Abstract
Ethiopia is claimed to be a center of diversity for cowpea production. The crop is the most drought tolerant and could help the country overcome the recurrent drought problem; however, the yield is very low due to lack of effort to develop varieties. This research was conducted to evaluate the stability of cowpea genotypes and to estimate the magnitude of genotypes by environment interaction (GEI) effect on grain yield. Sixteen cowpea genotypes were tested at seven environments in an experiment laid out in a 4 × 4 triple lattice design during 2016/17 cropping season. The combined analysis of variance over environments showed significant differences among genotypes and environments, along with significant effect of GEI on grain yield, days to flowering, days to maturity, plant height and pods per plants. Analysis of variance for grain yield from AMMI model indicated the contribution of genotype and environment, with GEI accounting for about 63.3, 5.3 and 29.7% of the total sum of squares, respectively. The result indicated that environments contributed much to the observed variations suggesting the need to test cowpea genotypes in diverse environments. Considering all stability parmeters, viz; deviation from regression (S2di), coefficient of regression (bi) from ER’s model, IPCA1, IPCA2 and AMMI stability value (ASV) from AMMI model, GGE biplot and variety TVU was identified as the most stable with mean yield above the mean grain yield of genotypes. Two genotypes: IT-99K-1060a (1398.8 kg/ha) and 86D-378 (1377.1 kg/ha) had first and second highest yield, identified as responsive to both environments but more to favorable environments suggesting the need to further test and develop as varieties. The other two genotypes: 95K-1095-4A and 93K-619-1, identified as unstable and highly responsive to environments suggested to consider the genotypes as candidate varieties where they performed best. Melkassa, Sekota and Jinka were identified as more descrimnating environments, whereas Arbaminch and Kobo were ideal for selecting superior genotypes; however, Babile and Meisso were non descrimnating environments. Key words: Additive main effects and multiplicative interaction (AMMI) stability value, Eberhart and Russell, deviation from regression and triple lattice.  
Highlights
Cowpea [Vigna unguiculata (L.) Walp] is an annual herbaceous legume that belongs to Fabaceae family
The present study was initiated to estimate the magnitude of genotype, environment and genotype by environment interaction for grain yield of cowpea and characterize yield stability of cowpea genotypes across different environments
analysis of variance (ANOVA) is important in revealing the presence of genotypes by environment interaction (GEI), but it does not indicate genotypes contribution to the interaction and which genotype was stable across environments
Summary
Cowpea [Vigna unguiculata (L.) Walp] is an annual herbaceous legume that belongs to Fabaceae family. The country has high potential for the production of the crop as more than 66.5% of the arable land is very suitable for cowpea production (Collaborative Crop Research Program (CCRP), 2015) It plays a critical role in the lives of millions of people in the developing world, providing them a major source of dietary protein that nutritionally complements low protein staple cereal and tuber crops. Ethiopia is known as a victim with recurrent droughts that causes partial or total crop failure, and subsequently, famine in the country In such situations, cowpea can be a potential crop to reduce the consequences of drought because of its drought tolerant nature more than other staple crops. Information on the effect of genotype, environment, and their interaction on cowpea grain yield under diversified agro-climatic conditions of Ethiopia is limited. The present study was initiated to estimate the magnitude of genotype, environment and genotype by environment interaction for grain yield of cowpea and characterize yield stability of cowpea genotypes across different environments
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