NITROGEN-FIXATION AND YIELD OF COWPEA (VIGNA-UNGUICULATA) AS INFLUENCED BY CULTIVAR AND BRADYRHIZOBIUM STRAIN

The nitrogen (N2) fixed and yield of three field-grown cowpea varieties, Ife Brown, Ife BPC and AFB 1757, uninoculated or inoculated with one of two cowpea Bradyrhizobium strains, were examined at physiological maturity. All plants were well nodulated, and the amounts of N2 fixed were not appreciably influenced by inoculation; the average values were 116, 87 and 74 kg N ha-1 for Ife BPC, Ife Brown and AFB 1757, respectively. While there were no varietal differences in the dry-matter yields of pods (with enclosed seeds), differences occurred in the total dry-matter yields, due to the large variations in vegetative matter, with Ife BPC generally giving the highest yield. Except for Ife BPC for which inoculation resulted in increased vegetative-matter yield, inoculation had no effect on dry-matter yields. Because of the higher vegetative yield of Ife BPC, the net soil N balance after removal of pods and seeds averaged + 30.3 kg N/ha, compared to -10.8 and -8.5 kg N/ha for Ife Brown and AFB 1757, respectively. Thus, although harvest index of Ife BPC was no higher than that of the other cultivars, nor was harvest index enhanced by inoculation, the economic returns, assessed in terms of pod yields plus the soil-N-restoring ability, was highest for Ife BPC, while inoculated Ife BPC was of greater economic value than when uninoculated. © 1990.