Yield and dry matter production of soybean response to late planting in southwestern Japan

The selection of the sowing date is one of the most important decisions in soybean production. It is important for farmers to know the expected seed yield when developing a sowing plan. The objective of this study was to quantitatively reveal the decreased seed yield ratio during late sowing in southwestern Japan under irrigated conditions. Total above ground dry matter and distribution were measured from July to August sowing in 2018 and 2019 in Fukuyama, Hiroshima, Japan. The harvest index in the August sowing did not decrease compared to the July sowing under irrigated conditions. The decrease in yield was related to the total aboveground dry matter rather than to the harvest index. The decrease in total aboveground dry matter was related to the total amount of solar radiation intercepted rather than to radiation use efficiency. A significant regression equation was obtained for the relative yield, which was calculated from the maximum seed yield observed from early to mid-July. When seeds were sown after mid-July, our result showed that the seed yield will decrease by 0.60% per day for Sachiyutaka A1 and by 0.64% for Akimaro, even under conditions of 100% seedling establishment and controlled soil water conditions. This regression equation can be used in fields as an indicator of seed yield where irrigation is possible. [GRAPHICS]