SOIL DISSIPATION AND BIOLOGICAL ACTIVITY OF METOLACHLOR AND S-METOLACHLOR IN FIVE SOILS

Authors: Shaner, Dale; BRUNK, GALEN - COLORADO STATE UNIVERSITY; NISSEN, SCOTT - COLORADO STATE UNIVERSITY; WESTRA, PHIL - COLORADO STATE UNIVERSITY

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2006
Publication Date: 7/1/2006
Citation: Shaner, D.L., Brunk, G., Nissen, S., Westra, P. 2006. Soil dissipation and biological activity of metolachlor and s-metolachlor in five soils. Pest Management Science.DOI:1002/ps.1215

Interpretive Summary: Research was done to compare the herbicidal activity of metolachlor (a 50:50 mixture of S-metolachlor and R-metolachlor) with S-metolachlor (an 85:15 mixture of S-metolachlor and R-metolachlor) on a gram per gram basis on five different soils. The objective was to determine how much of the biological activity of both formulations was due to the S-metolachlor isomers. The results showed that the herbicidal activity of both formulations was due to S-metolachlor. The results of this research are important to farmers when they are deciding which formulation to use for weed control. Formulations containing only metolachlor will not be as active nor will they have as long soil residual activity as formulations made with S-metolachlor.

Technical Abstract: The resolved isomer of metolachlor, S-metolachlor, was registered in 1997. New formulations based primarily on the S-metolachlor isomer are more active on a gram for gram metolachlor basis than formulations based on a racemic mixture of metolachlor containing a 50:50 ratio of the R and S isomers. The labeled use rates of S-metolachlor-based products were reduced 35% to give equivalent weed control as metolachlor. Recently, several companies have registered new metolachlor formulations with the restriction that they be used at the same rate based on the combined weights of the R and S isomers as the S-metolachlor-based products. This research was done to compare the soil behavior and the biological activity of metolachlor to S-metolachlor in different soils under greenhouse and field conditions. Although the Kd ranged from 1.6 to 6.9 across the five soils, there were no differences in the binding of metolachlor and S-metolachlor to soil, or in the rate of soil-solution dissipation, in a given soil. However, both greenhouse and field studies showed that S-metolachlor was 1.4-to 2.5-fold more active than metolachlor on barnyardgrass across five different soils and in three field locations within Colorado. When the rates of the herbicides were corrected for S-isomer concentrations in the formulations, the differences between the two formulations were not significant – suggesting that the R-isomer is largely inactive. A bioassay comparing the biological activity of S-metolachlor with metolachlor showed that the herbicidal activity of metolachlor is due to the S-isomer, which supports the conclusions from greenhouse and field experiments.