Chapter 10
Comparison of Foliar Dissipation and Turf Dislodgeable Residue Sampling Techniques
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J. E. Cowell, S. A. Adams, J. L. Kunstman, and M. G. Mueth The Agricultural Group of Monsanto Company, St. Louis, MO 63198
Several studies have been performed to evaluate foliar dissipation of turf applied chemicals and techniques for estimation of dislodgeable residues of these compounds. The experimental design and conduct of these studies are described herein. A number of parameters that have an effect upon foliar dissipation and reentry exposure have also been examined. Mass balance,turf intercept,turf density,and climatic conditions of foliar applications of two example compounds were investigated. Studies evaluating the difference in surface residues versus total (including endogenous) residues for a particular herbicide were performed. Data showing the effect of formulation type upon residue dislodgeability were compiled and several experiments comparing dislodgeable residue techniques and different types of dislodgeable sampling media were conducted. The goal of these experiments was to identify a procedure for realistic estimation of reentry exposure.
In the development of a new turf maintenance chemical, there are numerous human and environmental safety factors which need to be studied and evaluated prior to the actual commercialization decision. Hazard evaluations are made from results of toxicological studies. Exposure assessments usually based on field studies are then needed to complete the estimate ofriskto the subject. This area of study which traverses human and environmental safety from the standpoint of exposure is identified as "Reentry Protection" by the Environmental Protection Agency in Subdivision Κ of the Pesticide Assessment Guidelines (1). This Subdivision gives guidance in the conduct of studies which measure foliar dissipation and dislodgeable residues of pesticides. Numerous studies have been undertaken to evaluate exposure to humans entering and working in areas recently treated with pesticides (2-7). These studies can be directed at the actual work practices and in many cases do assess the specific exposure. However, in the case of pesticide applications to turf or lawns, the reentry activities of humans or animals cannot be exactly defined, and thus pesticide 0097-6156/93/0522-0100$06.00/0 © 1993 American Chemical Society
Racke and Leslie; Pesticides in Urban Environments ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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exposure assessment may be difficult Traditional means of assessing this type of exposure have been based on a foliar wash to measure removable pesticide residues. This approach is limited to measurement with respect to time of the amount of pesticide residues adhering to particulate matter that could be transferred from the foliage. Other researchers have evaluated the amount of turf maintenance chemical that can physically be removed or dislodged from the treated foliage (8). This latter approach takes into account the fact that there may be additional factors preventing complete foliage contact or residue removal. This paper presents the findings of our studies directed toward comparison of techniques and the assessment of this reentry exposure scenario for turf maintenance chemicals. FOLIAR DISSIPATION Traditionally in exposure measurement one encounters much variation. Although one plot of turf may look identical to another plot, there exist subtle differences which can affect measurements. In order to include as much of this variation in the measurements, experimental design should encompass sites representative of the climatic conditions expected in the intended use areas and representative sampling of the treated plots. Our method to address this representative sampling was to divide a relatively large plot of turf at each location into quadrants and columns producing 64 sampling plots which measured 4 feet by 9 feet as shown in Figure 1. Prior to each study, sets of four sampling plots were defined by computer selection to include one sample plot from each quadrant of the test plot with at least one sample plot from each column. This sampling technique attempts to account for differences in turf density, pesticide application variation, and environmental factors which affect chemical distribution within each treated plot Sets of four sampling plots were used to collect four replicates of each sampling type per time intervalfromthe treated test plot to allow statistical examination of the data. Each sampling plot was used only once in the study. Foliar dissipation of a turf maintenance chemical can be viewed from two perspectives: "surface residue" which is available unbound chemical and "total residue" which includes foliage absorbed chemical. A study to compare these two types of foliar residues was conducted with dithiopyr (Structure shown in Figure 2), a newly registered herbicide (Tradename: Dimension™) for control of crabgrass and other weed species in turf. Turf clippings were collected from four sampling plots at each sampling interval with a rotary lawn mower equipped with a grass catcher. Each turf sample was well mixed and subsamples analyzed by two different procedures. Samples of the fresh foliage were washed with a relatively mild 20% acetonitrile in water solution in order to determine the "surface residues". This solvent-wash system was compared to the traditional soap and water wash and found to be quantitatively equivalent but without the chromatographic interferences observed with the traditional approach. To determine "total residues", another subsample was extracted with a solution of 2.2% acetonitrile: 8.9% water: 88.9% isooctane. Table I shows comparative data of surface residues versus total residues at three climatologically stratified sites. The application of dithiopyr was made at a rate of 1.0 lb/A at each site and left to dry for one hour before the "0" sample was collected.
Racke and Leslie; Pesticides in Urban Environments ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
PESTICIDES IN
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^ 4 2 ft. ^ Column Β Column C
Column A
ENVIRONMENTS
Column D
9 ft
A
2
X
ft
1st Quad.
X
2nd Quad. 6
X
3rd Quad.
4th Quad.
X Figure 1. Plot Design
Figure 2. Structure of Dithiopyr
Racke and Leslie; Pesticides in Urban Environments ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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Foliar Dissipation and Residue Sampling Techniques
Table L Comparison of Foliar Surface Wash and Total Extraction Days After Treatment 0 1 3 7 14 30 60
2
Foliage Surface Total Surface Total Surface Total Surface Total Surface Total Surface Total Surface Total
Dithiopyr Residues in mg/m Columbus Atlanta 1.84 8.81 1.32 4.25 0.953 4.44 0.610 2.98 0.302 1.14 0.091 0.232 0.0086 0.0266
Cleveland
6.59 27.9 4.78 19.0 5.54 14.8 2.84 13.8 2.46 6.86 0.487 2.49 0.0302 0.322
2.37 14.6 1.83 10.6 1.86 12.0 1.01 4.17 0.327 1.83 0.052 0.398 0.0023 0.0138
Surface residues averaged 23% ± 8% (s.d.) of the total residues throughout the course of the study. Both surface and total foliar residues were found to decline to about 40% of initial levels within 7 days of the application. Dissipation was found to vary as a function of weather conditions with more rapid dissipation occurring in warm, dry conditions (Le. Atlanta) and slower dissipation occurring in cool, wet conditions (i.e. Columbus). As shown in Figure 3, total residues even from identical rate applications varied in magnitude from site to site. The same type of application equipment was used at all three sites and the same amount of chemical was mixed and applied Mass balance has been achieved (average > 92 %) in other studies with dithiopyr by placing polyurethane foam sheets on the soil surface as spray interception plates in the application area (9). Thus the residue magnitude differences at each site are believed to be due to differences in turf density and chemical intercept At these three sites, each of which had a different variety of fescue or bluegrass, we also measured turf surface area and turf density. Turf density was determined by cutting several one square foot sections of turf at the soil surface and weighing the collected grass. Leaf surface areas were determined for turf samples from several subplots at each site by measuring the leaf-lamina single surface areas of a weighed turf subsample with a LI-3000 Portable Leaf Area Meter similarly to the procedure of Goh et al (10). Surface areas per gram of fresh weight were determinedfromthe sampling plots. These leaf surface area and turf density measurements have been calculated and are shown in Table Π. Residue measurements also seem to be more consistent with greater surface area as exhibited by the mean Coefficient of Variation calculated at each site.
Racke and Leslie; Pesticides in Urban Environments ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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Table Π. Leaf Surface Area and Turf Density Location Atlanta Columbus Cleveland
2
Surface Area (cm /g) 104.7 129.7 129.5
2
Turf Density (g/ft )
Residue V.C. (%)
126.6 334.5 103.4
54 35 35
If the leaf surface area and turf density measurements from Table Π are applied to the total extraction datafromTable I, theresultsas graphically displayed in Figure 4 demonstrate in stark contrast to Figure 3, that the magnitude ofresidueon or in foliage is fairly consistentfromsite to site. Higher values at 1,7 and 14 daysfromthe Columbus site are believed to be due to the cooler, wet weather and the fact that dithiopyr, being somewhat hydrophobic, would partition under wet conditions into the waxy cuticle of the grass blades and therefore dissipate at a slower rate. In another study, applications of dithiopyrfromthree different types of formulations were made to turgrass at a rate of 2.0 lb/A at a plot at Troy, Missouri. Dithiopyr was applied as an emulsifiable concentrate (EC), a micro-encapsulated (ME) and a clay granule (GR) formulation. Figure 5 shows total foliar residues of dithiopyr for each formulation type over 60 days following application. Notice that the EC and ME dissipate similarly while the GR formulation does not leave much foliarresidueand does not dissipate as fast DISLODGEABLE RESIDUES Thompson, Stephenson and Sears in a 1984 article in Pesticide Science described their use of cotton cheese-cloth worn onresearchers'shoes to physically dislodge herbicide residue by scuffling vigorously back and forth over 1 square meter subplots (8). This technique was used in a study at a site in St Charles, Missouri to investigate the dislodgeable residue of amidochlor, a turf plant growthregulator,(Structureshown in Figure 6) and to compare theresiduesto those obtained by a polyurethane foam (PUF) covered paint roller technique. Results (Table ΙΠ)revealedthat only about 0.4% of amidochlor from a 4.0 lb/A application could be dislodged by the cotton gauze technique or the PUF covered paint roller. The paint roller techniqueresiduewas slightly lower, although not significantly, and someresearchershave suggested that additional roller weight might have improved this comparison. Because the comparison of two different techniques was made with two different sampling media, there was some uncertainty in comparingresults.In another study, the two sampling media, cotton gauze and PUF, were used to dislodge dithiopyr with the scuffling technique. Results shown in Table IV indicate that PUF is significantly better at
Racke and Leslie; Pesticides in Urban Environments ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
COWELL ET AL.
Foliar Dissipation and Residue Sampling Techniques
DAYS
Figure 3. Site Comparison of Total Extraction of Turf Residues
0.8
DAYS
Figure 4. Comparison of Total Extraction Residues by Surface Area and Turf Density Racke and Leslie; Pesticides in Urban Environments ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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Figure 6. Structure of Amidochlor
Racke and Leslie; Pesticides in Urban Environments ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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Foliar Dissipation and Residue Sampling Techniques
trapping and retaining dithiopyr than cotton gauze. Field fortification, transport and storage stability tests have demonstrated that PUF recovers >99% of dithiopyr that is trapped while similar studies with cotton gauze retained an average of only 77% of the applied dithiopyr (11). Table ΙΠ. Comparison of Dislodgeable Residue Sampling Media and Techniques 2
Amidochlor Residues in mg/m from 41b/A Trmt Days After Treatment 0 1 3 7 14 21
Shoe Pads - Cotton Gauze 2.21 0.19 0.017 0.008 0.003 0.001
Paint Roller - Polyurethane Foam 1.74 (±0.71) 0.14 (±0.026) 0.005 (±0.003) Not Sampled Not Sampled Not Sampled
(±0.09) (±0.004) (±0.005) (±0.004) (±0.002)
Table IV. Comparison of Dislodgeable Residue Sampling Media 2
Days After Treatment
Dithiopyr Residues in mg/m Cotton Gauze
0 1 3 7 14 30 60
1.62 0.283 0.148 0.0437 0.00353 0.00160
