Effects of water table management and row width on the growth and yield of three soybean cultivars in southwestern Japan

In southwestern Japan, soil water fluctuations from flooding to drought cause unstable soybean yields. Water table management (WTM) with sub-irrigation/drainage systems will overcome the soybean yield instability by inhibiting these fluctuations. Narrow row cultivation is expected to increase soybean yields. The effects of WTM and row width on soybean growth and yield in this region are not clear. We evaluated the effects of WTM with sub-irrigation/drainage systems and row widths (35 or 70 cm) on the growth and yield of one conventional (tall main stem) and two newly developed (short main stem) soybean cultivars. The WTM consisted of (1) fluctuation of the water table between the natural water table depth and that at 30 cm depth according to the growth stage and weather conditions, especially rainfall events (newly developed); (2) maintaining the water table at a 30 cm constant depth throughout the growth period (recommended in Japan); and (3) the natural water table with an underdrain (control). No significant interaction was observed between the WTM and cultivar or row width treatment, indicating that cultivars and row width treatments responded similarly to WTM. WTM 1 and 2 decreased the soybean yield by approx. 5% when the natural water table depth in control existed at 50-60 cm depths throughout the growing period, indicating that the natural water table depth in control was near optimum for soybean growth and yield. Before performing WTM, therefore, the natural water table depth should be measured and considered. The combination of newly developed cultivars with narrow rows had similar or greater yields than conventional cultivation (cultivar and row width), due mainly to an increase in pods m−2 and a decrease in yield loss without severe lodging. Thus, yield potential in southwestern Japan could be increased by narrow row cultivation, but cultivars with short main stem lengths should be cultivated.