As corn planting progress accelerates in many areas of Illinois this week, University of Illinois Extension weed specialist Aaron Hager says it's possible that some fields were planted before a planned soil-residual herbicide could be applied.
If the corn has not yet emerged, the application can proceed as originally planned. But what if the corn has begun to emerge and the soil-residual herbicide has not yet been applied? Can the application go on as planned, or will a different product need to be selected?
Hager says the answer depends on the respective herbicide. Many, but not all, herbicides that are most frequently applied prior to corn planting or emergence can be applied after the 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, so additional management procedures such as the addition of a herbicide that has postemergence activity, may be needed in situations where weeds also have emerged," Hager adds.
Growers should consult product labels for additional information, such as the need for tank-mix partners or spray additives to improve control of existing weeds. Hager also cautions growers about the potential for enhanced corn injury if these products are applied during periods of crop stress, such as stress caused by excessive soil moisture, cool air or soil temperatures, etc.
"Depending on the herbicide, tank-mixing other products or including various types of spray additives may be necessary to control existing weeds, but this may also increase the potential for corn injury," he says. "Attempting to save a trip across the field by applying a herbicide with a liquid nitrogen fertilizer solution, such as 28% UAN, as the carrier is not advisable if the corn has emerged."
Regardless of the scenario, growers need to remember that soil-residual herbicides must be moved into the soil solution 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 residual weed control.