Manage weeds before planting corn

• By Daniel Stephenson • 

early season weeds
Early season weeds can affect yield potential — photo courtesy University of Arkansas

Managing weeds in corn can be divided into three phases: preplant, in-crop and post-harvest, but I’ll save the post-harvest phase until later in the year.

Preplant is otherwise known as burndown. We suggested applying a burndown herbicide four to six weeks prior to planting. Typically, glyphosate at 1 to 1.25 lb ae/A is the first component with either 2,4-D at 0.5 to 1 lb ae/A and/or dicamba 0.25 to 0.38 lb ae/A in a tankmix. Sometimes LeadOff or Valor are included to provide some residual control.

Regardless of what you apply, fields need to be completely weed free prior to planting corn. Emerging corn is sensitive to weed competition; therefore, corn yield can be lost if fields are not weed-free.

If glyphosate-resistant Italian ryegrass is an issue, clethodim cannot be applied within 30 days of planting, which, based on history, we are within 30 days of planting in Louisiana.

Therefore, to manage glyphosate-resistant Italian ryegrass, your only option is paraquat at 0.75 to 1 lb ai/A tankmixed with atrazine at 0.5 lb ai/A followed by another paraquat application at the same rate 10-14 days later. If glyphosate-resistant Italian ryegrass is still alive after corn has emerged, little to no herbicide options are available.

Corn planting date influences when you should apply a herbicide treatment. If corn is planted late February through the first two weeks of March, herbicides can be applied either preemergence behind the planter or postemergence before 12-inch tall corn.

If corn is planted during the last two weeks of March or later, then a two-pass program — preemergence followed by a postemergence application — is needed. The choice of herbicide applied is not that difficult.

Based on data, weed control following a tankmix of glyphosate, atrazine and Dual Magnum is like many higher priced treatments. The only caveat is if morningglory is known to be a problem. Then, products that contain mesotrione, Halex GT for example, are the better choice. My research just does not justify spending a significant amount of money for early season weed control in corn.

Please call your local county agents if you need assistance.

Daniel Stephenson is an LSU AgCenter Extension weed scientist. He may be reached at

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