Effect of early sowing on growth and yield of determinate and indeterminate soybean (Glycine max (L.) Merr.) cultivars in a cool region of northern Japan

To increase Japanese soybean yield, both practical and genetic improvements are needed. Early sowing improved yields in the United States, where indeterminate cultivars may produce better yields than determinate cultivars, particularly with early sowing. However, the mechanism for the yield response is not fully understood. Furthermore, whether these results are valid for cool regions in northern Japan is unknown. This study tested the effects of early sowing on yield and its attributes in three years using a Japanese determinate cultivar (Ryuhou) and three indeterminate cultivars (Karikei-881 and Karikei-879, recombinant inbred lines from determinate * indeterminate crosses; UA4910, a high-yield American cultivar). The effects of early sowing varied among the years. Early sowing significantly increased pod number and seed yield by increasing cumulative intercepted solar radiation (thus, growth during early reproductive stages) in years with high precipitation during mid-season (July and August). High precipitation potentially inhibited canopy development from vegetative to early reproductive stages more strongly in normally sown plants than in early sown plants. In contrast, no significant effect of early sowing was observed in a year with low precipitation during mid-season, when low precipitation during vegetative stage promoted canopy development in both sowing dates. Therefore, early sowing could escape from excess water stress and ameliorate canopy development and thereby seed yield in this region. There was no significant sowing date * cultivar interaction for seed yield in any year; thus, the indeterminate cultivars did not benefit more from early sowing in northern Japan. UA4910 had greater seed yield than Ryuhou owing to its longer growth duration and greater aboveground biomass and pod number; Karikei-881, Karikei-879, and Ryuhou showed similar seed yields. Thus, indeterminate cultivars did not necessarily have higher yields in northern Japan. These results suggest that early sowing could be effective for increasing soybean yield in northern Japan.