I have received a few calls about the need for residual activity with postemergence herbicide applications in corn. I’m going to focus on grass weeds, which were the basis for the calls.  Of course, every situation differs.  In some northern areas, weeds had emerged before it rained to fully activate preemergence herbicides.  These fields may still get some benefit from the preemergence herbicides, but the escapes still need to be controlled. Other fields that are being treated entirely postemergence may have a greater need for residual activity as there was no residual herbicide in the system yet. 

 

Let’s consider corn planted on May 1 for an example.  We would typically like to have 8 weeks of residual weed control provided by a preemergence herbicide. By that time (July 1), the corn should be canopied and shading out most weeds emerging during the rest of the season. Plus, weed germination is also greatly diminished by that time of the summer. If we are making a postemergence application on June 1, we don’t need 8 weeks of residual activity anymore. Rather, about 4 weeks of residual activity should get us to the same July 1^st date. We can get that residual activity two different ways. One way is to get the residual activity from the postemergence herbicide. The postemergence grass herbicides differ in the amount of residual activity that they provide. Under conditions where a rain is received a couple days after application, which would activate their residual, I would rank the herbicides in the following general order: glyphosate (no residual), Liberty (< 1 week), Option (about 1 week), Accent and Stout (1-2 weeks), Steadfast and Resolve (2-3 weeks).  Of course, these estimates will vary on the weed species, their density, rainfall, etc. and should be used as a relative guide. So, some of these herbicides can provide some residual and may be sufficient especially if cleaning up escapes from a poorly activated preemergence herbicide. Others would benefit from additional residual activity.   

 

A second way to get residual activity is to add a preemergence herbicide to the tank mix.  A full rate is not needed anymore since 4 weeks have passed since planting in this example. A half rate should be adequate in many cases at this point in the season. Using a half rate also makes the addition of a residual herbicide more palatable when considering the cost. Herbicides to consider include those in the acetanilide family (i.e. Dual, Harness, Outlook, etc.) and Prowl. These herbicides do not have postemergence activity on grasses, but can provide the desired residual activity. Their benefit would be most obvious with herbicides like glyphosate and Liberty.

 

A wide range of field situations and needs certainly exist in corn fields across Wisconsin. However, if the time point in the season (the time between planting and canopy) and some basic principles are considered, appropriate decisions on the need and benefits of residual grass herbicides can be made.