Delayed Preemergence Herbicide Applications For Corn

Weed control plans are forced to change as wet weather keeps farmers from making timely preemerge applications.

Soggy fields this spring have not only delayed corn planting, but herbicide applications have been delayed too. The weed situation is causing a number of farmers to shift gears in their weed control plans. "Weeds are growing pretty fast now. Keep in mind that the larger they get, the more difficult they are to manage and the more important they are to control," advises Mike Owen, an Iowa State University Extension weed control specialist.

Given the way the season has developed, the best intentions to apply an early preplant (EPP) herbicide application prior to corn planting has gone out the window and it appears that applying a preemergence (PRE) application immediately after planting is also becoming a slim chance, notes Owen. Thus, many farmers who intend to use a soil-applied residual herbicide treatment in corn may be forced to make the application of an early postemergence (EPOST) to the weeds and possibly the corn.

Herbicide options available for corn

On the ISU Crop News Website is a handy reference table which explains the herbicide options available for corn. That table can be found at

The concern about the delayed PRE/EPOST herbicide treatment, regardless of whether or not residual products are included, is—can you make the application soon enough to protect crop yield? "It is typical that the herbicides will kill weeds that are larger but often after potential yield has been lost," says Owen.

Furthermore, some weeds, mainly winter annuals, become extremely difficult to control as they grow larger and the potential for crop injury also increases as the crop gets larger. Thus, the best option is to make the residual herbicide application immediately after planting. "If this is no longer an option, apply the treatment as soon as possible after planting," he advises.

Corn canopy can interfere with coverage

"You have to recognize that some herbicides are registered for application EPOST to the crop but do not have POST activity on weeds," says Owen. "Also, many of these products can be applied to corn that is considerably larger than appropriate to protect the potential yield and weed control may be variable due to the uneven distribution of the spray on the weeds." That is, the corn canopy interferes with the spray coverage.

Be sure to follow the label directions and understand the limitations of applications as the weeds and corn get larger. Also, it is highly unlikely that any of the EPOST treatments will meet expectations as a "season long, one pass treatment."

"Scout the fields prior to application and use the correct product to control the weeds that are present," he sums up. "Recognize that some of the herbicide labels are not clear about the size of weeds that may be controlled after they emerge."

TAGS: Extension
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