Cowpea yield variation in three dominant soil types in the Sudan Savanna of West Africa

Abstract

Cowpea (Vigna unguiculata) is a legume crop widely cultivated across the Sudan Savanna. Despite its importance for farmers’ food security in this region, the grain yields are highly variable among years and locations. To identify environmental and plant factors underlying this yield variation and select cowpeas with a stable yield, the grain yield variation of 16 cowpea genotypes in three dominant soils in the Sudan Savanna, namely, Ferric Lixisols (LXfr), Petric Plinthosols (PTpt), and Pisoplinthic Petric Plinthosols (PTpt.px), were analyzed in two consecutive years with different precipitation. In this study, the three soils were located near each other at the experimental site and thus the meteorological conditions were assumed to be identical for the soil types. Results of analysis of variance showed that the factor of year, representing the effect of precipitation, was the largest cause of annual yield variation although there was a significant interaction between year and soil type. Based on this result, environment was defined as each combination of year and soil type in a subsequent additive main effect and multiplicative interaction (AMMI) model analysis to detect the effects of the environment, genotype, and genotype-environmental interaction (GEI) on grain yield variation. Then, additive Bayesian network (ABN) analysis was performed to investigate the relationships between environmental and plant factors. The AMMI model revealed that the effect of GEI on grain yield was the complete opposite between the years even for the same soil type, indicating no single genotype achieved both stable and high yields across these soil types. The ABN analysis suggested that the primary cause of yield variation among the different soil types under same precipitation was the difference in soil total nitrogen and available phosphorus content rather than that in available soil water. The larger GEI in LXfr and PTpt was caused partly by their high water holding capacity, which led to excess water stress under high precipitation for LXfr and PTpt, and partly by deep soils, which led to drought stress under low precipitation for LXfr because of infiltration into the subsurface layer. However, the higher fertility of these two soils compensated for excess water/drought stress, and thus, the average yields were higher than that in PTpt.p × . PTpt.px showed the lowest mean yield and the smallest GEI because of poor soil fertility, although the soil with low water holding capacity was prone to drought stress even during short dry spells. The AMMI model uncovered two cowpea genotypes with stable yield across all the environments; however, the grain yields of these genotypes were not the highest in each environment. Selection of a genotype with a medium but stable yield would be favorable to improve long-term average yield in the Sudan Savanna, where multiple soils with a large GEI are distributed in mosaic patterns.