Precision Herbicide Application Technologies To ... - ACS Publications

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Precision Herbicide Application Technologies To Decrease Herbicide Losses in Furrow Irrigation Outflows in a Northeastern Australian Cropping System Aaron M. Davis* and Jordan Pradolin Centre for Tropical Water and Aquatic Ecosystem Research (TropWATER), James Cook University, Townsville, Australia ABSTRACT: This study compared water quality benefits of using precision herbicide application technologies in relation to traditional spraying approaches across several pre- and postemergent herbicides in furrow-irrigated canefarming systems. The use of shielded sprayers (herbicide banding) provided herbicide load reductions extending substantially beyond simple proportionate decreases in amount of active herbicide ingredient applied to paddocks. These reductions were due largely to the extra management control available to irrigating growers in relation to where both herbicides and irrigation water can be applied to paddocks, coupled with knowledge of herbicide toxicological and physicochemical properties. Despite more complex herbicide mixtures being applied in banded practices, banding provided capacity for greatly reduced environmental toxicity in off-paddock losses. Similar toxicological and loss profiles of alternative herbicides relative to recently regulated pre-emergent herbicides highlight the need for a carefully considered approach to integrating alternative herbicides into improved pest management. KEYWORDS: PSII inhibitors, ecotoxicity, Great Barrier Reef, precision agriculture

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mental impacts.14 The water quality benefits of band spraying have to this point, however, focused only on comparative load reductions of pre-emergent herbicides (i.e., the relative masses of specific active ingredient discharged in paddock runoff per hectare of crop land), with little quantification of the overall impacts of the often more complex herbicide mixtures associated with their use. In addition to nutrients and pesticides, water is one of the critical inputs to modern agriculture. Despite significant advancements in more efficient irrigation techniques and technologies, surface irrigation, mostly furrow irrigation, accounts for >60% of Earth’s 240 million irrigated hectares.15,16 Due to its simplicity and low capital cost, furrow irrigation is preferred particularly in many developing Asian and African countries17 and also in developed countries such as the United States, Europe, and Australia.18 Recent research in furrowirrigated farming systems highlighted significant reductions in herbicide loads via banding, substantially beyond simple proportionate decreases due to reductions in sprayed paddock area.13 Although results demonstrated the potential of shielded sprayers for minimizing migration of environmentally problematic herbicides in furrow-irrigated sugar cane, more realistic assessments of comparative environmental risks and integration of banding within the broader herbicide suite being used in the cane industry remain limited. Banded spraying is a rapidly evolving technology, and its adoption can entail a range of practice changes relating to alternative herbicide product selection and application of

INTRODUCTION Global agricultural expansion is currently dominated by development in tropical and subtropical regions, expansion that is likely to exert particularly heavy pressures on aquatic ecosystems.1,2 Because herbicides have become an integral part of modern agricultural systems, off-site transport of agricultural chemicals, such as herbicides, into freshwater and marine receiving environments is a worldwide concern. Herbicides are an established or emerging threat to freshwater3−5 and estuarine and nearshore marine habitats6−9 across much of the word’s tropical zone. Technologies that provide ecologically efficient food production and greater efficiencies for key farm inputs such as pesticides are a key priority for not only tropical regions2 but modern agriculture in general. In contrast to traditional broadcast applications of preemergent, residual herbicides in many crops, where herbicides are applied across the entire crop rows and inter-rows (resulting in 100% herbicidal coverage of the paddock), herbicide applications can be reduced by restricting application to a band across the top of the crop row. Although band spraying is not a new concept, economic and environmental drivers, particularly the emergence of precision farming techniques such as improved sprayer application technologies and weed sensing capacities, global positioning systems (GPS), and other guidance techniques have helped promote band spraying as a practical alternative tool in more sustainable pesticide use strategies.10 Some band sprayer approaches utilize banding on the crop row in conjunction with inter-row mechanical hoeing to remove weeds.11,12 Dual spray line systems have been recently developed to simultaneously apply two tank mixes at the same time, utilizing shielded sprays for herbicide application within the row in combination with a second simultaneous spray over the row with a separate spray line.13 In these dual applications, selection of herbicides on the basis of physicochemical properties can be used to minimize environ© XXXX American Chemical Society

Special Issue: Pesticide Fate and Effects in the Tropics Received: October 15, 2015 Revised: January 18, 2016 Accepted: January 21, 2016

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DOI: 10.1021/acs.jafc.5b04987 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry

cups located at the head of paddocks. Paddock tailwater is in some cases recaptured and recycled or otherwise leaves the farm via drainage channels. Herbicide treatments were a conventional broadcast application of herbicides across the beds and furrows from boom sprayers (conventional) and a banded application using a shielded sprayer where certain herbicides were applied to furrows (inter-rows) only and other herbicides to raised beds (rows) only (replicated two to three times). The shields were 900 mm wide, and the over-top sprays were 620 mm wide (Figure 1). Details of the herbicides applied are given in Table 2. The herbicides studied were chosen because they are commonly used in commercial sugar cane production in the Burdekin district and are relevant to local weeds, farming systems, and stage of crop cycle. Atrazine, a PSII herbicide currently regulated under Reef Plan,21 is one of the most widely used pre-emergent herbicides for residual weed control in the lower Burdekin and broader Australian sugar cane industry.20 Metribuzin is another PSII residual herbicide recently touted as a potential replacement for the currently regulated PSIIs in the sugar industry and is seeing increasing use across the region. Glyphosate, 2,4-D, and paraquat are postemergent (“contact” or “knockdown”) herbicides considered vital in industry sustainability shifts away from the traditional reliance on PSII herbicides.20 In the banded farming scenario, the residual (pre-emergent) herbicide (such as atrazine or metribuzin) is applied to the raised crop row and has limited contact with the applied irrigation water flowing in the furrow (crop inter-row). Herbicides with particular chemical properties that reduce environmental risk (such as shorter persistence, high soil binding affinity, or lower toxicity to nontarget organisms) are banded into the furrow where irrigation water travels (Table 3). Paraquat and 2,4-D can be applied over the entire paddock, whereas glyphosate (which will damage growing cane) can be applied under shields to the furrow, limiting contact with the crop. Also tested were both high-rate (label rate) and low-rate atrazine applications. Low-rate (“spike”) applications of atrazine well under label rate recommendations are a common practice for many lower Burdekin canegrowers, with the intent of “hotting-up” or improving the weed control efficacy of knockdown, postemergent formulations. All herbicide treatments were applied in November (6:30−10:00 a.m.) to designated blocks and irrigated simultaneously commencing 2 days after application (as per herbicide product labeling recommendations). Measurements. Water samples were collected as soon as tailwater flow commenced through pipes at the bottom end of paddocks (time 0) and then at the following time intervals: 0.5, 1, 2, 4, 6, 8, 10, and 12 h thereafter. All water pesticide samples were manually sampled into 1 L amber glass bottles for all herbicides except paraquat (collected in additional plastic bottles), and collected samples were then refrigerated at 4 °C until laboratory analysis. Water volumes leaving treatments was monitored manually by timing how long it took to fill a 20 L bucket at various time intervals throughout the tailwater flow event (ca. 30 volume measurements per event). This technique has been demonstrated to have good agreement with alternative methods previously used at the site such as installation of V-notch weir crates designed to cope with low flow discharges.13 Irrigation volumes applied to treatments were calculated by monitoring inflow rates to paddocks from fluming cups across the treatments over the course of the irrigation event (time taken to fill a 10 L bucket). Samples of irrigation inflow water were also collected from fluming cups prior to paddock application to test for herbicide presence in irrigation water. Herbicide Analyses. Analysis of concentrations of herbicide active ingredient in paddock runoff water was performed using liquid chromatography−tandem mass spectrometry by ACS laboratories (Kensington, Victoria, Australia) using in-house methods. Glyphosate and its metabolite (or degradation product) AMPA were analyzed using method ACS-AM-TM-029 (analytical detection limit of 1.0 μg L−1); 2,4-D was analyzed using method ACS-AM-TM-201.1 for “acid herbicides” (analytical detection limit of 2.0 μg L−1); atrazine and metribuzin were analyzed using method ACS-AM-TM-201 for “nonacid herbicides” (analytical detection limits of 0.5 and 2.0 μg L−1, respectively); and paraquat was analyzed using method ACS-AM-

additional herbicides in more complex mixtures to paddocks. The objective of this paper is to first evaluate the specific herbicide runoff dynamics of banded spraying practices, in comparison to their conventional counterparts in a furrowirrigated farming system and, second, to also quantify in a more integrated capacity the comparative water quality impacts of the more complex herbicide mixtures associated with band spraying, particularly in relation to their physicochemical and toxicological properties. Quantifying the relative runoff risks of herbicides and their scope for integration with precision application technologies and surface irrigation will provide valuable insights not only for the development of improved pesticide management strategies in the Queensland sugar industry but also for comparable farming systems globally.

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MATERIALS AND METHODS

Study Area; Trial and Experimental Design. Sugar cane (Saccharum officinarum L.) is the dominant crop in Australia’s Great Barrier Reef (GBR) catchment area (∼380 000 ha;19), with production particularly reliant on a variety of pre-emergent herbicidal applications.20 The identification of sugar cane as a major contributor to pesticide pollution in the GBR led in large part to the Reef Water Quality Protection Plan 2013 (Reef Plan) introducing targets to reduce end-of-catchment herbicide loads of several pre-emergent photosystem II inhibiting (PSII) herbicides (diuron, atrazine, hexazinone, and ametryn) by 60% by 2018.20,22 A recent package of legislation, extension, and research in 2009 known as the Reef Water Quality Program has been implemented to aid achievement of these targets.21 This includes the regulated use of diuron, atrazine, hexazinone, and ametryn (regulated PSII pesticides) on sugar cane properties in the GBR catchment area, where users of the regulated herbicides have to follow specific requirements including training, restrictions on use near water bodies, and revised allowable application rates. The use of precision herbicide application technologies is also attracting increasing research and funding attention as a strategy to meet Reef Plan targets.13,14,23 Shielded sprayer trials were conducted on a commercial sugar cane farm in the Mulgrave district of the lower Burdekin floodplain, a longterm study site in one of northern Australia’s largest sugar cane growing regions.13 Soil type on the trial paddock is a gray vertosol throughout the profile (Table 1), and the site had been laser leveled

Table 1. Properties of the Surface Soil at the Study Site soil physicochemical property type-texture depth coarse sand, fine sand, silt, and clay fractions soil pH cation exchange capacity field capacity electrical conductivity (1:5 extract, dS m−1) soil organic carbon (%)

value gray clay (medium heavy clay; type 2Ugh) 0−0.1 m 1:24:17:59 6.9 29 mequiv 100 g−1 0.35 (m3 m−3) 0.06 1.00

prior to raised bed formation (slope of 1 m/1000 m). Paddock row widths (center to center) were ∼1.52 m, with each study replicate block consisting of six rows and six raised beds (910 m long × 9.12 m wide), giving a total treatment area of 0.83 ha. Each end of the drainage channel for each bay was blocked off so the water draining from each bay was isolated and constrained to flow out through a pipe. Due to the region’s dry-tropical climate, annual irrigation inputs in the region are typically around 2000 mm per crop.18 Irrigation in the region, including on the study farm, is almost entirely applied by furrow systems involving lay-flat surface fluming and flow controlling B


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Figure 1. (Top left) Conventional boom sprayer widely used across GBR sugar cane growing for broadcast herbicide application across entire paddock where the herbicide spray covers the entire paddock area below the canopy (top right) and shielded dual herbicide sprayer (bottom) for banded herbicide application. Note the hood (bottom right), which restricts glyphosate application to inter-row (furrow) where irrigation water flows.

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TM-549.2 (analytical detection limit of 2.0 μg L−1). In-house methods were based on established methods.24−28 Statistical Analyses. Herbicide loads (active ingredient) were calculated from continuous time series discharge data and discrete point source water quality data collected from each treatment by linear interpolation using the Water Quality Analyzer program (version 2.1.2.4; eWater CRC, 201429). To more holistically compare potential environmental impacts of pesticide application strategies, an integrated “toxic equivalency quotient” (TEQ) for combined herbicide loadings leaving each treatment was also calculated.30,31 This relative toxicity measure (calculated relative to diuron, generally regarded as the most environmentally problematic herbicide in the GBRCA), was recently estimated for all sugar industry herbicides commonly used in the Great Barrier Reef (many of which have no relevant Australian water quality guideline data). The relative toxicity measure was developed using EC50 data for population growth and abundance end points over chronic exposure times (>72 h) collated from the Pesticide Ecotoxicity Database (PED), which includes all EPA-reviewed ecotoxicity end points for pesticides registered in the United States.32 Equivalency factors were calculated for aquatic plant species only, because all products included in the current study are herbicides, and based on the median value of all valid equivalency factors taken. To quantify combined total TEQ herbicide loads, the herbicide active ingredient loads leaving each treatment (g ai ha−1) were multiplied by their relevant TEQ, and individual toxic loads of all herbicides lost from each treatment were summed into a single index. Differences in mean herbicide loads and total toxic loads between conventional (broadcast) and banded treatments for each herbicide were performed using Welch’s t test for unequal variances.33 Due to the potential for runoff variations between treatments to confound results, relationships between pesticide load losses and runoff volumes from treatments with three or more applications of the same herbicide rate (i.e., 2,4-D, glyphosate, and atrazine (banded high rate)) were tested using linear regression. All statistical analyses were conducted using R.34

RESULTS AND DISCUSSION Irrigation Tailwater Outflow Analyses. Monitoring of treatment irrigation inflow volumes documented ∼53 mm (0.53 ML ha−1) of irrigation was applied to each treatment during the monitored irrigation event. The total volume of water leaving each irrigation bay is given in Table 4 and ranged from ∼6 to 19 mm. Irrigation Runoff Losses of Applied Herbicides. None of the monitored herbicides were detected in the irrigation water applied to the field, so all tailwater losses could be solely attributed to losses of herbicide applied to paddocks. Losses of atrazine and metribuzin applied conventionally (100% paddock coverage) ranged between ∼4 and 6% of active ingredient applied (Table 5), loss rates typical for pre-emergent herbicides in similar irrigation and application scenarios from the region.4,13 Banding produced significant load loss reductions of >80% for both atrazine (high rate; p < 0.001) and metribuzin applications (p < 0.05), despite only ∼60% less paddock area being treated compared to conventional broadcast applications for both herbicides. Further significantly (p < 0.05) lower atrazine load reductions (additional ∼50% decreases) were documented in the low-rate banded atrazine application consistent with the reductions evident with atrazine banding at higher rates and a lower application rate. Losses of metribuzin under both banded and broadcast applications were not dissimilar to those of atrazine, whether relating to total loads or proportionate losses. 2,4-D losses across all treatments were the most variable of monitored herbicides, ranging from ∼2 to 8% of applied product. Despite consistent application rates across all treatments, 2,4-D was lost at significantly lower rates from the atrazine banded low-rate treatment than the broadcast C


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Table 2. Details of Herbicide Treatments, Application Rates, and Amounts of Active Ingredient (ai) Applied for Each Trial treatment

product

chemical

product application rate (kg or L ha−1)

placement

% ai

ai g ha−1

% area of paddock sprayed

atrazine (broadcast)

Atradex Gramoxone Amicide 700

atrazine paraquat 2,4-Db

1 1.4 1

broadcast broadcast broadcast

0.9 0.25 0.70

900 350 700

100 100 100

atrazine banded (high rate)a 1 replicate

Atradex Gramoxone Glyphosate 450 Amicide 700

atrazine paraquat glyphosate 2,4-Db

1 1.4 1.5 1

onto row (hill) onto row (hill) under shield (into furrow) onto row and under shield (into furrow)

0.9 0.25 0.45 0.70

367 143 400 700

41 41 59 100

atrazine banded (high rate)a 2 replicates

Atradex Gramoxone Round-up attack Amicide 700


1 1.4 1.5 1


0.9 0.25 0.57 0.70

367 143 504 700

41 41 59 100

atrazine banded (low rate)

Atradex Gramoxone Glyphosate 450 Amicide 700


0.5 1.4 1.5 1


0.9 0.25 0.45 0.70

184 143 400 700

41 41 59 100

metribuzin (broadcast)

Soccer Gramoxone Amicide 700

metribuzin paraquat 2,4-Db

0.7 1.4 1

broadcast broadcast broadcast

0.7 0.25 0.70

490 350 700

100 100 100

metribuzin (banded)

Soccer Gramoxone Glyphosate 450 Amicide 700

metribuzin paraquat glyphosate 2,4-Db

0.7 1.4 1.5 1


0.7 0.25 0.45 0.70

200 143 400 700

41 41 59 100

a

Note slightly different applications of glyphosate in this treatment. b2,4-D present as dimethylamine and monomethylamine salts.

Table 3. Physicochemical and Toxicological Propertiesa for Herbicides Utilized in the Trial herbicide

CAS Registry No.

DT50 (typical)

solubility in water at 20 °C (mg L−1)

KOC

acute 7 day EC50, biomass (mg L −1)

acute 72 h EC50, growth (mg L−1)

diuron equivalencyb

atrazine metribuzin 2,4-D paraquat glyphosate diuron

1912-24-9 21087-64-9 94-75-7 1910-42-5 1071-83-6 330-54-1

75 11.5 10 3000 12 75.5

35 1165 23180 620000 10500 35.6

100 95 88.4 1000000 1435 813

0.019 0.008 0.58 0.037 12 0.0183

0.059 0.02 24.2 0.00023 4.4 0.0027

0.15 0.12 0.011 0.002 0.001 1

Chemical and toxicological properties compiled using the University of Hertfordshire “Footprint” Pesticide Properties Database 2.0 version: June 2013, http://sitem.herts.ac.uk/aeru/ppdb/en/index.htm (May 14, 2015). bDiuron toxicity equivalent29,30 a

application, but not the banded atrazine high-rate treatments. There was no significant difference in average 2,4-D losses from any metribuzin treatments. Glyphosate losses ranged between ∼1 and 5% of active ingredient applied to paddocks. Paraquat was not detected leaving in any water samples collected from treatments. There were no significant relationships (p > 0.05 for all linear regressions) between treatment runoff volumes and load losses of 2,4-D, glyphosate, or atrazine (banded high rate). This suggests variability in runoff volumes had minimal consistent effect on the load variations between treatments, which could instead be largely attributed to differences in herbicide application methods. Although explicit comparison is confounded by different application rates for the various applied herbicides, herbicide loss dynamics generally followed patterns expected on a priori

Table 4. Total Runoff Volumes from Each Herbicide Treatment treatment replicate 1: 2: 1: 2: 1: 2: 3: 1: 2: 1: 2:

atrazine banded (low rate) atrazine banded (low rate) atrazine (broadcast) atrazine (broadcast) atrazine banded (high rate) atrazine banded (high rate) atrazine banded (high rate) metribuzin (broadcast) metribuzin (broadcast) metribuzin (banded) metribuzin (banded)

runoff (mm) 11.0 6.4 9.5 8.9 11.0 13.0 18.5 14.4 8.6 11.0 8.3 D


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Table 5. Herbicide Runoff Data for Monitored Treatments Presented as Total Load Losses and Percentages of Applied Herbicide Lost from Paddocks in Surfacewater Runoff treatment replicate 1: atrazine banded (low rate) 2: atrazine banded (low rate) 1: atrazine (broadcast) 2: atrazine (broadcast) 1: atrazine banded (high rate) 2: atrazine banded (high rate) 3: atrazine banded (high rate) 1: metribuzin (broadcast) 2: metribuzin (broadcast) 1: metribuzin (banded) 2: metribuzin (banded)

2,4-D runoff load (g ai ha−1)

2,4-D (% applied lost)

atrazine runoff load (g ai ha−1)

atrazine (% applied lost)

glyphosate runoff load (g ai ha−1)

glyphosate (% applied lost)

metribuzin runoff load (g ai ha−1)

metribuzin (% applied lost)

14

2

3

2

4

1

15

2

3

1

6

2

37

5

32

4

39

6

32

4

22

3

6

2

6

2

25

4

6

2

15

3

33

5

8

2

25

5

53

8

25

5

27

4

27

6

22

3

6

2

5

3

47

7

6

2

4

3

Figure 2. Variance analysis for residual herbicide surfacewater loads (g ai ha−1) leaving each irrigation treatment (left) and relative overall toxic loads (right). Treatments: 1, atrazine broadcast; 2, atrazine banded (high rate); 3, atrazine banded (low rate); 4, metribuzin broadcast; 5, metribuzin banded.

knowledge of their respective physicochemical properties (see Table 3). The relatively soluble atrazine and metribuzin (for which pre-emergent efficacy is predicated upon plant uptake from soil pore water) and also 2,4-D, which all have relatively low propensity for soil-organic matter sorption, were all prone to off-paddock movement in applied irrigation water. Glyphosate, with one of the higher soil-binding capacities, was generally lost in lower proportions of product applied. Paraquat, a herbicide with a dominant tendency for soil binding, was not even detectable in paddock runoff, aligning with previous monitoring from the district.4 It should be noted that the paraquat analysis employed here detects the herbicide only in dissolved form, and losses of paraquat are possible in a sorbed form if significant amounts of soil are eroded during runoff events. Due to its ionic character, paraquat is readily adsorbed onto mineral particles, especially in the expanding lattice structure of swelling clays such as found at this study site.35 This necessitates severe extraction procedures for laboratory determination of soil paraquat residues, and its

bioavailability following even significant erosive losses from paddocks is questionable.35 Although not assessed in this study, with their low gradients and low erosive capacity, significant sediment losses are rare in lower Burdekin irrigated canefarming systems.4 Study results were not as pronounced as the ∼90% average comparative load reductions between banded and conventional applications of postemergent herbicides previously documented under similar practices.13 These differences (and possibly the variability in 2,4-D and glyphosate losses between some treatments) might be explained by paddock and runoff variations between study periods. Runoff volumes from treatments in this study ranged between ∼6 and 14 mm and encompassed more treatments. Runoff volumes were on average higher, but less variable, in previous site studies,13 ranging between 13 and 19 mm. The cooperating farmer noted that wet harvesting operations preceding this most recent study had produced considerable bed profile damage (and variable compaction) across the trial paddock, which may have E


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approaches comparing herbicides with common modes of action (such as PSII inhibition) have been demonstrated in several studies.41−43 However, these assumptions have not been tested for the remaining herbicides used in this study (glyphosate, 2,4-D, paraquat) that utilize different modes of action, and it is unlikely that a simple concentration addition model will fully explain mixture toxicity. However, concentration addition has been found to be a conservative measure in the absence of information on the interactions between different modes of action with relatively small likelihood of underestimating effects.44 Therefore, although this measure represents the best currently available assessment, there is a pressing need for future research to investigate mixture toxicity of herbicides relevant to the GBR on locally important species. Study outcomes largely align with similar recent research from the study area,13 with 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 herbicides and irrigation water can be applied to paddocks, a degree of control not available to growers in predominantly rainfed 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 proportional area of paddock receiving herbicide coverage.14,23 Despite more complex herbicide mixtures being applied to (and leaving) paddocks under banded applications, knowledge of herbicide physicochemical and toxicological properties can be used maximize water quality benefits. Although band-spraying technologies can be expensive in terms of construction and components, relatively low-cost band spraying technologies have recently been developed29 that could make precision technologies much more accessible to landholders in both developed and developing countries.

introduced some confounding effects (due to altered paddock runoff dynamics) on the direct comparison of results between different study periods. Toxic Load Losses of Applied Herbicide Mixture. Due to the lower toxicity of the postemergent (“knockdown”) herbicides, the significant load reductions evident in postemergent herbicides leaving banded treatments were similarly reflected in overall toxic loads leaving all treatments. Significant reductions were evident in herbicide mixture toxic load leaving banded treatments compared to conventional broadcast treatments for both atrazine and metribuzin (Figure 2). This suggests significant overall water quality improvements associated with banded spraying, despite the more complex herbicide mixtures leaving the shielded sprayer treatments. Toxicity load results are, however, also worth further discussion in the context of the changing face of herbicide products (product selection) used by GBR canegrowers. Recent desktopbased risk analyses of herbicides used in the Queensland sugar industry20 suggest a range of alternative herbicides to regulated PSIIs may pose similar environmental risks to regulated PSIIs. Metribuzin, for example, has been suggested as a viable alternative herbicide for GBR canegrowers desiring to shift away from continued reliance on now regulated PSII herbicides (diuron, atrazine, hexazinone, and ametryn) for residual control weeds. Metribuzin (itself a PSII herbicide) has a physicochemical and toxicological profile very similar to those of several of the regulated PSIIs (Table 3), with a recent risk assessment suggesting its environmental risk profile is very similar to those of the PSII herbicides it is touted to replace.20 Although not as toxic as diuron, metribuzin presents an ecotoxicological profile similar to that of atrazine.20 Study results here suggest that while metribuzin offers an alternative to regulated PSIIs in terms of weed control, its very similar loss dynamics from paddocks to that of atrazine suggest product shifts on purely environmental grounds may not be warranted (or at least require more refined ecotoxicological justification). Similarly, the documentation of glyphosate loss from paddocks (a non-PSII alternative that is now a cornerstone of global no-till agriculture and the world’s most widely used herbicide36) was noteworthy. Despite the reported strong sorption to soils and short soil half-life,37 researchers are nevertheless reporting frequent detections of glyphosate, and its main metabolite AMPA, in paddock runoff and in catchment waterways around the globe.38 Although not routinely screened in GBR catchment monitoring programs, the significant losses of glyphosate documented in this (ranging between ∼1 and 5% of ai applied) and other studies suggest further assessment of its environmental fate may be warranted. Recent research from the United States suggests the watersheds most at risk for the offsite transport of glyphosate are those with high application rates, rainfall, or irrigation that results in overland runoff and a flow route that does not include transport through the soil,38 all factors relevant to the GBR’s tropical cane farming context. Increasing cognition of greater than previously anticipated environmental impacts of glyphosate exposure (both the active ingredient but particularly the formulated product) in aquatic environments39 further highlights the future need to optimize glyphosate usage. Interactions between farming practices such as fertilizer application and glyphosate losses may also offer avenues to mitigate losses.40 The TEQ approach to quantifying impacts of overall herbicide mixtures leaving treatments would also similarly benefit from refinement. Consistent predictive success in TEQ

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AUTHOR INFORMATION

Corresponding Author

*(A.M.D.) E-mail: [email protected]. Author Contributions

A.M.D. provided advice on the experimental design, sampling, and analytical protocols, coordinated operations and personnel on- and off-site, collected and managed water sampling, and wrote the manuscript. J.P. coordinated operations and personnel on- and off-site, collected and managed water sampling and sample transport, completed initial processing and quality control of the data, and provided minor editing and consulting post activities. Funding

This work was funded by the Australian government as part of the Paddock to Reef Integrated Monitoring, Modeling and Reporting Program. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for assistance from co-operating landholders for hosting the on-farm study, maintaining crop management records, and providing ongoing support for the project. We F


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thank several anonymous reviewers for feedback that greatly improved an earlier version of the manuscript.

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NOTE ADDED AFTER ASAP PUBLICATION This article published January 27, 2016 with incorrect values in the last column of Table 3. The correct values published February 5, 2016.

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