Mechanism of Silwet L77 - induced antagonism of glyphosate efficacy on Paspalum dilataum: A dissertation submitted for partial fulfilment required for a Bachelor degree in Agricultural Science with Honours

Foliar uptake of the isopropylamine salt of ¹⁴C - glyphosate into Paspalum dilatatum leaves was reduced by the addition of a nonionic organosilicone surfactant, Silwet L77. The addition of 0.1 % (V/V) Silwet L77 reduced contact angles of glyphosate formulations to zero, allowing completed leaf surface wetting. Surface tension measurements were also reduced from 45.7 mNm⁻¹ to 24.1 mNm⁻¹ with the addition of Silwet L77. The antagonism to glyphosate uptake induced by Silwet L77 was attributed in part to the morphological characteristics of the P. dilatatum adaxial leaf surface, which is a smooth, trichome-free surface that has relatively few stomata (19mm⁻² ). It is not conductive to the mode of action of Silwet L77. The observed antagonism of P. dilatatum to the addition of Silwet L77 in glyphosate formulations was overcome by adding the humectant glycerine. The addition of 6% (V/V) glycerine, increased surface tension marginally to 25.6 mNm⁻², although this was not sufficient to prevent possible stomatal entry of the solution. A contact angle was measurable with the addition of glycerine (45.0°) and complete wetting of the leaf surface took up to 3 minutes, whereas without glycerine complete wetting occurred in 19 seconds. The removal of Silwet L77 induced antagonism was not related to the rate of droplet drying. The humectant properties of glycerine were lost with the addition of Silwet L77 and drying time of herbicide deposits were as rapid or more rapid than for Silwet L77 plus glyphosate. The droplet drying investigations were conducted on glass sides that were in no way representative of P. dilatatum's leaf surface, and hence may not accurately reflect droplet drying times on leaf surfaces. The benefit of glycerine to glyphosate plus Silwet L77 formulations was attributed to a complex interaction between the three components of the formulation and the leaf surface.