Successful preemergence corn herbicide programs require two key components. First, the programs need dry weather (with limited wind) in order to have the preemergence herbicides applied.  Second, the programs generally need 0.5 to 1 inch of rainfall to activate the herbicide after the application, which moves the herbicide from the soil surface into the zone where weeds are germinating. Earlier this year, we struggled to get dry weather to get into fields to do tillage, plant, and apply herbicides. After that tough start of the season, it seemed inappropriate to hope for rain, but we need rain now to activate these herbicides. At Arlington, after getting many of our corn trials planted and sprayed, we haven’t received the necessary rain over the past 2 weeks for good herbicide performance (Figure 1). The same may be true in other areas of the state. 

 

Figure 1. Rainfall for the month of May at Arlington, La Crosse, Eau Claire, and Green Bay.

 

As a consequence, we are seeing weeds emerging through preemergence herbicides treatments at Arlington. The rain predicted for the end of this week may activate some of these herbicides and reach back to control some of these weeds, depending on the species, their size, and the herbicide. However, the following actions may be very appropriate.

 

Action 1: Scout corn fields to determine how well preemergence herbicides were activated.

 

Action 2: Be prepared to apply a postemergence herbicide or cultivate if control is not satisfactory. Rotary hoeing would also be an option if done while the weeds are still emerging, but many growers may no longer have access to this technology.