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
]. Pesticide residues have been documented across virtually the entire continuum of GBR aquatic environments, including catchment irrigation drainage systems and waterways [4
]; estuaries [8
]; nearshore marine habitats [9
]; and coastal marine environments [1
]. 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
]. Improving the quality of water flowing from agricultural catchments is regarded as critical to increasing GBR ecosystem resilience to a growing range of pressures and supporting its recovery [2
The dominant intensively cultivated crop in Australia’s Great Barrier Reef catchment area (GBRCA) 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 particularly 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 also utilise PSIIs, such as ametryn, atrazine, and hexazinone [12
]. “Knockdown” herbicides, such as 2,4-D, paraquat, MCPA, fluroxypyr, and glyphosate, are also popular for post-emergent control (acting via contact with plants) in sugarcane [13
]. The identification of sugarcane as a major contributor to pesticide pollution in the GBR led in large part to the Australian and Queensland governments introducing an evolving package of legislation, extension and research, known as Reef Water Quality Protection Plans or ‘Reef Plans’. The various Reef Plans initially set specific targets to reduce end-of-catchment PSII herbicide loads of ametryn, atrazine, diuron, hexazinone, simazine and tebuthiuron by 60% by 2018 [14
]. Associated measures included new regulations on the allowed application windows and application rates for use of ametryn, atrazine, diuron, hexazinone, simazine and tebuthiuron (traditionally referred to as ‘priority’ PSII pesticides) within the majority of GBR sugarcane districts [16
]. Reef Plan targets were also recently and notably modified under the current Reef 2050 Water Quality Improvement Plan 2017–2022, from the simpler 60% PSII load-based reduction measure, to achieving herbicide concentrations at river mouths that protect at least 99% of aquatic species. This change was designed to provide more ecologically meaningful herbicide monitoring and management in the GBR catchment (acknowledging that PSIIs aren’t the only contributors to ecological risk), but also to align with approaches used in National, State and marine GBR water quality guidelines [17
]. Reef Plan pesticide focus has, accordingly, expanded substantially, now including a much broader range of 22 ‘priority’ pesticides (a combination of herbicides and insecticides), extending considerably beyond the customary PSII priority herbicide suite.
Due to its simplicity of use, and relatively low operating and capital costs, surface furrow irrigation accounts for the majority of Earth’s irrigated farmland across both developed and developing countries [18
]. Furrow irrigation is also a mainstay of Australia’s sugarcane industry, particularly in dry-tropical regions (the Burdekin and Mareeba/Dimbulah cane districts), which are almost totally reliant on full furrow irrigation of crops [21
]. Recent research into furrow irrigated sugarcane farming and precision herbicide application technologies in the GBRCA has highlighted significant reductions in herbicide loads, achieved via band spraying [23
]. Banded spraying is, however, a rapidly evolving technology, and its adoption can entail a range of practice changes relating to alternative herbicide product selection, often also requiring the application of additional herbicides in more complex mixtures to paddocks. Along with increased regulatory and monitoring focus on the popular Reef Plan ‘priority’ PSII herbicides, there have also been concurrent pressures for the industry to adopt an ‘alternative’ herbicide suite. Similar toxicological and paddock loss profiles of several alternative herbicides relative to recently regulated pre-emergent herbicides do, however, suggest the need for a carefully considered approach to integrating alternative herbicides into improved pest management for the sugarcane industry [13
]. The recent changes in Reef Plan in how to measure progress towards the new ecosystem protection targets, while sensible, will make evaluating the environmental benefits of herbicide practice change more complicated than the previous, and simpler, PSII load reduction targets.
GBRCA cane growers are, therefore, increasingly confronted with changes to regulatory agendas and product application rates and windows, new and often unfamiliar herbicide mixtures, uncertainties over the relative environmental impacts of practice change, and pushes towards precision application technologies [13
]. 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. Secondly, this paper attempts to quantify, in a more integrated capacity, the relative water quality impacts of the herbicide mixtures associated with band spraying runoff, aligning specifically with the new ecosystem protection-based appraisal on environmental benefits utilised in Reef Plan 2050 target setting and tracking.
The study outcomes largely align with similar recent research from the study area [23
], with the use of shielded sprayers (and herbicide banding) in furrow irrigated systems providing herbicide load reductions, extending substantially beyond simple proportionate decreases, in the amount of active herbicide ingredient applied to paddocks. These reductions are due to the extra management control available to irrigating growers in relation to where both specific herbicides and irrigation water can be applied to paddocks, a degree of control not available to growers in predominantly rain-fed cane farming systems. In rainfall reliant farming systems, where rainfall occurs across the entire paddock area, load reductions from banding are directly related to the associated reductions in the proportional area of paddock receiving herbicide coverage under band spraying [42
]. With furrow irrigation still being a dominant irrigation method on a global scale, the utility of band-spraying technologies offers significant potential for protecting downstream aquatic agro-ecosystems from tailwater runoff inefficiencies, and the agrochemical losses which are often inherent to this traditional irrigation method. Relatively cheap and easily manufactured dual herbicide band-spraying implements are increasingly being developed and utilised by GBRCA cane farmers [44
], that could also be adapted to other farming systems.
Pre-emergent herbicide loss dynamics in this study generally followed patterns that would be expected, on the basis of respective physico-chemical properties. All three pre-emergents have distinctly different modes of herbicide action; diuron—photosystem II inhibition; metolachlor—enzymatic inhibition of cell division and elongation; and imazapic—inhibiting the activity of the enzyme acetohydroxy acid synthase. All appear to act, however, by plant absorption, through either shoots and/or roots, and translocation through the plant [16
]. All three herbicides are relatively soluble (and persistent), with pre-emergent efficacy predicated upon plant uptake from soil pore water. This requisite combination of solubility, persistence (residual activity), and relatively low propensity for soil—organic matter sorption make all three pre-emergent herbicides prone to off-paddock movement in applied irrigation water (Table 5
). In contrast, paraquat, a herbicide with a dominant tendency for soil binding, was not detectable in paddock runoff, also aligning with previous monitoring from the district [7
]. It should be noted that the paraquat analysis employed here only detects the herbicide in its dissolved form, and losses of paraquat are possible in a sorbed form if significant amounts of soil are eroded during runoff events. Paraquat requires severe extraction procedures for laboratory determination of soil paraquat residues, and its bioavailability following significant erosive losses from paddocks is questionable [47
]. While not assessed in this study, with their low gradients and low erosive capacity, significant sediment losses are likely rare in lower Burdekin irrigated cane farming systems [48
While precision application technologies in some cases produced substantial reductions in edge-of-field toxicity, these toxicity decreases were not consistent across herbicides. Some of the greatest single reductions in PAF ratings were produced by changes in the pre-emergent herbicide applied. Results from edge-of-field suggest, for example, that even blanket application of the lower application rate imazapic delivered greater water quality benefits than band spraying of diuron. If their weedicidal efficacy is comparable to the higher application rate residual herbicides (i.e., diuron, metolachlor), changes in herbicide product selection alone could deliver some of the most significant water quality benefits for cane-growers. While not an exhaustive analysis, weed control efficacy assessments associated with this trial suggested comparable weed control across all herbicide treatments (unpublished data), and certainly no weed control failures. The almost identical load losses of metolachlor compared to diuron at edge-of-field, however, highlights important new issues for farmers to consider. Under the new, ecosystem-based, tracking and monitoring of progress, a much broader suite of herbicides will now face monitoring scrutiny. While in this case metolachlor seems to pose a lower ecosystem risk (at least relative to diuron), it could still leave paddocks at significant levels (almost identical to diuron) and present substantial ecosystem risk. A number of likely residual herbicide replacements to the traditional PSII suite (metolachlor, metribuzin, isoxaflutole, imazapic) are now captured specifically in PAF end-of-catchment monitoring, and their relative environmental dynamics have yet to be fully considered.
It should be noted that results from this trial (with EMCs and PAF calculated from concentrations in a single runoff event soon after application) also represent a ‘worst case’ water quality scenario, but one that is common under lower Burdekin irrigated cane farming practices in the first irrigation events following herbicide application [7
]. The intended use of ms-PAF at end-of-catchment scale GBR monitoring will be a considerably more conservative measure of pesticide ecosystem risk. The GBR ms-PAF metric calculates the risk posed by pesticide mixtures and expresses it as the percent of species affected for each water sample taken at a catchment monitoring site, and then, finally, as the average percentage of species that should be affected during the wet season (a standardised 182 day period) [26
]. Exposure is estimated over the course of a wet season, as this is when the majority of the rainfall and pesticide transport occurs, and aquatic organisms are exposed. Results from this current study instead represent a single ‘snapshot’ water quality event, not a longer-term appraisal of environmental risk. A useful area of research would be to increase our knowledge of the relationship between edge-of-paddock PAF and downstream risk assessments. The relatively recent registration of several newer herbicide products combining multiple pre-emergent herbicidal active ingredients (and modes of action) for use in sugarcane almost makes more holistic risk metrics, such as ms-PAF, a prerequisite for future assessments of the ecological benefits of practice changes.
While the upscaling of edge-of-field concentrations to catchment concentrations was admittedly crude, results did suggest that water quality improvements associated with practice change at more ecologically relevant concentrations were often variable between herbicides, and often affected significantly by the contribution of ‘knockdown’ herbicides included in mixtures. Broadcast diuron treatment concentrations, which were 100 times lower than the edge-of-field concentrations, still posed significant toxicity risks to large parts of the ecosystem (>40% PAF). Interestingly, the effects of diuron banding on water quality risk were more pronounced at these lower concentrations (reducing specific diuron PAF by 74%) than at edge-of-field. Banding of imazapic or metolachlor also markedly reduced the specific risk of these individual herbicides at projected catchment concentrations (81% and 68% PAF reductions, respectively), but produced minimal or even a slightly increased risk from total treatment mixtures, almost entirely due to the toxicity of MCPA at these lower concentrations. The risk profiles of knockdown herbicides (often regarded as fundamental to reducing industry reliance on the environmentally problematic PSII residual herbicides [13
]) have been rarely considered in paddock or catchment-scale appraisals of water quality risk and the likely benefits of practice change by cane growers. It should also be noted that glyphosate losses were associated with band spraying mixtures leaving paddocks (Table 4
), but could not be assessed using the currently available ms-PAF method. Edge-of-field EMCs for all treatments with glyphosate were, however, below the proposed 99% ecosystem protection guidelines for glyphosate of 140 g/L [38
], and assumed to have minimal impact on PAF results. Better quantification of the effects of lower concentrations of less well-studied herbicides, such as imazapic, MCPA and glyphosate, will undoubtedly improve future predictive understanding of the specific benefits of herbicide practice changes, as would understanding of changes to herbicide mixtures, particularly the utility of ‘knockdown’ herbicides typically associated with changes toward improved pesticide management.
A decade ago, the understanding of potential herbicide impacts in GBR aquatic environments, and relevant ecosystem protection water quality guidelines, was limited largely to a small suite of PSII herbicides [6
]. Substantial recent research, and investment in improvements in water quality monitoring, ecotoxicity data availability, risk assessment capacity, and broadened and improved water quality guidelines will no doubt improve current understanding, and ultimately future management, of herbicides in the GBRCA. Some early results of these new capabilities also suggest, however, that understanding the water quality impacts of on-ground practice change by farmers will be complex and technical. Results from this study highlight some of the new challenges now undoubtedly facing the cane industry, particularly with the recent changes from load-based reduction targets, focused on a specific suite of ‘priority’ PSII herbicides, to more environmentally relevant targets capturing the broader impact of a far greater range of the pesticides currently used in industry. Changes in the relative herbicide toxicity between edge-of-field and receiving ecosystem concentrations of different herbicide mixtures could also be complex and non-linear, complicating the appraisal of the ultimate benefits of on-farm practice change. How to best incorporate knockdown herbicides into management practices that produce genuine water quality improvements will also be a key research need. How to utilise new tools, such as ms-PAF, and the many more water quality guidelines to inform the industry of the likely environmental benefits of often costly practice change, particularly regarding how paddock-scale water quality improvements are translated to end-of-catchment scale, could be challenging from an industry engagement and extension perspective. The collective ecological impacts of the complex pesticide mixtures often applied to paddocks, rather than the historical focus on a limited range of PSII herbicides, will now have to be integrated into farmer decision-making regarding pest control strategies.
Long-term sugarcane industry aims should still focus on reducing long-term applications of all herbicides to paddocks, while still maintaining cost-effective weed control for growers [13
]. Future pesticide stewardship and weed management in the GBRCA will certainly require more integrated and strategic weed management systems that encompass minimised weed seed production and restricting the size of soil seed banks, particularly in the early crop cycle; improved farm hygiene; improved herbicide timing and application techniques, such as precision spraying; weed resistance management, incorporating rotational sequences and/or mixtures of herbicides with different modes of action within and between crop cycles; protection of the existing herbicide resource; and integration of non-herbicide weed management tools [13
]. One of the greatest emerging challenges for both industry, extension staff, government policy makers, and regulators is ensuring the requisite technical, environmental and extension support is available for growers to make informed decisions in what is now a very dynamic, monitored, complex and regulated herbicide management environment.