Effects of Row Spacing, Benomyl, and Duration of Sicklepod (Cassia obtusifolia) Interference on Soybean (Glycine max) Yields

Field experiments were conducted during a 3-yr period to study the effects of three soybean [Glycine max (L.) Merr. ‘Hill’] row spacings, four periods of sicklepod (Cassia obtusifolia L. # CASOB) competition, and timing of fungicide application for control of seed and foliar diseases of soybeans on soybean yields and growth of sicklepod Competition of sicklepod for 4 weeks reduced soybean yields in 2 of the 3 yr. Competition for the entire season significantly reduced the yield of soybeans regardless of row spacing. Average soybean yields were higher with 51-cm row spacings than with 102-cm row spacings regardless of fungicide treatment or period of sicklepod competition. Foliar applications of benomyl significantly increased soybean yields in only 1 of 3 yr. Sicklepod plants did not grow taller than soybeans until more than 96 days after emergence. Greatest rate of increase in weights of individual sicklepod plants and growth of axillary branches was in the period from 72 to 96 days after emergence. During this time, there was about an 8-fold increase in the number of leaves produced by sicklepod plants. The most rapid daily increase in height of sicklepod was in the period from 96 to 100 days after emergence when more than 50% of the total growth occurred. Height and weight of sicklepod plants were less when soybeans were grown in 25-cm row spacings than in 102-cm row spacings. No differences were noted in die internal microflora of soybean seed collected from fold plots with regard to the presence of sicklepod, duration of sicklepod interference, or timing of fungicide application.