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Title: Spatial variability of atrazine and metolachlor dissipation on dryland no-tillage crop fields in Colorado

Author
item BRIDGES, M - COLORADO STATE UNIVERSITY
item Henry, William
item Shaner, Dale
item KHOSLA, R - COLORADO STATE UNIVERSITY
item WESTRA, P - COLORADO STATE UNIVERSITY
item REICH, R - COLORADO STATE UNIVERSITY

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2008
Publication Date: 9/1/2008
Citation: Bridges, M., Henry, W.B., Shaner, D.L., Khosla, R., Westra, P., Reich, R. 2008. Spatial variability of atrazine and metolachlor dissipation on dryland no-tillage crop fields in Colorado. Journal of Environmental Quality. 37:1-9.

Interpretive Summary: This study was conducted to measure herbicide degradation in dryland cropping environments in Northeastern Colorado. Applying herbicides at variable rates and only to the areas that need herbicide may limit the total amount of herbicide applied and also reduce the potential for off-site movement. Before herbicides are applied site-specifically, we should first determine if the field has enough variability in it to justify a non-uniform application. Prior studies have determined that the corn herbicide atrazine is degraded by soil microbes in irrigated corn fields in Colorado. The objectives of our study were to determine 1) if the corn herbicides metolachlor and atrazine are degraded in dryland soils, and 2) to compare the rates of degradation across different soil zones. Herbicide degradation was evaluated in soils that were taken from three dryland fields with various cropping histories. Soils from these fields were classified into various zones with soil EC (Electrical Conductivity) analysis. Degradation was measured in the lab. We discovered that atrazine was rapidly degraded, with half lives ranging from 1.8 to 3.2 days in the laboratory. Rapid atrazine degradation was attributed to the prior history of atrazine use on these fields. Although the degradation rates did vary with respect to the different soil zones throughout the field, these differences while statistically significant, would likely not be of agronomic importance to the farmer. The bigger problem was that regardless of soil zone, the herbicide degraded so quickly that weed control likely would be inadequate.

Technical Abstract: An area of interest in precision farming is variable rate application of herbicides to optimize herbicide use efficiency and minimize negative off-site and non-target effects. Two commonly used soil applied herbicides in dryland corn production are atrazine (6-chloro-4-(ethylamino)-6-isopropylamino)-s-triazine) and metolachlor (2-chloro-N-(6-ethyl-o-tolyl)-N-(2-methyloxy-1-methylethyl) acetamide). Because both of these herbicides are frequently found in surface and shallow ground water, variable rate application could help alleviate this problem. However, accelerated dissipation of atrazine has been discovered recently in irrigated corn fields in eastern Colorado. The objectives of this study were to i) compare the rates of dissipation of atrazine and metolachlor across different soil zones from three dryland no-tillage fields under laboratory incubation conditions, and ii) determine if rapid dissipation of atrazine and/or metolachlor occurred in dryland soils. Herbicide dissipation was evaluated at time points between 0 and 35 days after soil treatment using a toluene extraction with GC/MS analysis. Differential rates of atrazine and metolachlor dissipation occurred between two soil zones on two of three fields evaluated. In addition, accelerated atrazine dissipation occurred in soil from all fields of this study with half-lives ranging from 1.8 to 3.2 days in the laboratory. A history of atrazine use on all fields likely contributed to the rapid atrazine dissipation rates.