Weed Control in Soybeans by Glyphosate Applied in the Recirculating Sprayer

Field experiments were conducted to study the feasibility of applying glyphosate [N-(phosphonomethyl)glycine] postemergence for the control of johnsongrass [Sorghum halepense (L.) Pers.], redroot pigweed (Amaranthus retroflexus L.), and hemp sesbania [Sesbania exaltata (Raf.) Cory] in soybeans [Glycine max (L.) Merr.]. Herbicide sprays were directed across the row to weeds growing taller than soybeans in June, July, and August. Herbicide not sprayed on weeds was trapped and reused. Glyphosate at 1.12 and 1.68 kg/ha effectively controlled johnsongrass with little soybean injury and with greatly increased soybean yields. The use of 0.1% surfactant frequently increased the toxicity of glyphosate at 1.12 and 1.68 kg/ha to soybeans, but this adverse effect was overcome by the use of 0.1% anti-drift polymer in sprays. Control of redroot pigweed with glyphosate at 1.12 kg/ha was improved by the use of 0.1% surfactant, but surfactant did not increase control with glyphosate at 1.68 kg/ha. Glyphosate at 1.68 kg/ha, with 0.1% surfactant, was required to provide more than 80% control of hemp sesbania. Glyphosate applied at 1.12 kg/ha in the recirculating sprayer provided equal johnsongrass control, increased soybean yields and caused less soybean injury than when glyphosate at 0.56 kg/ha was applied over-the-top in water or in foam.