University of Illinois

No. 13/June 18, 1998

Monitor Soybean Fields Treated with Soil-Applied Herbicides

The excessive precipitation across much of Illinois may reduce the soil-residual activity of many soil-applied herbicides used in soybeans. Although preemergence herbicide applications require precipitation or mechanical incorporation to move the herbicide into the soil solution, too much precipitation may move the herbicide too deeply into the soil, or possibly out of the field through the process of soil erosion. Additionally, the degradation of dinitroaniline herbicides, such as Prowl (pendimethalin) and Treflan (trifluralin), is accelerated under anerobic (no-oxygen) soil conditions, such as those created with excessive precipitation.

On the "good" side, fields previously treated with a soil-applied herbicide often have much less weed pressure than fields where a postemergence-only approach is to be utilized. Fields where no herbicide has yet been applied may have a tremendous amount of "alternative vegetation" present by the time the postemergence herbicide applications are made. Be certain to scout fields before postemergence herbicide application so you can determine the best herbicide option for a particular field, as well as select the appropriate rate and spray additive based on the size of weeds present.

Aaron Hager ( and Marshal McGlamery (, Crop Sciences, (217)333-4424