Soybean growth traits suitable for forage production in an Italian ryegrass living mulch system

Forage soybean, like alfalfa, is a source of high-protein feed, but expansion of forage soybean cultivation has been prevented by restrictions on the use of herbicides. Although the use of living mulch (LM) is considered as an effective technique for weed control without requiring the use of herbicides, LM sometimes suppresses not only weed but also main crop growth. In the present study, field studies using eight soybean varieties were conducted to evaluate suitable traits and varieties of soybean for forage production in LM systems. Italian ryegrass was sown as a living mulch in April, and soybean was sown without tillage after the ryegrass was mowed in June. The whole-plant yield of soybean was decreased by LM for most varieties, and this yield reduction was larger in early-maturing than in late-maturing varieties. The whole-plant yield in the LM system was higher for late-maturing varieties, but the variety with the highest yield, Fukuyutaka, showed severe lodging, implying that marked yield loss would occur during harvest by farming machinery. In contrast, the variety with the second-highest yield, Tachinagaha, had strong lodging resistance. Whole-plant yield was highly correlated with the vegetation cover ratio of soybean at flowering, which was affected by earliness of soybean and competition with LM. The crude protein content of soybean varieties which reached the beginning of the maturity stage was >20% and was higher than that of imported alfalfa hay ranked as premium grade. Thus, soybean varieties with high whole-plant yield, crude protein content, and lodging resistance are suitable for forage soybean production in LM systems.