SELECTION OF COWPEA (Vigna unguiculata (L). Walp) FOR HIGH YIELD UNDER LOW SOIL PHOSPHORUS CONDITIONS
ABSTRACT
Nitrogen (N) and phosphorus (P) are among the main edaphic factors that limit crop production in Sub-Saharan Africa (SSA). As a nitrogen fixing legume, cowpea production is not significantly affected by N, but P deficiency in soils. Two hundred (200) cowpea genotypes composed of both landraces and improved varieties were screened in the field under low P soil conditions in order to (i) determine genetic variability and heritability of traits associated with low P tolerance, (ii) assess the genotype by environment interactions for yield and its components under low P conditions, (iii) identify superior cultivars for use as varieties and/or as parents in improvement programs and (iv) determine the effects of planting date on the cycle and yield of local cowpea varieties in Niger. A participatory rural appraisal (PRA) was conducted to obtain farmers‘ knowledge and perceptions on low soil fertility and preference for cowpea varieties. The PRA results revealed that 11, 49 and 40 % of respondents claimed that their farms are respectively very low, low and high in fertility. About 75 % of farmers indicated that their cowpea production was low. Drought and low soil fertility were identified as the major production constraints. In terms of varietal preferences, early maturity and high yield potential were the traits preferred by farmers. High diversity with regards to yield and yield related traits under low soil P conditions was detected in the evaluated germplasm. High estimates of heritability, genetic variance, phenotypic variance as well as GCV and PCV were obtained in rainy season in comparison to dry season. The mean number of days to flowering, days to 50 % maturity, plant height, fodder yield, harvest index and hundred seed weight were the most highly heritable traits with 92.51 %, 85.02 %, 70.56 %, 80.6 %, 80.72 % and 71.94 % broad-sense heritability estimates, respectively. Grain yield was positively correlated with shoot dry weight, plant height and shoot color ; it was however negatively correlated with days to flowering and days to 50 % maturity. Genotype x Environment interaction was only significant for days to flowering, days to 50 % maturity and fodder yield. The contribution of genotypes to the variation was high regarding fodder yield and days to flowering, but medium with regard to grain yield. Genotypes with high grain and fodder yield under low P soil conditions were detected; nevertheless, only G150 was stable across the test environments. The largest contribution of genotypes to traits variation was observed in fodder yield (46.76 %) while the smallest was in shoot dry weight (1.5 %). Genotypes were also responsible for a significant variation with regard to days to flowering (21.57%) as well as grain yield (15.06 %). The environment was the main cause of the variability detected in shoot dry weight, plant height, days to 50 % maturity and grain yield with 82.45 %. 72.04 %. 58.69 % and 36.55 %, respectively. The GEI part of the variation was high in days to flowering (36.85 %). fodder yield (22.73 %) and days to 50 % maturity (18.66 %). The G x E interaction although not significant for grain yield, contributed considerably (15.85 %) to the yield variation. Planting date had a significant effect on yield and yield related traits of local cowpea cultivars in Niger. Significant reduction in days to flowering and days to 50 % maturity with subsequent decrease in grain and fodder yield was observed when varieties were planted late. The first planting dates recorded the highest mean grain yield in both 2016 and 2017 while the lowest belonged to last planting dates. G56 and G34 were the best and worst performing genotypes with respectively 40.47 and 1.78 g grain yield per plant across the test environments. Superior genotypes in terms of fodder yield were G35, G78 and G74 with 178.50, 173.63 and 167.37 g per plant, respectively. Cultivars tolerant to both low P soil conditions and striga were also identified.The population developed for recurrent selection and QTL detection is at F2 generation. Candidate Genotypes for release as variety were identified.