Translocation of14C-Glyphosate in Soybeans (Glycine max) and Johnsongrass (Sorghum halepense)

Abstract

Absorption and translocation of 14C-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 14C-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 14C-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 14C-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 14C translocation. The addition of surfactant plus KH2PO4 to the treatment solution increased glyphosate absorption and translocation in soybeans, but addition of surfactant alone reduced translocation and addition of KH2PO4 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 14C-glyphosate into treated leaves of the same six cultivars ranged from 23% to 74% of the amount applied.