Towards the optimisation of Coarse sprays to reduce drift and improve efficacy in Australian cropping systems

Abstract

The optimisation of Coarse sprays for control of insects, weeds, and diseases in agriculture involves understanding how those technologies are classified and selected for use. Coarse sprays are the focus of this thesis, because Coarse is a spray size classification that balances adequate coverage, while reducing pesticide spray drift. Spray drift is detrimental, and by selecting coarser droplet sprays, spray drift can be minimised. The initial research study in this thesis sought to identify the repeatability of each nozzle type by randomly selecting five units to test consistency of droplet size measurements across nozzle type. The optimisation of Coarse sprays involves understanding differences that separate uniform technologies, especially when uniform nozzles of the same droplet size classification category are selected. When using plants and efficacy studies alone to quantify spray coverage, specific coverage values are difficult to determine. This consideration prompted studies with artificial collectors to quantify droplet coverage and provide the droplet distribution across the collector to help aid in technology selection. Results from these studies showed that nozzles which produced the same spray quality based on the droplet size distribution did not result in similar coverage across collector types. Nozzles that resulted in the highest coverage were further studied in the subsequent chapters.The optimisation of Coarse sprays involves understanding how sprays move within plant canopies. Consistent spray coverage that is evenly distributed throughout the canopy is necessary to control pest populations that negatively affect yield. As applicators are switching to coarser sprays to reduce risk and liability of pesticide spray drift, concerns about efficacy loss are increasing. Results showed that droplet number densities were inversely related to the droplet size produced by the nozzles, yet coverage was influenced more by application volume rate than droplet size. The optimisation of Coarse sprays involves ensuring that measurement methods and techniques are precise for the selection of the best technologies. A freely available smartphone application (app), SnapCard was developed to provide for in-field analysis of spray collectors. Results from this study showed consistent agreement between Image J and the new app, SnapCard, both of which were used consistently in this study. The optimisation of Coarse sprays requires using novel techniques for characterising sprays, notably artificial collectors. The use of artificial collectors for spray characterisation is well accepted, but there is no literature to bridge the knowledge gap to understand how each major artificial collector type differs. Using all collector types (water sensitive paper, Kromekote® and Mylar®) in tandem was useful to properly characterise the sprays, in addition to helping to quantify the differences between each collector type for coverage and droplet number density analyses. An optimal Coarse spray should not drift in appropriate application conditions, thus measuring the drift potential of Coarse sprays was required. The method of using droplet size alone and laser diffraction as the primary method to determine drift potential may not properly characterise technologies as thoroughly as other methods. Switching from a Fine spray quality (e.g. a TCP type nozzle) to a Coarse spray quality (e.g. an AIXR nozzle) can reduce the drift potential by as much as 85 % and reduce the small droplet fraction of the spray with diameters below 100-200 µm by a similar quantity. This reduction in drift potential without losing efficacy makes a strong case to select DRTs in nozzle and adjuvant technologies. Finally, an optimal Coarse spray should be one that not only reduces spray drift, but also maintains or improves herbicide efficacy against finer sprays. Four efficacy studies in varying grass species sought to compare the effect of spray quality on herbicide efficacy across ten different herbicides and found varying results. In each study, Fine, Coarse and Ultra-Coarse sprays were examined, and across all studies, Coarse sprays resulted in similar dry weight reductions to Fine sprays. The results from these studies and the research thesis will give a grower confidence to select a Coarse spray which can reduce drift potential by 85 % while maintaining or improving herbicide efficacy. This is an optimal Coarse spray.