Farmer-named yam (Dioscorea spp.) diversity, cultivar preferences and access to improved planting material in Oyo State, Nigeria

Abstract Nigeria produces the majority of the world’s yam, yet national tuber yields remain far below the crop’s biological potential, and on-farm uptake of the improved varieties released since 2003 has stayed low. We characterised the yam genotypes actually cultivated across five yam-growing centres of Oyo State, south-western Nigeria, and related that inventory to farmers’ reasons for their choice of planting material. Twenty-eight farmers in nine cooperative-based groups were interviewed with a structured instrument supported by focus group discussions, and on-farm accessions were assigned to species using a standard yam descriptor list. Thirty-five genotypes were recorded: 24 Dioscorea rotundata Poir., 9 D. alata L. and 2 D. cayenensis Lam. The descriptors separated the three species cleanly but resolved almost no variation within D. rotundata , where 24 accessions with distinct farmer names shared an identical state across eleven of twelve characters. Genotype richness ranged from 5 to 15 per farmer group and was highest at Ibadan; the first two principal components of the location × genotype matrix accounted for 70.9% of the variation. No farmer grew improved genotypes alone: 85.7% cultivated landraces exclusively. Only one improved genotype, VA Yam, was encountered, at low abundance, despite recording the highest mean tuber weight of any genotype weighed (3.9 kg). Farmers identified unavailability of seed yam, rather than unwillingness, as the operative barrier, while an introduced landrace of foreign origin had spread widely through farmer-to-farmer exchange alone. In this system the binding constraint on adoption is seed access rather than farmer conservatism.