Influence of rapeseed oil ethoxylate surfactants on retention and biological efficacy of glyphosate spray solutions in selected weeds

Abstract

Chemical weed control plays a major role in increasing productivity of modern cropping systems. Surfactans are used to improve the biological efficacy of foliar-applied herbicides. However, questions have been raised about the safety of the commonly used active ingredients and surfactans. As a consequence of the increasing pesticide safety demands and restrictive regulatory limitations on pesticide use, researchers are looking for new adjuvants with improved biological and ecotoxicological profiles. With this in mind a newer group of vegetable oils, namely rapeseed oil ethoxylates (RSO), was studied. Rapeseed oils with 5 (RSO 5), 10 (RSO 10), 20 (RSO 20), 30 (RSO 30), and 60 (RSO 60) ethoxylation units were added to glyphosate lutions and applied to different weed species (Amaranthus retroflexus, Datura stramonium, Setaria viridis, Viola arvensis). The influence of these oils on spray retention as well as the biological efficiacy of glyphosate was examined, using electron microscopy, spectroscopy, chlorophyll fluorescence, and dry matter determination. Scanning electron microscopy pictures of the weed surfaces were taken and displayed fundamental differences in the epidermal, cuticular and epicuticular structures of the leaf surfaces. Beside different habits and leaf structures the results were regarded as reasons for quantitative differences in the mean spray solutiondeposited. Further tests demonstrated that there was no correlation between retention behavior and biological efficiacy. Maximum chlorophyll fluorescence measurements showed similar or higher impact of glyphosate when rapeseed oils were added in comparison with treatment of Roundup Ultramax®. Good correlation was shown between hydrophilicity/ethoxylation of the solution and dry matter content, In D. straminium and V. arvensis increasing ethoxylation led to descreasing dry mass. The opposite could be shown in S. viridis. Amaranthus retroflexus had lowest dry mass when glyphosate was applied with RSO 20. The rapeseed oil ethoxylates used showed promising impacts on glyphosate spray solutions but further studies needd to be undertaken to clarify other processes of the foliar application of pesticides