The efficacy of the organosilicone surfactant, Silwet L77, and the herbicide glyphosate on a range of grass species: A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Improved herbicide efficacy with the addition of the organosilicone surfactant, Silwet L77, was not demonstrated for all grass species. A pot trial experiment investigated the effect of the addition of 0.25% Silwet L77 on the efficacy of formulated glyphosate in controlling a range of grass species grown in New Zealand. Physical characteristics of surface tension and contact angles of droplets were determined. Scanning electron microscopy was used to investigate leaf surface characteristics, and fluorescence microscopy used to investigate herbicide uptake, via stomatal and cuticular routes. Species responses to Silwet L77 were related to their leaf surface characteristics, and the physical characteristics of the solutions. The inclusion of 0.25% Silwet L77 produced varied responses to the efficacy of formulated glyphosate. Efficacy was improved in the species Tall Fescue (Festuca arundinacea), Cocksfoot (Dactylis glomerata), Timothy (Phleum pratensis), and the two perennial ryegrass (Lolium perenne) cultivars, Yatsyn and Greenstone. Their leaf surfaces were highly ridged, rough and stomatous, and in the case of Cocksfoot, covered in long trichomes. The abaxial leaf surfaces of Yatsyn and Greenstone were astomatous. Timothy had relatively smooth leaf surfaces and a high stomatal density. Silwet L77 had no influence on the efficacy of glyphosate on the species Prairie Grass (Bromus wildenowii), Mountain Brome (Bromus marginatus), Phalaris (Phalaris aquatica), and Paspalum (Paspalum dilatatum). The leaf surfaces of these species were relatively smooth and stomatous, apart from Prairie Grass which was ridged and trichomeous. Antagonism towards glyphosate, induced by Silwet L77, was measured on the species Grazing Brome (Bromus stamineus), which also possessed smooth leaf surfaces. The contact angle of deionised water with the adaxial leaf surface did not vary greatly between species, ranging from 133° for Grazing Brome to 140° for Timothy. The addition of 0.25% Silwet L77 gave a non-measurable contact angle for all species. Glyphosate solution surface tension was reduced to 20.8 24.2mNm⁻¹ in the presence of 0.25% Silwet L77. The addition of the humectant, glycerine, produced contact angles similar to those of the glyphosate-alone solutions, but increased solution surface tension to 25.8-32.2mNm⁻¹. The effect of addition of glycerine in bioassay studies was not evaluated. Fluorescence microscopy confirmed that stomata! uptake of the Silwet L77-containing solutions was occuring in Tall Fescue and Prairie Grass. These two species exhibited extreme contrasts in their response to Silwet L77 in the initial pot trial experiment. Glyphosate efficacy was improved with Silwet L77 on those species which possessed typically difficult to wet leaf surfaces, i.e rough, ridged, stomatous leaf surfaces and not improved in those species with relatively smooth, easy to wet leaf surfaces. Not all species fitted the pattern. The response to Silwet L77 could not be wholly attributed to the presence of trichomes, or the adaxial and/ or abaxial leaf stomata! density. The wax characteristics of the species Prairie Grass, Phalaris and Paspalum could be limiting the uptake of glyphosate, through a reduction in cuticle damage as a result of the improved droplet spread associated with Silwet L77. The physical characteristics of the leaf surface may, in part, be influencing the response to Silwet L77 solutions. However, the results suggest that other factors that were not identified may also play a part in determining the response to Silwet L77.