Postemergence Corn Herbicide Considerations

The eastern Corn Belt's early corn crop has started to – and so have the weeds. So now is the time for growers to consider postemergence herbicide applications, says Travis Legleiter, a Purdue Extension weed specialist. In the northern part of the Corn Belt, some of the crop is as far along as the V3, or third leaf, growth stage. Development is even further along in some fields in the southern part of the region. That means corn farmers need to prepare to apply the right postemergence herbicide at the right time, likely within the next few weeks.

"Unlike postemergence soybean herbicides, there are a large number of herbicides available beyond glyphosate products for weed control in corn," says Legleiter. "The large number of products is a positive when considering glyphosate-resistance management and prevention, but also can make timing and product-application decisions more complicated."

Most herbicides are effective on some weed species and only to certain weed heights. According to Legleiter, controlling all of them likely will require a combination of products or a pre-package of active ingredients.

Also making the decision complex is corn ear development, which can be affected if postemergence herbicides are applied too late in the growing season. Therefore, growers need to consider not only the sizes and types of weeds present but also crop growth stage.

"The type and amount of injury from an application beyond the labeled window is dependent upon the herbicide, other environmental stresses and exact timing of application," Legleiter says.

He says injury symptoms could include ear pinching, ear bottlenecking, internode stacking, onion leafing, rat tailing, brace root malformation and green snap.

When growers are tank-mixing products, Legleiter says it's important to follow the most restrictive label to determine the right crop growth stage restriction. Other ways to avoid crop injury include:

• Avoid using contact herbicides just prior to rain, or when there is a heavy dew, to avoid washing the herbicide down into the whorl.
• Avoid applying growth regulator herbicides after several nights of temperatures 45 Farenheit or cooler.
• Do not use UAN solutions as the carrier when applying atrazine pre-mixes to spike stage corn.
• Do not mix growth regulator herbicides with chloroacetamide herbicides and apply postemergence. (These mixtures are fine if applied pre-emergence.)
• Do not apply ALS inhibitors past the V6 growth stage.

More information about choosing the right herbicides is available in the 2012 Weed Control Guide for Ohio and Indiana, produced by Purdue Extension and Ohio State University Extension. The guide is available for free download from Purdue Extension: The Education Store. It also can be purchased in hard copy from Ohio State University Extension'seStore. The guide includes information about herbicide and weed management in multiple crops, including corn, soybeans, popcorn, grain sorghum, small grains and forages.

Corn growers also can find more information about herbicides and crop growth stages in the May 11 issue of Purdue Extension's Pest and Crop Newsletter.