Recent changes in weed spectrums and an increasing frequency of weed populations that are resistant to glyphosate have prompted a shift back to the use of soil-applied residual herbicides, especially in soybeans. That was one of the topics University of Illinois Extension weed specialist Aaron Hager addressed at a March 31 meeting in Davenport, Iowa, regarding how to deal with the increasing problem of weed resistance in corn and soybean fields.
The one-day conference was sponsored by Iowa State University Extension and University of Illinois Extension. A number of herbicide industry and seed company representatives and researchers attended. A total of around 100 people were present at the day-long conference. Hager, along with Iowa State University weed scientist Mike Owen, were on the program.
"Soil-residual herbicides can provide many weed management benefits, but several factors influence their effectiveness," says Hager. "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 some key considerations and suggestions for improving the effectiveness of soil-residual herbicides.
- Select a soil-residual herbicide product that offers the best solution for the problem weed species encountered in each field.
- Pay careful attention not only to what products are contained in a premix, but how much of each active ingredient will be applied at the intended use rate.
- 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.
- 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.
- 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.
- 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.
- Herbicide selectivity arises from the crop's ability to metabolize (break down) the herbicide to a nonphytotoxic form before it causes much injury. When the crop is growing under favorable conditions, it rapidly metabolizes the herbicide before excessive injury occurs.