Foliar absorption of some glyphosate formulations and their efficacy on plants

Uptake of various doses (0.27-4.06 g litre-1) of glyphosate acid formulated with various concentrations (0.27-5.42 g litre-1) of two polyethoxylated alcohol surfactants ('Genapol' T-150, 'Genapol' T-250) in wheat (Triticum aestivum L. cv. Ralle) 24 h after application was studied. Both surfactants greatly enhanced glyphosate uptake at concentrations up to 1.35 g litre-1. Uptake could be slightly further improved by 'Genapol' T-150 up to 5.42 g litre-1 while 'Genapol' T-250 at this high concentration antagonized uptake a little. Uptake of glyphosate-monoammonium formulated with a quaternary ammonium surfactant ('Trimao' (8PO)) in wheat and white mustard (Sinapis alba L. cv. Alba) was also studied. Uptake of glyphosate-monoammonium in wheat was enhanced at surfactant concentrations up to 1.35 g litre-1. Beyond this concentration uptake continued to increase slightly but not significantly. Generally, glyphosate-monoammonium formulated with 'Trimao' (8PO) was not taken up as extensively by mustard as by wheat, and the optimum surfactant concentration is probably higher on mustard. The optimal combinations of surfactant concentration and glyphosate dose based on uptake 24 h after application were used in a time-course study. Results from uptake studies were compared with the efficacy of similar formulations on wheat. The surfactant concentration range which gave the most pronounced effect on glyphosate uptake was found to be similar (0-1 g litre-1) in both studies. However, the surfactant concentration providing maximum herbicide uptake was found to be higher in the uptake study compared to the surfactant concentration providing optimal efficacy in the dose-response experiments. A dose rate of 0.5 g litre-1 of 'Tween' 20 (sorbitan monolaurate surfactant) improved efficacy of a formulation with an optimal mix of glyphosate-monoammonium and 'Trimao' (8PO). Ammonium sulfate (5.0 g litre-1) did not improve this formulation further.