Aaron Hager, University of Illinois Extension weed specialist, said the answer depends on the respective herbicide. Many, but not all, herbicides that are typically applied prior to corn planting or emergence may be applied after corn has emerged.
"Even if a soil-residual herbicide can be applied after crop and weed emergence, not all soil-residual herbicides will control emerged weeds," Hager said. "If weeds also have emerged, additional management procedures, such as the addition of a herbicide with post-emergence activity, may be needed."
He reminds farmers to consult the product label for additional information regarding the need for tank-mix partners or spray additives to improve control of existing weeds.
"Corn injury can be enhanced if these products are applied during periods of crop stress, such as stress caused by excessive soil moisture, cool air, or cool soil temperatures," he said. "Most herbicide labels also caution not to use nitrogen fertilizer as the herbicide carrier if corn has emerged."
Soil-residual herbicides need to be moved into the soil solution in order to be available for uptake by weed seedlings. A herbicide that remains on the soil surface after application and is not moved into the soil profile by precipitation or mechanical incorporation may not provide adequate weed control.
For more information about delayed application of soil-residual herbicides, read the April 29 edition of The Bulletin online at http://ipm.illinois.edu/bulletin.