield potential of varieties, as well as increasing levels of field and storage pests and diseases associated with intensification of cultivation. The labor requirements in yam cultivation for mounding, staking (especially in the forest zone), weeding, and harvesting exceed those for other starchy staples such as cassava. These account for about 40% of yam production costs while 50% of the expenditure goes to planting materials. The seed yams are also perishable and bulky to transport. If farmers do not buy new seed yams, they must set aside up to 30% of their harvest for planting the next year. Increasing pressure from a range of insect pests (e.g., leaf and tuber beetles, mealybugs, scales), fungal (e.g., anthracnose, leaf spot, leaf blight, tuber rots), and viral diseases, as well as nematodes contribute to suboptimal yields and the deterioration of tuber quality in storage.
Our work on yams
Our effort at IITA is geared towards the genetic improvement of yam, primarily on white and yellow Guinea yams, and water yam. The principal objectives of this include high and stable yield of marketable tubers as well plant morphology for reduced labor use in yam-based production systems. The breeding program has access to 2216 accessions of Guinea yams and 816 of water yam in IITA’s genebank. Our efforts in interspecific hybridization among the three species are aimed at the transfer of complementary traits from the one to the other. Our scientists have made progress in finding and analyzing the genetics of host plant resistance to anthracnose and virus diseases. The sources of resistance have been used in the development of improved populations, which have been delivered to research partners. These have been included in national and subregional collaborative trials leading to recent varietal releases by NARS partners in Nigeria and Ghana. We are increasing our efforts in the development and application of molecular markers for genetic improvement. Currently expressed sequence tags (EST) are under development through collaboration with the Virginia State University, USA and SSR markers are being used to characterize the Dioscorea core collection at IITA.
Among other things, tissue culture facilitates delivery of improved germplasm to our research partners. We use meristem culture routinely for elimination of pathogens, principally viruses, from germplasm intended for international distribution. The plantlets produced are tested with diagnostics (a combination of ELISA, PCR, and IC-RT-PCR) for the key viruses known in West and Central Africa such as potyviruses (Yam mosaic virus and Dioscorea alata virus), a badnavirus (Dioscorea alata virus), and a Cucumovirus (Cucumber mosaic virus). Other techniques of tissue culture used routinely include in vitro micropropagation using single nodal cuttings and in vitro production of microtubers. Minitubers are produced annually in insect-proof screenhouses, following efficient postflask management of plantlets, for delivery to partners who prefer these to in vitro plantlets. Ongoing research includes improvement of protocols for the rescue of immature embryos in support of our work on interspecific hybridization. Current effort in rapid field propagation includes the use of vine cuttings.
In food science and technology we conduct research towards improved understanding of the relationships between physicochemical properties of the tuber and the range and quality of products derived from it. We also aim to improve on screening methods to facilitate assessment of tuber textural quality at early stages of the selection cycle. Our current work also includes screening germplasm for tuber contents, and retention in yam food products, of micronutrients (carotenoids, iron, and zinc), ascorbic acid, tannins, and phytic acid.
Yams export substantial proportions of nutrients from the soil with the harvested produce. Increasing tuber yields through breeding and selection will thus lead to soil nutrient depletion if not accompanied by soil and nutrient management strategies. We use an integrated approach to soil fertility management. Current activities include assessing the nutrient requirements of yams, establishing varietal differences in nutrient use and response efficiency, and evaluating the impact of including cover crops in yam-based cropping systems. We have also established variations in white Guinea and water yams for the extent of root colonization by arbuscular mycorrhizal fungi (AMF) in different agroecologies. Studies are in progress to establish the contribution of AMF to mineral nutrition of yams.
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. This includes studies on seed systems, yam consumption patterns, industrial demand, export, marketing channels, effects of trade and market liberalization, and general competitiveness of the yam subsector.
We work with a broad range of partners within subregional organizations (e.g., CORAF), national research institutes (NARI), Universities, and advanced research institutes (ARI). Our principal NARI partners in research on agronomy, economics, crop protection, varietal development, and postharvest technology of yams include: the Crops Research Institute, Ghana; Institut National des Recherches Agricole du Benin (INRAB), Benin; Savanna Agricultural Research Institute (SARI), Ghana; Institut Togolais de Recherche Agronomique (ITRA), Togo; Institute of Agricultural Research for Development (IRAD), Cameroon; National Root Crops Research Institute (NRCRI) Umudike, Nigeria; Centre National de Recherche Agronomique (CNRA), Côte d’Ivoire; and Kizimbani Research Station, Zanzibar. Specific studies on yam trade are conducted with Centre Ivoirien de Recherche Economique et Sociale (CIRES), Côte d’Ivoire and Nigerian Institute of Social and Economic Research (NISER), Nigeria.
Partnerships with African universities involve training of postgraduate research fellows and specific research activities. We have ongoing collaborative studies on: (1) marketing with the University of Development Studies, Ghana and the University of Dschang, Cameroon; (2) agronomy with University of Ibadan, Nigeria; (3) postharvest technology with Kwame Nkrumah University of Science and Technology, Ghana; University of Buea, Cameroon; University of Abomey-Calavi, Benin; Bowen University, Nigeria; (5) varietal development and crop protection with Universities of Cocody and Abobo-Adjame, Côte d’Ivoire.
Our partnership is strong in varietal development with the Centre Suisse de Recherches Scientifiques, Côte d’Ivoire. We have active collaboration with CIRAD, France and Tokyo University of Agriculture, Japan in agronomy research in West Africa, and with the Virginia State University, Petersburg, USA in the development of molecular markers for genetic improvement. For technology testing, we have links with large development programs on root and tuber crops in West and Central Africa e.g., the Root and Tuber Improvement and Marketing Program, Ghana; the Roots and Tubers Expansion Program, Nigeria; the Roots and Tubers Development Programme, Bénin; and the Roots and Tubers Market-Driven Development Project, Cameroon.