ANTAGONISM OF THE FOLIAR UPTAKE OF GLYPHOSATE INTO GRASSES BY ORGANOSILICONE SURFACTANTS .1. EFFECTS OF PLANT-SPECIES, FORMULATION, CONCENTRATIONS AND TIMING OF APPLICATION

Radiolabelled glyphosate was used to investigate the antagonistic effe cts of an organosilicone surfactant, 'Silwet'(R) L-77(TM), on the foli ar uptake of the formulated herbicide into four grass species, over a 24-h time-course. While the effect of L-77 addition was generally to i ncrease the initial rate of uptake of glyphosate, thereafter the rate was slowed or even halted, such that an overall decrease in uptake was evident as early as 4 h after application in one of the species teste d. Further extensive studies on wheat investigated L-77 effects on upt ake of the herbicide as both the pure mono-isopropylammonium salt, and as the formulated product containing a tallow amine polyethoxylate su rfactant. The addition of L-77 initially increased, but after 1 h redu ced the rate of glyphosate uptake, regardless of formulation. There wa s no evidence that antagonism resulted from any surfactant interaction between the organosilicone adjuvant and the tallow amine co-formulant . Antagonism was directly related to the concentration of L-77, and in versely related to that of the formulated herbicide and of its tallow amine co-formulant. Applied prior to glyphosate application, L-77 redu ced the amount of herbicide in the foliage, and failed to increase sub stantially the initial rate of uptake. When applied after the a.i., in stant enhancement of glyphosate uptake took place with some stomatal i nfiltration observed, but thereafter no significant cuticular penetrat ion occurred. The mechanisms by which L-77 could interfere with the cu ticular penetration of glyphosate are discussed in light of these resu lts.