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Washoff of residual Photosystem II herbicides from sugarcane trash under a rainfall simulator Aaditi Dang, Mark Silburn, Ian Craig, Melanie Shaw, and Jenny Foley J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b04717 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 11, 2016

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Journal of Agricultural and Food Chemistry

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Washoff of residual Photosystem II herbicides from sugarcane trash under a rainfall

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simulator

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Aaditi Dang1, 2, 3, Mark Silburn1, 3, Ian Craig2, Melanie Shaw4, Jenny Foley1,

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[email protected]

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University of Southern Queensland, Toowoomba, Qld 4350 [email protected]

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Department of Natural Resources and Mines, 203 Tor Street, Toowoomba, Qld 4350

School of Civil Engineering and Surveying, Faculty of Health, Engineering and Science,

National Centre for Engineering in Agriculture, University of Southern Queensland,

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Toowoomba, Qld 4350

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Park Queensland 4102

Department of Natural Resources and Mines, EcoSciences Precinct, Boggo Road, Dutton

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ACS Paragon Plus Environment


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Abstract

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Herbicides are often applied to crop residues but their fate has not been well studied. We

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measured herbicide washoff from sugarcane trash during simulated rainfall, at 1, 8 and 40

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days after spraying (DAS), to provide insight into herbicide fate and for use in modeling.

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Herbicides included are commonly used in the sugar industry, either in Australia or Brazil.

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Concentrations of all herbicides and applied Br tracer in washoff declined exponentially over

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time. The rate of washoff during rainfall declined with increasing DAS. Cumulative washoff

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as a function of rainfall was similar for most herbicides, though the most soluble herbicides

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did have more rapid washoff. Some but not all herbicides became more resistant to washoff

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with increasing DAS. Of the total mass washed off, 80% washed off in the first 30 mm (~40

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minutes) of rainfall for most herbicides. Little herbicide remained on the trash after rainfall

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implying near complete washoff.

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Keywords: Great Barrier Reef, water quality, ametryn, tebuthiuron, hexazinone, atrazine,

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metolachlor, diuron, bromide

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Introduction

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Sugarcane is one of the largest intensive land uses in the Great Barrier Reef (GBR) catchment

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and is estimated to contribute 94% of the total load of photosystem II herbicide export in

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rivers from the GBR catchment1. These herbicides that inhibit functioning of photosynthesis

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at photosystem II in plants (PSII herbicides) have been identified as a concern for the GBR2,

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particularly considering their additive ecological effects in the marine environment 3. Over

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the last few decades the sugarcane industry in Australia has moved toward farming practices

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with reduced tillage and retention of crop residues on the soil surface (green cane trash

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blanketing (GCTB). These practices result in substantially reduced rates of soil erosion 4.

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However, they have also increased reliance on herbicides for the control of weeds 5.

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Herbicides applied to a field with GCTB will be intercepted by the crop residues. The

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herbicides need to be washed off the crop residues by rainfall (or overhead irrigation) to

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become active in the soil and perform their role in weed control. The presence of crop

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residues has been shown to be effective in enhancing infiltration compared to bare soil6.

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However, excessive rainfall could lead to runoff and herbicide movement off-site. Herbicide

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washoff can be affected by two major factors: (i) the sorption of the herbicide on the crop

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residues, and (ii) the half-life of the herbicide 7. Susceptibility to wash-off can decline with

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time 8 9 as the compound enters into or is more sorbed to the crop residue and also dissipates;

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the readily dislodgable fraction maybe be more easily dissipated. It is therefore useful to

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have some understanding of washoff with time after application.

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Much of the data currently available for washoff is for insecticides and knockdown

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herbicides on live plants due to an interest in rain-fastness, rather than herbicides on crop

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residues10. Wauchope, et al. 11 found that a majority of foliar chemical residues can be easily

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washed off if rainfall occurs within a few days after application.



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Estimating washoff and runoff of herbicides under a complex sequence of applications and

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rainfall events is the domain of models such as GLEAMS 12, RZWQM

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compiled databases including washoff parameters for many pesticides, leading to

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developments such as the Pesticide Properties Database (2009)16. However, the washoff

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parameters were seldom measured under the same conditions in all tests undertaken or using

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rainfall relevant to field conditions, and are often not measured for herbicides on crop

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residues. Use of locally derived parameters relevant to field conditions generally gives

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improved modeling results14, 15. Therefore, it is important to characterize herbicide behavior

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for the local conditions where they are used and need to be managed 17.

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This study was conducted to determine the concentrations of different herbicides in washoff

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from sugarcane crop residue (referred to as trash) during rainfall, as influenced by days after

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spraying (DAS), and to derive washoff parameters for modeling.

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Materials and methods

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In this study, we used a rainfall simulator to quantify washoff of six herbicides, including

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atrazine, ametryn, diuron, and hexazinone which are PSII herbicides commonly used in the

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GBR catchment

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because they are commonly used in the Brazilian sugar industry

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bromide (KBr) was used as a surrogate compound to study the washoff processes. As KBr is

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not subject to the degradation processes that affect herbicides, Br may provide a useful

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control against which washoff of other compounds can be compared.

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Treatments, rainfall simulator and experimental design

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and HowLeaky?14,

which require dissipation half-lives and washoff parameters. Thus modeling groups have

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. Tebuthiuron, also a PSII herbicide, and S-metolachlor were chosen


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. Initially, potassium

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Sugarcane trash from a crop (cv. KQ228) planted in the previous year in Bundaberg,

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Queensland, was obtained. The trash was air-dried gently to ensure that there was no stored

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moisture that could affect washoff. It was packed into porous square metal trays 7 cm deep

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and 0.56 m2 in area. Stainless steel mesh was used at the bottom of the trays to prevent loss

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of the trash through the open squares. The amount of trash used (280 g/tray or 5000 kg/ha) in

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each tray was calculated to provide 100% cover.

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We used an oscillating boom laboratory rainfall simulator based on a design by Loch et al.,

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2001

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trash trays. This simulator produced intermittent rain, with intensity regulated by the number

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of times per minute a spray passed over the tray 22. When small sample areas are wetted by

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simulated rain, a significant proportion of the rainfall may be lost over the plot boundary as

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splash, and the relative importance of this edge effect will increase as plot size is reduced 23.

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To minimize splash losses, we created an additional area of trash surrounding the target tray

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so the spray would still land in trash that was included in the analysis.

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The rain had a range of drop sizes with a mean drop diameter of 2.1 mm 23. All trays were

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exposed to two hours of rain at a constant intensity of 50 mm/h providing 100 mm of

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equivalent rain. This rainfall intensity was considered to be most suitable by examining the

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recurrence interval of different rainfalls in Mackay and Tully – two of Australia’s major

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sugarcane growing areas. A rainfall of 50 mm/h for one hour duration tends to occur at least

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once a year and a rainfall of duration of two hours tends to occurs once every two years.

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Rain water was used for rainfall simulation. Washoff water falling through the trash was

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collected into a Teflon tube where it could be sampled.

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with 2 flat fan Veejet 80100 nozzles set 1 m apart and positioned 2 m above the

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Initial tests were conducted with the control compound bromide as KBr at a rate of 100 kg/ha

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of Br (0.1 L/m2) with 1 L/ha of KBr sprayed, using a garden sprayer with multiple passes to



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ensure uniformity and homogeneous application onto the trash placed in the square metal

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trays. The trays were placed under the rainfall simulator at a slope of 5 degrees. The KBr

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testing was replicated four times to ensure consistency in results.

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Herbicides, with a range of physio-chemical properties (Table 1), were applied in a spray

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cabinet in commercial formulations at recommended rates for application in sugarcane (Table

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2). The sprayer moves horizontally to and fro to ensure that each tray received the same

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amount of each herbicide at the same rate. A total of 8 trays were sprayed, with each

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herbicide applied one by one. The sprayed trays were kept in a greenhouse until the rainfall

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events were applied. The simulated rainfall events were applied onto trash in duplicate either

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1 day after herbicide application, considered a worst case scenario, or 8 or 40 days after

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herbicide application. Therefore two trays were created for each testing period: control, Day

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1, Day 8 and Day 40 to ensure results consistency.

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Control testing

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Control trays with no herbicides or KBr were also run, with two replicates. Washoff water

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was analyzed for major cations and anions including K and Br and the herbicides used in the

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washoff study. The chemicals found in washoff from control trash samples were dominated

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by chloride, bromide, calcium and sulfate (data not shown). The concentration of bromide in

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washoff from the cane trash was only about 5.5 mg/L, which would have little effect on

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washoff results from the KBr-treated trays. No herbicides were detected in the washoff

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samples from the control trays.

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Sampling and analysis

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Washoff water samples from each tray and simulated rainfall event were collected at regular

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intervals throughout the hydrograph (6 times, between 1-2, 6-8, 12-15, 35-40, 70-80 and 90-


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100 minutes intervals) using Teflon pipe to ensure that there was no loss of herbicide

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Water samples for herbicides were collected in 1 L glass amber bottles with Teflon lined lids.

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Prior to each simulated rainfall event, four samples of cane trash were collected and

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composited for herbicide analysis. Similarly, trash samples were collected at four locations

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on the tray and composited following the simulated rainfall. Trash samples were stored in foil

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lined ziplock bags. All water and trash samples were chilled immediately following

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collection and transported to the laboratory on ice overnight.

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Water samples were analyzed using liquid chromatography–mass spectrometry (LCMS) at

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the Queensland Health Forensic and Scientific Services laboratory as described by Lewis, et

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al.

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Melbourne for analysis by ultra-high pressure liquid chromatography (UPLC).

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For preliminary experiments, K and Br concentrations were analyzed at the Soil and Water

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laboratory, Department of Science, Information Technology and Innovation, Brisbane.

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Bromide was analyzed by High Performance Ion chromatography

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analyzed using Inductively Coupled Plasma 26.

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Washoff data were fitted to an exponential equation used in RZWQM 13:

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.

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. Trash samples were sent to the Analytical Consulting Services (ACS) Laboratory in

Cr = Cr0 Frwo exp (-Prwo I ∆t)

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and potassium was

(1)

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where Cr is the mass per unit area of pesticide on the crop residue; Cr0 is the initial mass per

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unit area of pesticide on the crop residue; Frwo is the fraction of pesticide mass which is

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readily washed off; Prwo is the washoff coefficient (mm-1); I is the rainfall rate (55 mm hr -1)

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and ∆t is the time (hrs). Due to difficulties in attaining mass balance relative to the rates of

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application, Frwo was calculated by two methods. Firstly, Frwo1 was calculated from the

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masses of herbicide on the trash before and after rainfall. Secondly, Frwo2 was calculated as



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the ratio of total herbicide mass in washoff to the sum of the total herbicide mass in washoff

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plus the mass in trash after rainfall.

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Statistical analysis

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Herbicide washoff from the simulated rainfall events was analyzed by general analysis of

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variance (ANOVA) for a completely randomized design, using the statistical package

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GenStat Release 14 27. Herbicide washoff concentration data were fitted with an exponential

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equation and the cumulative mass of washoff was fitted using Eqn 1 using Sigma Plot 12.5.

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Results

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Bromide concentrations in washoff

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The concentrations of Br in washoff were initially very high, declining exponentially as a

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function of time (Figure ) and applied rainfall volume (data not shown). The relationship

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between bromide concentrations in washoff and time had a negative exponent of 0.53 with a

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constant determined by the initial washoff concentration. Thus Br washed off more rapidly

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than all of the herbicides (discussed below). The cumulative mass of Br washoff was 60-70

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percent of the amount applied with the majority of this washed off in the first 10 minutes or 7

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mm of rain.

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Herbicide concentration in washoff

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The concentration in washoff water underwent an exponential decay in concentration with

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time for all herbicides. In most cases, a large fraction (~ 80%) of the final amount of washoff

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occurred in the first 35-40 minutes of simulated rainfall for all three DAS studied. On

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average, a quarter of the herbicide washoff occurred in the first 8-10 minutes of rainfall

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(Figure ). In most cases, the washoff rate of all the herbicides for all three DAS decreased

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significantly with increasing time of simulated rainfall only for the first 35-40 minutes of


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simulated rainfall followed by a non-significant decrease to 90-100 min i.e. the concentration

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can be considered constant after 40 minutes of rainfall. Increasing DAS from 1 to 8 or to 40

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significantly (P

Washoff of Residual Photosystem II Herbicides ... - ACS Publications

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