The Basis of Bentazon Antagonism on Sethoxydim Absorption and Activity

Abstract

The antagonistic effect of bentazon on sethoxydim adsorption and activity was studied in quackgrass. The diffusion of14C-sethoxydim into and through an isolated tomato fruit cuticle was inhibited in the presence of the sodium salt of bentazon. Bentazon also increased the partitioning of14C-sethoxydim into CH2Cl2and water; however, it decreased partitioning into ethyl acetate. Removal of epicuticular wax from quackgrass leaf surfaces did not prevent the antagonism. Addition of sodium acetate or sodium bicarbonate to the sethoxydim spray solution at 10 mM reduced uptake of14C-sethoxydim by quackgrass similar to the effect of bentazon. Sodium ions in the bentazon formulation appeared responsible for the antagonism by exchanging with the H+of the sethoxydim hydroxyl group to form a more polar sodium salt of sethoxydim. The addition of Li+, K+, Cs+, Ca++, and Mg++cations associated with a weak acid also reduced14C-sethoxydim absorption. Addition of organic acids to the spray solution overcame the antagonism by preventing the formation of sodium salt of sethoxydim. In the field, the addition of a 3000 ppm sodium acetate solution delivering 0.56 kg/ha produced the same antagonism as bentazon on quackgrass control with sethoxydim.