Effects of Environment on the Toxicity of Glyphosate to Johnsongrass(Sorghum halepense)and Soybeans(Glycine max)

Johnsongrass [ Sorghum halepense (L.) Pers.] and soybeans [ Glycine max (L.) Merr. ‘Lee 68’] were treated over-the-top with glyphosate [ N -(phosphonomethyl)glycine] at 0.2 and 0.4 kg/ha with surfactant in spray solutions and 0.4 and 0.7 kg/ha without surfactant in spray solutions. Treated plants were maintained in the growth chamber for 2 weeks after treatment at 24, 29, and 35 C; 45 and 100% relative humidity (RH); and 12 and 20% soil moisture. Glyphosate at 0.4 kg/ha applied without surfactant controlled johnsongrass better at 100% than at 45% RH. Johnsongrass control was better with 20% than with 12% soil moisture. Averaged over all evironments, the order of glyphosate toxicity to johnsongrass at 0.4 and 0.7 kg/ha was 35 C > 29 C > 24 C. The environmental condition of 45% RH and 12% soil, moisture resulted in least johnsongrass control regardless of temperature. The order of glyphosate toxicity to soybeans when applied without surfactant was 24 C > 29 C > 35 C. Glyphosate applied without surfactant was more toxic to soybeans at 100% than at 45% RH and more toxic at 12% than at 20% soil moisture. Surfactant in spray solutions increased glyphosate toxicity to johnsongrass and soybeans.