Characterisation of glyphosate uptake into grass species

The effect of surfactant ethylene oxide (EO) content, droplet spread area, and the active ingredient (a.i.) dose on the leaf surface on glyphosate uptake into 3 grass species (wheat, ryegrass, and barnyard grass) was investigated. It was found that glyphosate uptake into wheat was only moderately correlated (R^2 = 0.66) with surfactant EO content (range 5–20 units per molecule). By contrast, a highly negative correlation (R^2 = 0.98) was found between the uptake and the spread area of the treatment formulations on the leaf surface. The influence of droplet spreading on glyphosate uptake was further investigated using a superspreading surfactant, Silwet L-77, and varying a.i. concentrations. It was demonstrated that the determining factor for glyphosate uptake into all 3 grass species was the a.i. dose (in (mu)g/mm^2) formed on the leaf surface after droplet spreading. Droplet spread area per se had only an indirect effect on the uptake through diluting the a.i. dose. In the presence of a non-spreading surfactant, Mon 0818, glyphosate uptake depended not only on the a.i. dose but also the surfactant dose on the leaf surface. In the absence of surfactants, high a.i. dose alone was not enough to ensure a high level uptake. The results are discussed in relation to the current knowledge on the interactions between surfactants and herbicide uptake.