Yield and late sowing limits of soybean in southwestern Japan

The response to sowing date is one of the most important aspects of soybean production. When developing a sowing plan, it is important for farmers to know the final maturation day, as well as the expected seed yield. This study aimed to determine the physiological limit for late soybean sowing and evaluate seed yield in southwestern Japan. Four sowing dates, from late July to early September, were established in 2021 and 2022, in Fukuyama, Hiroshima, Japan. Cultivar ‘Sachiyutaka A1 gou’ was cultivated, and growth stage, seed yield and total aboveground dry weight were measured. Machine harvesting was conducted for the third and fourth sowings. From the first to the third sowing, pods turned to brown normally and matured. In the fourth sowing, however, leaves were damaged by frost in December, and pods were dried in the field, with green pods remaining in both years. At the fourth sowing, both the harvest index and total aboveground dry weight significantly decreased. Among yield components, the 100-seed weight significantly decreased as sowing was delayed. During machine harvesting, most pods were discharged from the harvester without threshing. It is suggested that the physiological limit of late soybean sowing in the Chugoku region of southwestern Japan is between late August and early September. After late August, plants do not normally reach maturity, and machine harvesting becomes impossible. These results can be applied when developing cropping plans.