Effect of glycerol on spreading and drying of vision(R) droplets containing Silwet(R) L-77: Relevance to rainfastness and herbicidal activity of glyphosate on trembling aspen (Populus tremuloides michx)

Literature information indicates that Silwet(R) L-77 acted as a rainfastening adjuvant and provided increased herbicidal activity on quackgrass. However, previous results on trembling aspen were inconclusive, probably due to the high dosage rate (1.0 kg glyphosate/ha) used, or due to excessive spreading and rapid drying of droplets. This paper describes a laboratory study conducted in 1990 using a low dosage rate of 0.4 kg glyphosate/ha to optimize the concentrations of a humectant adjuvant (glycerol), and a field study conducted in 1991 using the optimum concentration of glycerol to examine rainfastness and herbicidal activity of glyphosate under field conditions. The laboratory study involved application of Vision(R) (a formulation concentrate of glyphosate) droplets with and without Silwet and glycerol, onto trembling aspen leaf discs to determine the optimum spray mix based on maximum droplet spreading, drying time and rainfastness. The optimum spray mix (Vision + 0.15% of Silwet and 0.25% of glycerol) was applied onto young aspen trees in the field. For comparison, Vision alone, and Vision + 0.15% Silwet were also applied. Simulated rainfall (10 mm) was applied at 24 h after herbicide treatment. Glyphosate residues on foliage were analyzed using a fluorometric method, and the amount washed off in rain was determined, The data indicated that initial deposits on foliage were nearly quantitative (ca 95%). The average pre-rain residues were 81% of the applied amount for all three mixes. The post-rain residues were: Vision 25%, Vision + Silwet 39%, and Vision + Silwet + glycerol 49% of applied. The mortalities of the aspen trees were: Vision 42%, Vision + Silwet 49%, and Vision + Silwet + glycerol 69%. The data indicated significant increase in mortality of the trees due to the addition of glycerol to the Vision-Silwet spray mix.