Effects of surfactants and the kinetic energy of monodroplets on the deposit structure of glyphosate at the micro-scale and their relevance to herbicide bio-efficacy on selected weed species

The deposit pattern of foliar-applied agrochemicals, and its relation to their bio-efficacy, has major practical importance. Thus, in our experiments, we evaluated the relevance of the deposition properties of glyphosate for its bio-efficacy. The deposition pattern of glyphosate monodroplets was influenced by using surfactant and by applying the droplets with or without kinetic energy to the plant foliage. Monodroplets (1 mu L) of glyphosate, formulated with or without ethoxylated rapeseed oil surfactant (RSO) having on average 5, 10, 30 or 60 ethylene oxide units (EO), as well as one commercial glyphosate product (CGP), were applied either by carefully placing the droplet on the foliage with a pipette (kinetic energy assumed to be near zero) or by a monodroplet generator (with kinetic energy). We selected two easy-to-wet (Stellaria media and Viola arvensis) and two difficult-to-wet (Chenopodium album and Setaria viridis) weed species as target plants. The deposit structure was determined using a scanning electron microscope with energy dispersive x-ray microanalysis. The kinetic energy of the droplet had no consistent effect on the deposit structure or the bio-efficacy of glyphosate formulations. In contrast, surfactants differing in EO unit, affected both the deposit structure and the bio-efficacy of the formulations, depending upon the species. In easy-to-wet species, the increase in EO unit of RSO surfactant failed to affect the deposit area of glyphosate and its bio-efficacy. However, in difficult-to-wet species, the increase in EO unit of RSO surfactant reduced the deposit area of glyphosate and enhanced its bio-efficacy.