Deficit irrigation during the reproductive stages of soybean (Glycine max (L.), Merrill)

The results of the study showed that as the level of deficit irrigation increased from 20 to 40 percent, significant reductions were observed in the average incremental increase in plant height (24.2-32.6 percent), total vegetative dry matter production (25.7-34.8 percent), pod dry matter production (25.8-37.4 percent), total reproductive dry matter production (18.8-27.6 percent), number of pods per square meter (14.9-27.1 percent) and grain yield (18.7-30.3 percent). However, reductions were generally higher when water deficits were imposed during seed enlargement stage (17.4-36.7 percent) than when they were imposed during flowering stage (10.2-23.6 percent) or pod formation stage (7.7-15.2 percent). The number of seeds per pod and the one hundred seed weight were reduced by deficit irrigation but such reductions were insignificant. The length of growing period of soybean plants under deficit irrigation was shorter by almost 2 days than that of the fully irrigated plants. However, no significant differences were observed between the length of growing period of the 20 and 40 percent water deficit plants. Plants under deficit irrigation during the flowering and pod formation stage also matured in about 2 days earlier than the fully irrigated plants. Deficit irrigation carried out during the seed enlargement stage failed to shorten the growing period of soybean plants. Deficit irrigation significantly improved the average water-use efficiency of soybean plants when this was accomplished during the flowering (127-132 percent) and pod formation (130-133 percent) stages compared to the fully irrigated plants. Plants under 20 percent deficit irrigation had the highest average WUE while the fully irrigated plants had the lowest. The practice of deficit irrigation during the seed enlargement stage did not improve the WUE over that of full irrigation. The economic feasibility of deficit irrigation was determined through the potential net returns from soybean cropping. The average potential net returns of soybean cropping under 20 percent and 40 percent deficit irrigation were, respectively, higher and lower than that of the full irrigation practice. Deficit irrigation reduced the potential net returns of soybean cropping when it was imposed during the seed enlargement stage. The highest potential net return of P61,093.65 was obtained when a 20 percent deficit irrigation was imposed during pod formation stage. Of the different yield components, only the number of pods affected the grain yield of soybean plants and consequently, the potential net returns. This study confirmed that a 20 percent deficit irrigation can be beneficially practiced during pod formation stage and , to a lesser extent, during flowering stage