Influence of Two Polymeric Adjuvants on Bioavailability of Glyphosate in Vision® Formulation: Relevance to Rainwashing of Deposits from Foliar Surfaces

The influence of two polymeric adjuvants, Sta-Put^® and Silwet^® Y-6652, on glyphosate washoff from trembling aspen foliage was studied using Vision^® formulation at a dosage rate of 0.356 kg of active ingredient (ai) in 25 1/ha. End-use formulations were prepared by adding adjuvant concentration levels ranging from 0.05 to 1.5% v/v to diluted Vision^® formulation containing ¹⁴C-glyphosate. Two types of studies were carried out: one for investigating the rate of uptake, foliar washoff, translocation, and bioavailability of glyphosate; and the other for optimizing adjuvant concentration levels to maximize foliar uptake and to minimize washoff, without causing reduction in translocation and bioavailability. Young seedlings were used for the first study using end-use formulations containing 0.05% of the adjuvants. The data indicated that foliar uptake of glyphosate is a slow process since the amount washed off the foliage was >67% at 8 h after treatment and >50% at 48 h. Similar to the foliar uptake, translocation also occurred slowly, only about 2% of the applied amount was translocated in 8 h, and about 10% in 48 h. Nevertheless, there was no evidence of reduced bioavailability because of the presence of the adjuvants at the concentration level used.

Branch tips were used in the second study for treatment of end-use formulations containing 0.05 to 1.5% of the adjuvants, and were harvested at only one time period (48 h). The data indicated that both adjuvants contributed to reduced glyphosate washoff even at the low concentration level of 0.05%. The amount washed off decreased progressively as the adjuvant concentration increased from 0.05 to 1.5%. However, the amount that translocated into the untreated parts of the branch increased initially, passed through a maximum at an adjuvant concentration range of 0.3 to 0.6%, and decreased gradually at higher concentrations of 1.0 to 1.5%. The data thus indicated an optimum adjuvant concentration range beyond which reduced translocation and bioavailability would likely occur. Thus, the study indicated the need to optimize adjuvant concentration levels for every application condition. Otherwise, the rate of uptake and translocation is likely to be impaired, resulting in reduced herbicidal effectiveness.