Effect of Variety and Environment on Grain Yield of Cowpea in the Guinea Savannah Agroecology of Nigeria

The study was conducted at three locations; Makurdi, Abuja and Zaria in 2016, 2017 and 2018 cropping seasons within the Guinea Savanna agro-ecological zone of Nigeria. Six improved cowpea varieties (IT99K-573-1-1, IT99K-573-2-1, IT89KD-288, UAM09-1055-6, UAM09-1046-6-1, and UAM09- 1051-1.) were planted across the three locations. The experiment was laid in split plot design with planting dates as main plot and varieties as subplots, with three replications per treatment. Grain yield was analyzed with the Genotype plus Genotype × Environment model (GGE) to generate biplots for genotype compatibility to cultivation environment and ranking of genotypes based on mean grain yield and stability. The results showed that, in the Genotype plus Genotype × Environment (GGE) biplot view, environment E4 (Makurdi, 2016) possessed the most discriminating power for grain yield, whereby environments, E9 (Zaria, 2018), E3 (Abuja, 2018) and E5 (Makurdi, 2017) were the most representative in the mega environments. Environment E6 (Makurdi, 2018) and E8 (Zaria, 2017) are considered the least environment for selecting adapted varieties as they were located at the vertices of the polygon in ‘which won- where’ GGE biplot, indicating poor stability and specific adaptation for these varieties. G4 (UAM09-1046-6), G5 (UAM09-1051-1) and G2 (IT99K-573-2-1) were located close to the origin, indicating that these varieties were least responsive to differential environmental conditions. These varieties: G4 (UAM09-1046-6), G5 (UAM09-1051-1) and G2 (IT99K-573-2-1) possessed suitable grain yield across the test environments. In the contrary, G3 (IT89KD-288) and G6 (UAM09-1055-6) are considered the most variable among the evaluated varieties. Significant variety-by-environment interaction effect was detected for grain yield among the test varieties. Cowpea varieties G4 (UAM09-1046-6), G5 (UAM09-1051-1) and G2 (IT99K-573-2-1) were identified as ideal for grain yield, in that order. The selected cowpea varieties are recommended for production in Guinea savannah or in similar agro-ecologies, and for incorporation in future production and breeding programs targeting genetic improvement for grain yield.