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United States Department of Agriculture

Agricultural Research Service

Title: Dissipation of the Defoliant, Tribufos, in Cotton Producing Soils

Authors
item Potter, Thomas
item Reddy, Krishna
item Millhollen, Eddie - LOUISIANA STATE UNIV.
item Bednarz, Craig - UNIVERSITY OF GEORGIA
item Bosch, David
item Truman, Clinton
item Strickland, Timothy

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 25, 2002
Publication Date: December 1, 2002
Citation: Potter, T.L., Reddy, K.N., Millhollen, E., Bednarz,C., Bosch, D.D., Truman, C.C., Strickland, T.C. 2002. Dissipation of the Defoliant, Tribufos, in Cotton Producing Soils. Journals of Agricultural and Food Chemistry. 50: (13):3795-3802.

Interpretive Summary: The price that cotton growers receive for their lint may be significantly reduced by "trash"; i.e. dried leaves and twigs that are mixed with it during machine picking. This is why efficient chemical defoliation is critical. Through use of products that cause the plants to drop their leaves prior to picking, "trash" can be minimized and profitability maintained. Defoliants that contain tribufos are favored in Southeast and Mid-South states. The product is highly effective over the broad range of environmental conditions that growers face in the region. Risk assessments recently prepared by the U.S. Environmental Protection Agency (EPA) concluded that tribufos runoff from treated fields presented high risk to aquatic life. Exposures were predicted with simulation models. The models assumed that tribufos degrades very slowly making it available for runoff long after application. We examined tribufos degradation in cotton producing soils collected in GA, MS and LA and at a field site in GA and found that degradation rates were 40 to 700 times faster than values used by EPA. Incorporating more rapid degradation rates in the simulation models will reduce runoff risk estimates and improve risk assessment accuracy. This will help cotton producers by contributing to efforts to maintain current registrations for this important active ingredient.

Technical Abstract: Soil dissipation of the cotton defoliant, tribufos, was measured in laboratory incubations and on research plots. Computed 50 % dissipation times (DT50) using non-linear and linear kinetic models ranged from 1 to 19 days and data indicated that exchangeable soil aluminum inhibited tribufos degrading soil organisms. Measured DT50 were 40 to 700 times less than the aerobic soil half-life (t1/2) used in recent tribufos risk assessments. The measured DT50 suggest that risk estimates were overstated, nevertheless some aquatic risk was indicated. Edge-of-field runoff concentrations exceeded invertebrate LOECs. Field data also indicated that volatilization may be a significant soil dissipation pathway. We conclude that volatilization should be included in simulation models used for pesticide registration. This will likely improve accuracy of model outputs for products like tribufos. Potential volatilization losses emphasize the need to evaluate tribufos atmospheric behavior on a regional basis.

Last Modified: 4/18/2014
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