Environ. Sci.
Techno/. 1993. 27. 2250-2252
Potential Environmental Risks Associated with the New Sulfonylurea Herbicides John S. Fletcher, Thomas 0. Pfleeger,' and Hllman C. Ratsch U.S. Environmental Protection Agency, Environmental Research Laboratory, 200 SW 35th Street, Corvaiiis. Oregon 97333
Introduction The first sulfonylurea herbicide, chlorsulfuron (Figure l ) , was introduced in 1982. Members of thisnew herbicide class are known for their high toxicity toward plant growth (100times more toxic than herbicides used prior to 1982), low application rate (8.8-26 g/ha), extremely low toxicity to humans and other animals, and phytotoxicity reported due to the inhibition of a single enzyme (acetolactate synthase) (I). All these characteristics make the sulfonylurea herbicides ideal candidates for replacing some of the older herbicides in efforts to reduce the quantity of chemicals used (Z),to eliminate herbicides with relatively high animal toxicity, and to replace compounds such as atrazine which have contaminated the groundwater in many agroecosystems (3). Unfortunately, there are anecdotal claims from certian regions of the United States that drifting sulfonylureas have caused crop losses by disruptingnormal reproductive processes. For example, in south-central Washington, orchard growers attribute low cherry yields during the late 1980stosulfonylureadrift because the losses coincided with the first extensive use of sulfonylureas in that portion of the state. If low concentrations of drifting sulfonylurea compounds do disrupt plant reproduction, then it follows that expanded use of these chemicals around the world could have a devastating impact on the productivity of nontarget crops and the makeup of natural plant communities and wildlife food chains. We have used fruit yield on cherry trees as a model system to evaluate the influence of sulfonylureas on plant reproduction. Materials and Methods
Our experiments were conducted on 20-year-old Royal Anne Cherry (Prunus auium L.) trees a t the Oregon State University Lewis-Brown Horticulture Farm, located 5 mi southeast of Corvallis, OR. The influence of the chemical on fruit yield was determined by comparing the weight of fruit collected from previously marked 50-cm segments of treated and control branches. All chemical treatment.8 and the control (sprayed with the water carrier and no chemical) included 0.05 % Unifilm 707, a surfactant. The chemical was applied until wetness in the late afternoon with a dual nozzle spray wand. Treated branches (both chemical and control treatments) were contained in Tyvex bags (spun polyethylene) during and for the next 12 h following chemical application to prevent cross-contamination between branches and chemical removal by rain which followed spring treatments. Single applications of chlorsulfuron (2-chloro-N-[[(4methoxy-6-methyl-1,3,5-triazin-2-yl)aminol carbonyl]henzenesulfonamide) were administered a t five different concentrations (0, 2.3 X 4.6 X 2.3 X lo4, and 2.3 X l0-S M). Each concentration was administered to five individual branches on five replicate trees. These
* Corresponding author. 2250 Envlron. Sci. Technol.. Voi. 27. NO. 10. 1983
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Flgure 1. ChemicalstructureofchlorsultuOn(2-chloro-N-[ [(CrrmmOxy6-memyl-l,3.5-triazin-2-yl~mino]carbonyl] benzenesulfonamlde).
Spring Application
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46x107
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Treatment time and concentration (M) Flgure 2. Influence of a single spring application of chiorsulfuron on freshweight yield of cherry huns. Developmental stages of chemical applicatlon wereas follows: A. swollen floralbudlno expanded leaves: E, in flowerlleaves 1110 expanded; C. postflowerlleaves 114-113 expanded; D. fruit pea-sizedlleaves 112-314 expanded, E. fruit 112 full-sizelleaves fully expanded. The five treatments were divlded
between two successive springs (1991 and 1992)wlth treatment C common to bolh years. The standard error of any treahent mean for treatments applied at A = 15.6, B = 16.7. 1991 C = 13.5. 1992 C = 50.0. D = 25.9,and E = 37.0. An asterisk Indicates that it is slgnificant at the 0.05 level.
five concentrations represent 0,0.001,0.002,0.01, and 0.1 of the recommended tank mixture of chlorsulfuron for use on small grain crops in Washington, Oregon, and California. Separate experiments were conducted at five differentphenological stages spanning the developmental transition in spring from floral bud to fruit as described in the legend of Figure 2. Multiple applications were made by making three sequential applications of 4.6 X l W 7 chlorsulfuron to the same branches on five trees at 1-week intervals during the fall of 1991or the spring of 1992. Three different multiple application experiments were conducted, with the first applications made on Sept 19, Oct 10,or Apr 6, and cherry yield was determined during June 1992. In all experiments, data collected from five replicate branches of each control and treatment were subjected to statistical analysis using the Dunnett's test of an CI level of 0.05. The number of fruit on control branches varied between trees, and as a result apparently major yield reductions caused hy some treatments were not significant (i.e., Figure 2, B treatments at 2.3 and 4.6 x 1W7 M).
This article not s u b m to US. Copyright.
Published 1993 by
me American Chemical Socletv
A B C
A B C
A B C
A B C
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Treatment time and concentration (M) Figure 3. Influence of a single fall application of chiorsulfuron on cherry fresh weight yield during the subsequent spring. Dates of fall 1991 applications were as follows: A, Aug 15; B, Sept 12; C, Oct 10. The standard error of any treatment mean for treatments applied at A = 43.6, B = 48.1, and C = 43.0. An asterisk indicates that it is significant at the 0.05 level.
Results and Discussion The nature and extent to which cherry tree reproduction was influenced by a single application of chlorosulfuron (Figure 2) depended on both the concentration and the time of chemical treatment. The response to the lowest concentration (2.3 X M) was inconclusive, because there was no clear pattern in the data. At the next higher concentration (4.6 X M), a statistically significant reduction was observed when the chemical was applied a t two of the five different stages; and treatment C, which was included in both the 1991 and 1992 experiments, caused a similar reduction both years. The yield reductions brought about by the 4.6 X M applications were accompanied by almost no observable vegetation damage, Le., a few leaves on some of the treated branches showed modest chlorosis and cupping. When the two higher concentrations were provided, the yield was reduced to a greater extent, but this was accompanied by pronounced leaf chlorosis, cupping, reduced leaf expansion, and eventual death of the 2.3 X M treated branches. One of the developmental stages examined responded quite differently from the others. When chlorosulfuron was applied in the spring at the time when floral buds were swollen and no leaf expansion had occurred (Figure 2A), an increase in fruit yield was observed but was not statistically significant. There were other isolated occurrences of increased yields, but none of them were statistically significant. A single application of chlorosulfuron during the late summer and early fall of 1991, when the next year’s floral buds were developing (41, also influenced cherry tree reproduction (Figure 3). Significant reductions were observed on five occasions at the two higher concentrations and once a t the lowest concentration. The fall applications had no visible infuence on the leaves except for the 2.3 X 1WM application made on Aug 15where a modest increase in the normal rate of senescence (leaf yellowing) was observed. The influence of multiple applications (i.e., chronic exposure) was examined by providing three sequential
treatments with 4.6 X le7 M chlorosulfuron to the same branch a t 1-week intervals. Two fall experiments (September and October) and one spring experiment (March) were conducted. Exposure of the developing buds to a low level of chlorosulfuron in September caused next spring’s yield to be reduced to 15% of the controls (SE = 83.9). When this treatment was administered during the spring of 1992,the yield of treated branches was only 40% that of controls (SE = 29.2) While not statistically significant, the October treatment also reduced fruit yield (60% of the controls, SE = 89.4). In all of the multiple exposure experiments, there was no visible alteration of leaves on the treated branches. The results from this series of experiments showed that M) reproduction a t a low level of chlorosulfuron (4.6 X of cherry trees was reduced without visible disruption of vegetative organs. Thus, at certain times of the year when particular phases of bud or flower development were occurring, chlorosulfuron was 100-1000 times more effective in altering cherry tree reproduction (flower and fruit set) than has been reported for gibberellic acid, a plant hormone known for this property (5, 6). The high sensitivity cherry trees display toward chlorosulfuron indicates that small quantities of the chemical, such as might be found in airborne particles traveling long distances, may change plant reproduction without altering vegetative growth. If the effect of chlorosulfuron on cherry trees is characteristic of other sulfonylurea herbicides and the cherry tree’s response is characteristic of other plant species, drifting sulfonylureas may severely reduce both crop yields and fruit development on native plants, an important component of the habitat and foodweb for wildlife. These possibilities, especially involving chronic exposure of plants to chemically undetectable amounts of sulfonylureas, warrant a concerted research effort to gain a clearer understanding of the environmental behavior and effects of more toxic herbicides. This effort is necessary to avert potential environmental problems as these new compounds gain a larger market share of the approximately 400 million pounds of herbicides used annually in the United States on 270 million acres of cropland (7,8). Expanded use of sulfonylurea herbicides may occur very rapidly because of growing pressure to replace other classes of herbicides which are more toxic to animals or contaminate groundwater.
Acknowledgments We thank Don Stufflebeem, Bob Hayes, and Lisa Ganio for technical assistance and Karl Arne for constructive suggestions during the investigation. The information in this document has been funded wholly by the U.S. Environmental Protection Agency. It has been subjected to the Agency’s peer and administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Literature Cited M.; Duffy, M.F.;Hay, J. V.; Schlueter, D. D. In Herbicides-Chemistry, Degradation, and Mode of Action; Kearny, P. C., Kaufman, D.D., Eds.; Marcel Dekker: New
(1) Beyer, E.
York, 1988; pp 117-189. Envlron. Sci. Technol., Vol. 27, No. 10, 1993 2251
(2) Pimentel, D.; McLaughlin, L.; Zepp, A.; Lakitan, B.; Kraus, T.; Kleinman, P.; Vancini, F.; Roach, W. J.; Graap, E.; Keeton, W. S.; Selig, G. Bioscience 1991, 41, 402-409. (3) Kross, B. C.; Seiim, M.; Hallberg, G.; Bruner, D. R.; Cherryholmes, K. Environ. Int. 1992, 18, 231-241. (4) Ryugo, K. Fruit Culture; John Wiley: New York, 1988; pp 69-106. (5) Goldwin, G. K.; Webster, A. D. Hort. Sci. 1983,58,505-576. (6) Facteaw, T. J.; Rowe, K. E.; Chestnut, N. E. Scientia Hort. 1989,38, 239-245.
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(7) Waddell, T. E.; Bower, T. B.; Cox, K. Managing Agricultural Chemicals i n the Environment; T h e Conservation Foundation: Washington, DC, 1988; Chapter 3. (8) Agricultural Resources Inputs Situation and Outlook Report: U.S. Department of Agriculture Economic Research Service, AR-17, 1990. Received for review May 3, 1993. Revised manuscript received July 19, 1993. Accepted July 20, 1993.
