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Title: FIELD HISTORY AND DISSIPATION OF ATRAZINE AND METOLACHLOR IN COLORADO

Author
item Shaner, Dale
item Henry, William

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2006
Publication Date: 1/1/2007
Citation: Shaner, D.L., Henry, W.B. 2007. Field History and Dissipation of Atrazine and Metolachlor in Colorado. Journal of Environmental Quality. Volume 36:pp 128-134

Interpretive Summary: Atrazine and metolachlor are widely used herbicide for controlling many broadleaf and grass weeds in corn, grain sorghum, and sugarcane. Farmers in Colorado have commented recently that atrazine was no longer providing the length of weed control that it used to give. In this study we found that atrazine is dissipating very rapidly in corn fields in eastern Colorado to which atrazine had been applied for multiple years. There was no measurable level of atrazine in the field after 45 days. Metolachlor, on the other hand, dissipated much more slowly. Laboratory studies confirmed that the rate of degradation of atrazine is correlated with years of use of the herbicide in the field, whereas there did not appear to be such a relationship with metolachlor. These results suggest that the lack of long term weed control by atrazine is due to rapid degradation of the herbicide by soil microbes.

Technical Abstract: Field and laboratory studies were conducted to determine the rate of dissipation of atrazine and metolachlor in soil from fields in Colorado. The published half-lives of atrazine and metolachlor are 60 d and 56 d, respectively. In the field studies, the half-lives of atrazine and metolachlor in the top 15 cm of the soil ranged between 3.1 to 6.6 d and 17.8 to 28.9 d, respectively. In the laboratory studies, the half-life of atrazine varied from 1.2-22.1 d with the shortest half life occurring in soils which had been treated with atrazine for at least 5 years. The longest half-life was in a soil that had never received atrazine. The half-life of metolachlor in these same soils varied from 9.8 to 43.1 d. There was no apparent relationship between the half-life of metolachlor and the half-life of atrazine in the laboratory studies.