herbicide timing

Editor’s note: The following was written by Bob Hartzler, Iowa State University professor of agronomy, for the university’s Integrated Crop Management News website.

Preemergence herbicides are the foundation of herbicide-based weed management systems, and effective use of these products is essential to protect crop yields and reduce selection pressure for herbicide-resistant weeds.

In a perfect world, applying preemergence herbicides immediately after planting would provide maximum performance, but equipment and labor availability limit many farms from using this approach.

Early preplant

For early preplant — applications made more than 7 to 10 days prior to planting — applications are typically completed before summer annual weeds begin to germinate, thus increasing the likelihood of timely activation by rainfall. This may provide a weed-free seedbed at planting.

Application is also completed before planting, spreading out the workload.

However, residual activity into the growing season is shorter than when the product is applied near planting. Use of layered residual (including a residual with postemergence application) reduces this risk.

Final seedbed preparation tillage may “dilute” the herbicide within the soil profile if the tool is run too deep and may result in uneven distribution of the herbicide.

Planter units may move herbicide out of the crop row, allowing weed escapes

If planting is delayed, much of the value of the herbicide may be lost.


Applications made within a week before or after planting mean the product is applied near the time that summer annuals initiate emergence and residual control is extended later into the growing season than early preplant applications.

But if rain doesn’t occur within a week of planting, early-emerging weeds may escape control due to lack of herbicide activation.

Planter units may move herbicide out of the crop row if applied preplant, allowing weed escapes.

Delayed preemergence

Applications made more than a week after planting spread the workload and residual control is extended later into growing season. But herbicide options may be reduced if the crop has emerged before application.

Summer annual weeds likely will have emerged at application, requiring additional post-emergence product to control these weeds. Application delays can result in early-season competition between crops and weeds and may allow weeds to exceed optimum size for post-emergence control.

Rainfall is needed within a few days of the application to activate the product.

Herbicide-resistant weeds have limited our ability to “rescue” fields when weather delays preemergence applications following planting. Thus, consider how you can ensure that all acres are protected with appropriate preemergence herbicide applications when prolonged wet periods or other factors interfere with field operations.