Stephen Lewis1, Mark Silburn2, Melanie Shaw2, Aaron Davis1, Dominique O'Brien1, Danni Oliver3, Jon Brodie1, Jenny Andersen3, Rai Kookana3, Emilie Fillols4, Sam Rojas-Ponce2, Jack McHugh5, Craig Baillie5
Few studies have quantified the water quality (and agronomic) benefits of improved management practices (e.g. spot spraying, hooded/shielded application) largely aimed at reducing the loss/use of 'residual herbicides' from sugarcane paddocks. In the Great Barrier Reef catchment area such work is critical to examine whether water quality targets in Reef Plan 2013 (i.e. a 60% reduction in pesticide loads to the Great Barrier Reef) are achievable under what are considered 'current best practice' or 'aspirational practices'. Another key consideration is the evaluation of the purported 'new/alternative' herbicides that have become available to the sugarcane industry as 'softer' alternatives to diuron and atrazine. In this study, a series of spot spray rainfall simulation trials were conducted across the Burdekin and Mackay Whitsunday regions to examine the potential of improved spray technology (i.e. spot spraying using weed seeking technology, banded/shielded spraying) to reduce the surface runoff losses of herbicides from the cane paddocks. The effectiveness of banded/shielded spray application was also examined on a commercial furrow-irrigated paddock in the Burdekin region. Finally, the data are used to examine the relative risk of the key herbicides (diuron, atrazine, ametryn, hexazinone, metribuzin, metolachlor, pendimethalin, isoxaflutole, imazapic, glyphosate, paraquat, 2,4-D, fluroxypyr) used in the sugarcane industry based on their relative toxicity and their runoff in the dissolved phase from paddocks.
The results show that the application of less herbicide on the paddock translated to a proportional reduction in the surface runoff losses of herbicides from the paddock. While this result was expected, it demonstrated that marked reductions in residual herbicide application could be made (without any detrimental impact on the industry) and that the current Reef Plan targets are achievable under best practice scenarios. The data also provide insights on how the different herbicides runoff from a paddock with the vast majority being lost in the dissolved phase. Exceptions such as pendimethalin and imazapic were predominately transported attached to particles, while glyphosate, 2,4-D and diuron all had some affinity (~ <20%) for the particulate phase. Hence, the herbicides with the affinity for particulates could be managed by controlling soil loss from the paddock, while management of the hydrology of the paddock (i.e. reducing the amount of water leaving the paddock) could reduce losses for the herbicides transported in the dissolved phase. Another trial examining the effectiveness of banded spraying in the Burdekin region was conducted to determine the reduction of PSII inhibitor herbicides (atrazine and diuron) where furrow irrigation was applied to the paddock. These results showed that for these irrigation systems diuron and atrazine loads leaving the paddock could be reduced by ~ 90%. If this practice was widely adopted across the Burdekin region, the results suggest that freshwater ecosystems such as Barratta Creek would benefit greatly with concentrations maintained below ecosystem protection guidelines for most of the year (as opposed to concentrations being above guidelines for ~ 6 months).
The relative risk analysis suggests that, for the herbicides examined in the rainfall simulation trials, the alternative/new herbicides have a generally lower risk than diuron. However, trifloxysulfuron sodium (Krismat), s-metolachlor (Dual Gold, more toxic than metolachlor) and the metabolite of isoxaflutole (Balance) (DKN) need further analysis before we can have full confidence in these results. Additional Great Barrier Reef toxicity data would also help confirm our model. This preliminary analysis suggests that provided the residual herbicides are applied with banded/shielded sprayers on the raised bed or hill (50 to 60% less than what would be applied as a broadcast application), residual herbicides can be adequately managed in the sugarcane industry with far less impact on the Great Barrier Reef.