Abstract
This study compared water quality effects of using precision herbicide application technologies and traditional spraying approaches across several regulated ‘priority’ and alternative pre- and post-emergent herbicides in a northern Australian cane farming system. Use of herbicide banding spray technologies resulted in pre-emergent herbicide load reductions, extending substantially beyond simple proportionate decreases in the amount of herbicide ingredient applied to paddocks. Aquatic risk assessment from resultant chemical mixtures leaving paddocks, and upscaled to local catchment concentrations, highlighted that precision application technologies could markedly reduce the ecological risk of pre-emergent herbicides. These risk reductions were, however, often complicated by the additional toxicity of post-emergent herbicides in mixtures, some associated with the adoption of band-spraying weed treatments. While the currently regulated priority herbicide, diuron, posed the greatest risk to the environment, alternative herbicides could still pose significant environmental risks, although these relative risks were lower at more ecologically relevant concentrations, typically found in the local freshwater ecosystems. Results underline the need for a carefully considered approach to integrating alternative herbicides and precision application technologies into improved weed management by irrigating cane farmers. Recent government changes to the appraisal of water quality improvement progress, from load-based to ecosystem-based targets, involving a much broader suite of herbicides, also appear likely to complicate assessment of the environmental impacts of practice change adoption for the industry.
Highlights
The decline in fresh and marine water quality, associated with land-based runoff from adjacent agricultural catchments, is a major cause of the current poor state of many of the coastal ecosystems of Australia’s Great Barrier Reef (GBR) World Heritage Area [1,2,3]
The objective of this paper is to firstly evaluate the specific herbicide runoff dynamics of banded spraying practices of several priority and alternative herbicides, in comparison with conventional spraying practices at a commercial scale in a furrow irrigated farming system
Herbicide detections in the irrigation inflow water applied to the field and pre-spray soil samples were negligible, so runoff tailwater herbicide losses could be solely attributed to losses of recent herbicide applications to paddocks
Summary
The decline in fresh and marine water quality, associated with land-based runoff from adjacent agricultural catchments, is a major cause of the current poor state of many of the coastal ecosystems of Australia’s Great Barrier Reef (GBR) World Heritage Area [1,2,3]. The pesticides historically most commonly detected in the GBR catchment area (GBRCA) and marine environments are herbicides that inhibit electron transport at photosystem II (PSII) in plants, and include ametryn, atrazine, diuron, hexazinone, simazine and tebuthiuron [1,9,10]. Is sugarcane (Saccharum officinarum L.), with ~380,000 total ha being grown, predominantly in a strip within ~50 km of the Queensland coast [11] Due to their efficacy, convenience and cost-effectiveness, the Australian sugarcane industry has become reliant on PSII herbicides (predominantly as systemic, pre-emergent, “residual” herbicides, with continued activity in the soil for a period of time, reducing weed seed germination and/or growth in the soil). The herbicide diuron is traditionally widely used, but cane farmers utilise PSIIs, such as ametryn, atrazine, and hexazinone [12,13]
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