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Weed Control Options in Corn After the Preemergence Application Window

May 3, 2002
With the recent rains and forecasts of more to come, getting corn planted has been a challenge. In many cases this mad rush to plant and the extreme winds that have been plaguing us over the past few weeks have put us in the situation again where it is going to be very difficult to get soil-applied herbicides sprayed in a timely fashion. This will leave many growers and applicators with the decision of what to do for weed management strategies on those acres that won't be treated until the corn crop has emerged. There are essentially two different approaches that can be taken in this situation. The first option is to use the soil-applied herbicide program that was initially planned, and the second option is to switch to a total postemergence strategy.

The option of using a delayed application of a soil-applied herbicide requires consideration of several factors. Many, but not all, soil-applied corn herbicides can be applied after corn emergence. Keep in mind that not all of these herbicides will control emerged weeds. Additionally, there are a number of soil-applied herbicides that can cause significant crop injury if they are applied after corn emergence. So in some instances, additional management strategies may need to be implemented to control existing vegetation. These strategies could include the use of a rotary hoe or the addition of a herbicide that has postemergence activity. Table 4 contains information about which "traditional" soil-applied corn herbicides can be applied postemergence and some considerations to remember if these herbicides are applied to emerged corn. For additional information, consult the respective product labels.

If you are considering the option of switching to a total postemergence herbicide program, there are also some points that should be addressed. First, there are a number of good postemergence corn herbicide options available to growers, and information on these products can be found in Chapter 2 of the Illinois Agricultural Pest Management Handbook. However, a number of these herbicides do not provide any soil residual control, and often the timing of herbicide application is critical for providing season-long weed control. Another point to consider is that, while we often stress a maximum corn height or developmental stage for postemergence herbicide applications, some postemergence corn herbicide labels indicate a minimum size or developmental stage the corn should be at before an application is made. This is a particularly important consideration when deciding on a postemergence herbicide program when corn is in its early developmental stages. Table 5 lists the postemergence corn herbicides that have minimum corn size label restrictions. So, remember, there are a number of considerations that need to be made when deciding on weed management strategies for early-season corn.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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