Yield Stability and Adaptability of Black Soybean Line BSCM41 Support Bi-Directional Seed Rotation System in Northeast Thailand

Black soybean is a high-value alternative crop that can offset declining yellow soybean prices in Thailand. Currently, “Sukhothai3” is the only commercial variety produced through conventional breeding over the past three decades. Its low first-pod height (FPH) limits mechanized harvesting and reduces production efficiency. Recently developed elite RILs have shown improved FPH, grain yield, and protein content (34.88–41.22%), but their yield stability and adaptability across environments remain unevaluated. This information is essential for establishing an effective bi-directional seed rotation system while maintaining seed quality, quantity, and genetic purity. This study evaluated five black soybean RILs across eleven trial environments in the major production zones of Northeast Thailand, where the tropical savanna climate poses significant constraints on crop production. Field experiments used a randomized complete block design with four replications per location. Yield stability and adaptability were assessed using AMMI1 and GGE biplot analyses. BSCM41 emerged as the most stable and widely adaptable genotype, consistently outperforming the commercial check across environments and seasons. It exhibited adequate FPH for mechanized harvesting (>10 cm), rapid vegetative development, and short growth duration (88–91 days), reducing exposure to late-season heat stress and excessive rainfall. Higher seeds per pod, greater 100-seed weight, and a superior harvest index further support yield formation under marginal conditions. BSCM41 maintained high yields in both dry and rainy seasons (1524.73 and 1633.26 kg/ha, respectively), meeting the requirements for an efficient bi-directional seed rotation system. These findings can inform future breeding strategies and production management in comparable tropical environments.