Breeding Black Soybeans for High Yield and First Pod Height Is a Promising Approach to Improving Thai Commercial Soybean Varieties

Black soybeans are rich in beneficial substances like anthocyanins, which help combat free radicals, and also have a high protein content. However, the soybean production system in Thailand struggles with issues relating to the use of machinery for harvesting. For efficient harvesting with machinery, the first pod of the soybean cultivar should be positioned at a height greater than 10 cm. Thailand has not yet reported black soybean cultivars with the height of the first pod measuring 10–15 cm above the ground. Therefore, the aim of this experiment was to improve the commercial soybean varieties KKU35, SJ5, NSW1, and CM60 in Thailand by increasing their first pod height and developing black grains through crossbreeding with the KKUSB–108 soybean germplasm. Subsequently, the pedigree selection method was used to evaluate and select plants with black grains, good growth performance, and a first pod height exceeding 10 cm from the F2 to F5 generations. The selected line of F5 was selected, while the grains of the F6 generation were designated as recombinant inbred lines (RILs). Eight soybean RILs, namely KKU35xKKUSB–108–12–4–3, KKU35xKKUSB–108–24–5–7, SJ5xKKUSB–108–25–2–1, SJ5xKKUSB–108–30–3–7, NSW1xKKUSB–108–49–3–3, NSW1xKKUSB–108–49–3–6, CM60xKKUSB–108–41–1–7, and CM60xKKUSB–108–64–4–8, together with the Sukhothai 3 black soybean commercial variety, were laid out in a randomized complete block design (RCBD) with three replications at the Agronomy Field Crop Station, Khon Kaen University, over two seasons: the rainy season in 2021 and the dry season in 2022. The results revealed that the first pod height of all RILs in the rainy and dry seasons was higher than Sukhothai 3. The RILs showing a consistently high yield in both the rainy and dry seasons were SJ5xKKUSB-108-25-2-1 (1.85 and 1.86 T/ha), SJ5xKKUSB-108-30-3-7 (1.65 and 1.72 T/ha), NSW1xKKUSB-108-49-3-6 (1.52 and 1.83 T/ha), and CM60xKKUSB-108-64-4-8 (1.60 and 1.61 T/ha). Moreover, the RIL, NSW1xKKUSB-108-49-3-6, has a protein content of up to 44.21% in the dry season and shorter maturity than other RILs. This RIL can be used for cropping rotation systems in areas with limited time and water resources. This work provides a resource of black soybean RILs with high yield and first pod height for soybean breeding programs in the future. However, yield and protein content were affected by season (S), genotype (G), and the S × G interaction, indicating that RILs require a diverse environment for regional yield trials in the future.