Dissipation of the Defoliant Tribufos in Cotton-Producing Soils THOMAS L. POTTER,* ,† KRISHNA N. REDDY, ‡ EDDIE P. MILLHOLLEN, § CRAIG W. BEDNARZ, | DAVID D. BOSCH, † CLINT C. TRUMAN, † AND TIMOTHY STRICKLAND † Southeast Watershed Research Laboratory, Agricultural Research Service, USDA, Tifton, Georgia; Southern Weed Science Research Unit, Agricultural Research Service, USDA, Stoneville, Mississppi; Red River Research Station, Louisiana State University, Bossier City, Louisiana; and Coastal Plain Experiment Station, University of Georgia, Tifton, Georgia Soil dissipation of the cotton defoliant tribufos was measured in laboratory incubations and on 0.2-ha research plots. Computed 50% dissipation time (DT 50 ) using nonlinear and linear kinetic models ranged from 1 to 19 days. Data indicated that exchangeable soil aluminum inhibited tribufos-degrading soil organisms. Nevertheless, measured DT 50 values were 40 to 700 times less than the aerobic soil half-life (t 1/2 ) values used in recent tribufos risk assessments. DT 50 values suggest that risk estimates were overstated. However, edge-of-field runoff concentrations measured on research plots exceeded invertebrate LOECs, thus some aquatic risk is indicated. Field data also suggested that volatilization may be a significant soil dissipation pathway. From this result, we conclude that volatilization should be included in simulation models used for pesticide registration. This will likely improve the accuracy of model outputs for products such as tribufos. Potential volatilization losses indicate a need to evaluate the atmospheric behavior of tribufos. KEYWORDS: Defoliant; tribufos; soil; dissipation; kinetics INTRODUCTION Cotton producers routinely apply chemical defoliants to their crops prior to machine picking. In the United States, the most widely used active ingredient is tribufos (1). Its structure is shown in Figure 1. In crop-year 2000, an estimated 1.8 × 10 6 ha were treated, with application rates averaging 1 kg ha -1 (1). Recently the U.S. Environmental Protection Agency (USEPA) issued an Interim Reregistration Eligibility Decision (IRED) for tribufos (2). No major limitations to re-registration were identified; however, concerns were raised regarding acute risks to estuarine and marine fish, and acute and chronic risks to freshwater, estuarine, and marine invertebrates in “rainbelt” cotton-producing areas in southeastern and Mississippi delta states. Contributing factors were (a) high rates of precipitation in the region, which promotes pesticide runoff, (b) tribufos toxicity to aquatic life, and (c) the persistence of tribufos. Potential for tribufos movement from treated fields in runoff and its relatively high toxicity to aquatic life is well-documented (3-6), but conclusions regarding its persistence are not. They were based on a single proprietary study (7). The soil half-life value reported in this study, 745 days, was used in the USEPA risk assessment. This value suggests that the compound is exceptionally stable and that it may accumulate in soil and aquatic environments and impact wildlife for extended periods. The USEPA risk assessment document noted that the compound was “unusually persistent”. Ambiguity in tribufos soil degradation rates pointed to a need for further study. Accurate data are required to ensure accuracy of risk assessments. Further, if degradation data used by USEPA are correct then tribufos may be accumulating at unacceptably high rates in soil and in aquatic environments impacted by runoff. This could have longterm negative ecological conse- quences and adversely impact crop yields. Thus, this study was conducted to evaluate tribufos dissipation kinetics in laboratory incubations with cotton-producing soils from Georgia, Missis- sippi, and Louisiana, and on field research plots located in south- central Georgia. MATERIALS AND METHODS Soil. Soils used for laboratory incubations included Tifton loamy sand (Tift County, GA), Tunica silty clay loam and Dundee silty clay loam (Stoneville, MS), and Norwood very fine sandy loam (Bossier City, LA). These soils support extensive cotton production (8). * To whom correspondence should be addressed. Phone: 229-386-7073. Fax: 229-386-7294. E-mail: tpotter@tifton.cpes.peachnet.edu. † Southeast Watershed Research Laboratory. ‡ Southern Weed Science Research Unit. § Red River Research Station. | Coastal Plain Experiment Station. Figure 1. Tribufos (S,S,S,-tributylphosphorotrithioate). J. Agric. Food Chem. 2002, 50, 3795-3802 3795 10.1021/jf011673j CCC: $22.00 © 2002 American Chemical Society Published on Web 05/23/2002