Studies on effects of mineral fertilizer, organic manure and cultivar on the yield and storability of Yam ( Dioscorea rotundata   Poir)

Yam is an important staple food crop in the humid and sub-humid tropics. Despite inadequacies in supply and availability of fertilizer to boost yam production in Nigeria, there have been reports and arguments that yams grown with chemical fertilizer are susceptible to pathological deterioration in storage while those grown with organic manure store better and have longer shelf life. Therefore, two experiments were conducted to investigate this controversy. The first field experiments tested the responses of five yam cultivars; Amula, Nwaopoko, Ezakwukpolo Danacha and Pepa to fertilizer and organic manure treatments. Statistical differences were not detected in all cases of manuring treatment although tuber yield appeared less where no fertilizer or organic manure was applied. While organic manure appeared to have a better effect on tuber yield with Danacha, NPK fertilizer had better yield effect with Ezakwukpolo, Amula and Pepa. The second experiment tested the effect of manuring treatments on the storage life of the five yam cultivars. Dormancy period was extended in Nwaopoko cultivar when organic manure was used than when no manuring or NPK fertilizer was applied. Tuber weight loss varied significantly among cultivars with different treatments. Rotting was significantly higher in Amula with the application of NPK fertilizer compared with Danacha, Nwaopoko, Ezakwukpolo and Pepa. The combined effects of cultivar and manuring treatments on rotting incidence varied with cultivar type. For example, Amula cultivar had the highest rotting incidence with or without manuring treatments, whereas Danacha and Ezakwukpolo had statistically similar rotting incidence with any of the manuring treatments. In conclusion, evidences in this study show that post harvest losses of yam are, in part due to field management but mostly dependent on species and cultivars. Nwaopoko and Danacha cultivars had better keeping qualities than all other cultivars tested with or without manuring treatment. It is recommended that plant breeders should use the traits in these two cultivars to improve others.