The recent cool, wet weather can influence herbicide application decisions for weed management in corn. Numerous acres of corn in Louisiana were damaged by frost this past weekend and a majority of that corn ranges V3 to V5 (3 to 5 visible collars). The damage is seen as leaves that are necrotic. Add this to the corn that has been sand-blasted, wind-blown, and/or had its growth hampered by cool temperatures and wet soils, corn in Louisiana does not look as normally expected for late April. The current condition of the Louisiana corn crop has to be considered when appling a postemergence herbicide application.
One issue is plants that were damaged by a frost often appear less developed than they really are. Many herbicide labels contain application restrictions based upon corn development stages (growth stages) or plant height. In a normal year, Louisiana corn producers apply their herbicides well before any restriction specified by a label. Imagine you want to apply a herbicide whose label specifies application prior to V5 (five visible collars). However, two leaves were lost due to frost damage earlier in the season, so the corn is actually at V7. If applied, the application would be off-label. Care should be taken to count corn collars to ensure the herbicide application follows the label.
Another issue to consider is the potential for herbicide injury to the frost-damaged corn. In many of the phone calls I have had, the leaf in the whorl is relatively healthy (green and intact). Therefore, proper growth can be expected if the environment cooperates. However, I suggested that producers delay their herbicide application until new growth is observed (i.e. the new leaf has unfurled out of the whorl). Secondly, the addition of adjuvants that contain oil-based additives to the tank should be avoided to minimize phytotoxicity. With the past environmental conditions already hampering the growth of the crop, avoid further injure the corn crop with a herbicide application.
The cool environmental conditions that are affecting the corn crop may also have an effect on the weeds. Frost does not play favorites, so it may injure a weed as well. Frost damaged leaf tissue will not absorb and translocate a herbicide properly, which may lead to less than expected control. Like corn, ensure the weeds are actively growing prior to a herbicide application. Please understand that I am not advocating waiting until the weeds are bigger to make the application. It is always easier to kill a small weed versus a large weed.
The LSU AgCenter and the LSU College of Agriculture