Corn Post-emergence Weed Control
Timing Important for 3 Reasons

Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension

Many pre-emergence herbicide plans had to be scrapped this year because of frequent rains and warm temperatures that provided for rapid emergence.   For these situations, and those where a post-emerge program was the game plan all along, timing of weed control will be important for three reasons: yield reduction, effective weed control, and crop injury.  Let’s take a closer look at each.

Yield reduction

WHEN weeds are controlled is nearly as important as IF weeds are controlled.   As indicated in the Minnesota study below (Tables 1 and 2), delaying foxtail control can cut yields.
 

The study provides further evidence that when foxtails were controlled too early (0.5 to 1.5 inches), control was reduced and yields suffered due to late season flushes of the weed.  However, when control was late (>6 inch foxtail), control was good but again corn yields were reduced resulting from early season weed competition.  We did a similar study in Fond du Lac County in 1997 and found much the same results.  A good objective is to treat foxtail when the tallest plants 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.

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.  Beacon and Buctril are the exceptions to this rule in that if applications are made too early, the risk of injury also increases.  The following list provides the range of maturity stages that corn can be treated based on label recommendations.  The pre-emergence grass herbicides (Dual II Magnum, Frontier, Guardsman, Harness/Surpass/Top Notch, Micro-tech/Partner, and Prowl) listed below will not control emerged grasses, but will provide residulal grass   contol when applied.  If the corn exceeds the size listed on the label, switch to a herbicide that allows application at a more advanced stage of growth or use drop nozzles if permitted.  With drop nozzles, Accent, dicamba, or Northstar can be applied up to 36 inches, Beacon before tassel, and 2,4-D before tassel.  The V-staging system counts the number of visible collars, which is the area where the leaf blade joins the leaf sheaf.  For more information on corn V-staging consult How A Corn Plant Develops.

* Applies only to resistant hybrids

For more information contact Mike Rankin