Effect of the soil-binding adjuvant Grounded® on herbicide efficacy and runoff losses in bare soil in ratoons : ASSCT peer-reviewed paper
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To reduce the impact of pesticides, in particular pre-emergent herbicides, on fresh and estuarine water bodies of the Great Barrier Reef catchment, while maintaining productivity, the sugar industry is exploring innovative options to reduce the movement of herbicides off site. Previous research work has shown the oil-based adjuvant Grounded® added at 3 L/ha to the herbicide tank reduced runoff losses by 17 to 40% across all tested herbicides at 48 h and 3 weeks after product application, when applied on bare soil in a tilled plant cane in far northern Queensland. Herbicide efficacy was maintained above 90% for 200 days after product application with or without the addition of the adjuvant. Conversely, Grounded® did not reduce runoff loss when added to herbicides applied in trash blanketed ratoon. This paper presents additional research work carried out to assess the impact of Grounded® on pre-emergent herbicide efficacy and on runoff losses when applied to ratoon cane on bare soil. This scenario is typical of the Burdekin and New South Wales regions. Two trials were conducted in untilled ratoons after burning the trash blanket in far northern Queensland. Grounded® was added to six registered pre-emergent herbicides: imazapic (94.5 g/ha), hexazinone (472.5 g/ha), isoxaflutole (150 g/ha), amicarbazone (700 g/ha), atrazine (1350 g/ha) and pendimethalin (1001 g/ha). Herbicide efficacy trials were implemented as randomised complete blocks with three replicates and adjacent untreated controls. Losses of the tested pre-emergent herbicides in runoff were monitored using replicated rainfall simulations, delivering 80 mm of simulated rain, 48 h or 3 weeks after herbicide application. Both runoff trials generated similar herbicide concentrations in runoff. As expected, higher concentrations for all herbicides were found in runoff 48 h after spraying compared to 3 weeks after spraying. The adjuvant Grounded® added to the spray tank did not decrease herbicide loss via runoff in both trials. Topsoil samples taken before and after rainfall, generally showed higher percentage herbicide in topsoil after rainfall when Grounded® was added to the tank mix compared to no added adjuvant. However, this slight binding improvement to the soil did not result in lower herbicide loss in runoff. These runoff and soil results mirrored previous research results when Grounded® was applied on trash blanketed ratoons. In both efficacy trials, weed control varied at each site between herbicide treatments depending on the environmental conditions and the weed species. However, the addition of Grounded® to each herbicide treatment did not affect the efficacy of any herbicide treatment in both trials. These results show that the oil-based adjuvant Grounded® is unlikely to improve the quality of runoff water leaving sugarcane paddocks when applied to untilled ratoon cane on bare soil.