Impact of application depth and slot closure on runoff losses of imidacloprid

Imidacloprid represents the Australian sugar industry’s best canegrub-management tool, but it has been detected in many water bodies, including groundwater, creeks, rivers and marine environments, posing a potential risk to the health of the Great Barrier Reef. In ratoon cane, it is commonly applied in liquid form with coulters within the cane row. Imidacloprid product labels state that, when applied in ratoons, the product must be placed at 100–125 mm depth and the slot must be covered; however, it is not uncommon to observe application equipment that does not maintain the desired depth or fails to close the slot appropriately. To investigate the best application methods to reduce imidacloprid runoff, two rainfall-simulation trials were established in the Burdekin and in the Wet Tropics to assess the impact of depth and slot coverage on imidacloprid runoff when the liquid formulation is applied with a stool-splitter tine implement. An additional runoff trial under overhead irrigation was set up in the Wet Tropics to test the efficacy of the StoolZippa™ to close the slot and reduce imidacloprid runoff losses when the product is applied at the correct depth of 100 mm. Results from the rainfall-simulation trials showed higher imidacloprid concentration in runoff from a shallow application at 50 mm compared to the recommended minimum 100 mm application depth. A press wheel reduced the imidacloprid concentration to nil when the product was applied at the correct depth of 100 mm; however, it slightly increased the concentration in the case of the shallow application. In the overhead-irrigation trial, the StoolZippa™ increased the imidacloprid concentrations in runoff versus the slot left open, but these concentrations were still extremely low and not of environmental concern. These trials indicate that ensuring the product is consistently applied at 100 mm depth is the best way to reduce imidacloprid loss via runoff when the product is applied with a stool-splitter tine implement. As trials were only conducted in loam soils at two locations, further trials are recommended over a range of soil types and geographic locations. Key words Imidacloprid, stool splitter, application, depth, slot