Corn Post-emergence Weed Control Timing

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

Wet weather has made most field operations a struggle to accomplish in a timely manner.  Spraying herbicides on corn has been no exception.  This year there will be extreme variation in both crop and weed growth.  It will be important to monitor both so that post-emerge herbicides can be applied at the optimum time.  Timing of post emerge herbicides is important for several reasons.  These are explained as follows:

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 as little guys compared to big guys.  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 (for example, Buctril and Beacon).    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.  With drop nozzles, Accent, dicamba, and NorthStar can be applied up to 36-inch corn, Beacon and 2,4-D before tassel, and Shotgun up to 12 inch corn.  The V-staging system counts the number of visible collars, which is the area where the leaf blade joins the leaf sheaf.