Among the ramifications of extended wet weather is the need to sometimes shift from Plan A weed control option to Plan B.  Often this means forgetting about the pre-emergence application in favor of a total postemergence program.  Post emerge herbicide programs can be tricky in terms of timing because too early or too late can both result in yield reductions or possibly crop injury. 

Yield reduction

The main reason for weed control is to protect crop yield.  WHEN weeds are controlled is nearly as important as IF weeds are controlled.  If weeds are controlled too early (0.5 to 1.5 inches), yields suffer due to subsequent weed flushes.  However, when herbicides are applied late, control may still be good but corn yields are reduced from early season weed competition.  A good "rule of thumb" is to apply herbicides when the tallest weeds are about 4 inches in height.  This will insure both good weed control and help minimize yield losses. 

Effective weed control

This one is hopefully obvious, but it’s a simple agronomic fact that most annual weeds kill better when they are small compared to when they are big.  You can’t expect miracles from herbicides.  Whatever your product of choice, know the maximum weed size for those species that are the primary targets and monitor weed growth closely.   If weeds emerge after early post-emergence applications, effective weed control can be maintained and corn yields can be protected by a timely cultivation. 

Crop injury

            Correctly staging the corn crop is important to avoid crop injury.  In general, corn is more tolerant as a small seedling and the risk for crop injury increases as the plant becomes larger.  However, there are a few exceptions (Buctril, Beacon, NorthStar, Distinct, and Celebrity Plus).    Table 1 provides the range of corn stages that can be treated with different herbicide options.  If the corn exceeds the size listed on the label, switch to a herbicide that allows application at a larger stage of growth or use drop nozzles if permitted.  

            Crop injury can sometimes occur if early post mixtures of growth regulator (e.g. dicamba) and ALS herbicides are mixed.  Several ALS grass herbicides carry the warning that tank mixtures with dicamba or other growth regulating herbicides to stressed corn that is V3 or smaller may cause the corn to rattail later in the season.  This type of injury is most prevalent in cool, wet conditions. 

            The V-staging system counts the number of visible collars, which is the area where the leaf blade joins the leaf sheaf.  

 *Always consult the pesticide label for exact application recommendations.