Recent changes in weed spectrums and an increasing frequency of weed populations resistant to glyphosate have heralded a shift back to the use of soil-residual herbicides, especially in soybeans, said University of Illinois Extension weed specialist Aaron Hager.

“Soil-residual herbicides can provide many weed management benefits, but several factors influence their effectiveness,” he said. “Factors such as product selection, application rate, and when the herbicide is applied in relation to crop planting are largely under the control of the farmer, whereas soil moisture content at application and the interval between application and the first precipitation event are factors largely beyond the farmer’s control.”

Hager offers a few considerations and suggestions for improving the effectiveness of soil-residual herbicides.

1.) Select a soil-residual product that offers the best solution for the problem weed species encountered in each field.

2.) Pay careful attention not only to what products are contained in a pre-mix, but how much of each active ingredient will be applied at the intended use rate.  

3.) Herbicides applied close (within 14 days) to crop planting generally control weeds longer into the growing season compared with applications made several weeks before planting.

4.) Some residual herbicides commonly applied to the soil also can be applied after the crop has emerged. Applying these products following crop emergence may extend residual weed control for a few additional weeks.  

5.) Higher application rates generally provide a higher level of weed control longer into the growing season. However, don’t assume that a higher application rate will provide season-long weed control.

6.) In order for a soil-applied herbicide to be effective, the herbicide needs to be available for uptake by the weed seedling. If no precipitation is received between application and planting, mechanical incorporation can help move the herbicide into the soil solution.  

7.) Herbicide selectivity arises from the crop’s ability to metabolize (break down) the herbicide to a non-phytotoxic form before it causes much injury. When the crop is growing under favorable conditions, it rapidly metabolizes the herbicide before excessive injury occurs.  

For more information about soil-residual herbicides, read The Bulletin at http://bulletin.ipm.illinois.edu/.