Translocation of¹⁴C-Glyphosate in Soybeans (<i>Glycine max</i>) and Johnsongrass (<i>Sorghum halepense</i>)

Abstract

Absorption and translocation of 14 C-glyphosate [ N -(phosphonomethyl)glycine] in johnsongrass [ Sorghum halepense (L.) Pers.] and soybeans [ Glycine max (L.) Merr. ‘Lee 68’] were evaluated under various environmental conditions. The toxicity of unlabeled glyphosate applied similarly was determined in soybeans under similar conditions. In johnsongrass, absorption of 14 C-glyphosate nearly doubled and translocation increased as air temperature increased from 24 to 35 C. Translocation in johnsongrass and soybeans was often greater at a relative humidity (RH) of 100% than at 45% and at a soil moisture level of 20% (field capacity) than at 12% (near the wilting point). In soybeans, absorption of 14 C-glyphosate increased about 100%, translocation increased between 100 and 1200%, and glyphosate toxicity increased more than 200% as temperature decreased from 35 to 24 C. At 35 C, translocation in soybeans was often greater at 20% than 12% soil moisture. Absorption of 14 C-glyphosate in soybeans was greater when the herbicide was applied to mature rather than immature leaves and stems, but the age of the treated tissue had less effect on 14 C translocation. The addition of surfactant plus KH 2 PO 4 to the treatment solution increased glyphosate absorption and translocation in soybeans, but addition of surfactant alone reduced translocation and addition of KH 2 PO 4 alone increased translocation. Injury to six soybean cultivars from application of unlabeled glyphosate at 0.4 kg/ha ranged from 37% to 95% while total absorption of 14 C-glyphosate into treated leaves of the same six cultivars ranged from 23% to 74% of the amount applied.