Effects of Temperature and Relative Humidity on the Toxicity of Glyphosate to Bermudagrass (Cynodon dactylon)

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Abstract
Glyphosate [N-(phosphonomethyl)glycine] toxicity to bermudagrass [Cynodon dactylon (L.) Pers.] increased significantly with each rate increase from 0.14 to 1.12 kg/ha. Under greenhouse conditions approximately 50% bermudagrass control was obtained at 0.56 kg/ha glyphosate. Visible toxicity and fresh wt of treated plants and regrowth of plants clipped at the soil surface 24 h after treatment were used as indices for penetration and translocation of glyphosate. Visible injury to bermudagrass with 0.56 kg/ha glyphosate was greater at 100% than at 40% relative humidity (RH) at both 22 and 32 C. Fresh wt data indicated that 0.56 kg/ha glyphosate was more toxic at 32 C than at 22 C at 40% RH, but no difference was observed at 100% RH. Less than 10% of the applied 14C-glyphosate penetrated the treated bermudagrass leaf at 22 C and 40% RH; whereas, more than 70% penetrated the treated leaf at 32 C and 100% RH. Five to six times more 14C-label was translocated into the plant at 100% than at 40% RH. Significantly more 14C-label translocated out of the treated leaf and into the plant at 32 C than at 22 C at 40% RH but no significant increase was observed at 100% RH.