Genetic and environmental variation for seed yield, protein, lipid and amino acid composition in cowpea (Vigna unguiculata (L) Walp)

The genetic and environmental variability for seed yield (kg ha(-1)), protein content (g kg(-1) DM), lipid content (mg g(-1) DM) and essential amino acid composition (g per 16 gN) of cowpea seeds using 15 cultivars grown in three locations (Kano (12 degrees 00' N 8 degrees 31' E), Mokwa (9 degrees 17' N 5 degrees 04' E) and Ago-Iwoye (6 degrees 58' N 4 degrees 00' E)) that differ markedly in climate was studied. The environmental effect accounted largely for the variability observed in yield (93%), protein (71%), lipid (100%), threonine (96%), cystine (80%) and arginine (81%) while the genotypic effect accounted largely for the variability observed in methionine (56%) and lysine (51%) contents. Correlation coefficients (pooled data) from the three locations indicated that yield was negatively correlated to protein content (r = - 0.87, P < 0.001) and positively correlated to lipid content (r = 0.73, P < 0.001). Protein content was negatively correlated to lipid content (r = - 0.67, P < 0.001) and positively correlated to threonine (r = 0.66, P < 0.01), cystine (r = 0.69, P < 0.001) and methionine (r = 0.88, P < 0.001) contents. Similarly, yield was positively correlated to threonine (r = 0.22, 0.01 < P < 0.05), cystine (r = 0.38, 0.001 < P < 0.01) and methionine (r = 0.67, 0.001 < P < 0.01). Yield showed a strong negative correlation with protein content (r = - 0.81) and a strong positive correlation with lipid content (r = 0.97) at the environmental level. Cystine, methionine and to some extent, threonine were the limiting amino acids in the three environments. This research established the degree of variability for these characters in cowpea and indicated that concurrent selection for yield, lipid content, threonine and sulphur amino acids compositions is feasible.