Author : Prapto Yudono

Summary : A greenhouse experiment was conducted at the Central Experiment Station of the University of the Philippines Los Banos (UPLB) to evaluate the effects of various defoliants (handpicking as control, Sodium chlorate, 2,4 D Ester, and Glyphosate) and time of application on soybean. The study also compared the effectiveness of the defoliants in accelarating harvest time. Defoliants were applied at different stages of maturity: a) nearly all of seeds have not reached their physiological maturity (when 90 percent of pods were still green); b) half of seeds have already reached their physiological maturity (when 50 percent of pods have turned to yellow); c) when all of seeds reached their physiological maturity (100 percent of pods turned to yellow); d) at post physiological maturity (when 90 percent of pods yellow and the rest had turned to brown). Application of defoliants prior to physiological maturity, significantly reduced one hundred weed weight, germination rate, vigor index, total dry weight of normal seedlings and gave a higher percentage of abnormal seedlings. While application of defoliants at the time of physiological maturity and at post physiological maturity gave a higher one hundred seed weight, germination rate, vigor index, total dry weight of normal seedlings but a lower percentage of abnormal seedlings. The yield components such as the number of pods, the number of productive branches, the number of nodes, were not significantly affected by the application of defoliants. The only seed yield component, affected by the application of defoliants was the one hundred seed weight. Compared with untreated plants, application of defoliants significantly reduced harvest-age. Untreated plants needed 101 days from seed sowing to harvesting. The shortest harvest-age (80 days) found was from the application of sodium chlorate at the time when 90 percent of pods were still green. The application of glyphosate when most of the pods had turned to yellow gave the longest harvest-age (93 days). The application of defoliants shortened harvest-age at the range of eight to twenty one days. Application of 2 and 4 D ester and glyphosate prior to physiological maturity resulted in higher abnormal seedlings exhibiting stunted hypocotyl and defective primary leaves, and terminal buds.