We have received a number of phone calls this week from growers who have planted corn and did not have the chance to apply their preemergence herbicide before the corn emerged. These growers have essentially two different approaches that can be taken. The first option is to use the soil-applied herbicide program that was initially planned; the second 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, 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 a herbicide that has postemergence activity. Table 1 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, here also are some points that should be addressed. First, growers have a number of good postemergence corn herbicide options available to them, and information on these products can be found in Chapter 2 of the Illinois Agricultural Pest Management Handbook or Chapter 16 of the Illinois Agronomy 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, although we often stress a maximum corn height or developmental stage for postemer-gence herbicide applications, some postemergence corn herbicide labels indicate a minimum size or developmental stage at which the corn should be before an application is made. This is a particularly important consideration when deciding on a postemer-gence herbicide program when corn is in its early developmental stages. Table 2 lists the postemergence corn herbicides that have minimum corn size label restrictions. So remember, a number of considerations are needed when deciding on weed management strategies for early-season corn.--Christy Sprague and Aaron Hager