Considerations for Early-Season Weed Control in Corn

April 28, 2000
With over 31% of the corn crop planted in Illinois and the remainder probably being planted in the next couple of weeks, there are likely to be some situations where the corn will emerge before any herbicide applications have been made. A number of these acres may have been scheduled to receive a soil-applied herbicide application but for some reason were not treated prior to corn emergence. Knowing that delays in the field may result in delays for soil-applied herbicide applications, what are some weed-control options once the corn crop has emerged? There are essentially two different approaches a grower has 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.

In examining the first option of using a delayed application of a soil-applied herbicide, several factors should be considered. 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, a number of soil-applied herbicides 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 an herbicide that has postemergence activity. Table 3 contains information about what "traditional" soil-applied corn herbicides can be applied post-emergence and some considerations to remember if these herbicides are applied to emerged corn. For additional information, consult the product labels.

If you are considering the option of switching to a total postemergence herbicide program, some issues should be addressed. First, a number of good postemergence corn herbicide options are 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. The second point to consider is that while we often stress that there is a maximum corn height or developmental stage for postemergence herbicide applications, some postemergence corn herbicide labels indicate that there is a minimum size or developmental stage the corn should be 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 4 lists the postemergence corn herbicides that have minimum corn size label restrictions.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague