Timing, Product Choice Important For Postemergence Herbicide Applications

Weather has prevented Indiana corn farmers from applying herbicides before much of the crop emerged from the ground. But that doesn’t mean it’s too late, a pair of Purdue Extension weed scientists say.

Most of Indiana’s corn crop was planted May 4-9, but windy conditions followed by days of rain kept growers from applying pre-emergence herbicides for weed control. The good news, however, is that many of these herbicides are still safe for farmers to use after corn has emerged so long as they pay close attention to product selection and application timing, Bill Johnson and Travis Legleiter said.

“Many of the pre-emerge herbicides can also be applied post-emerge, and there are many herbicides for postemergence weed control in corn,” Johnson said. “The large number of products is a positive when considering glyphosate-resistance management and prevention, but they can also make timing and product application decisions more complicated.”

This is especially true because postemergence herbicides can affect corn ear development if those products are applied too late in the growing season.

When growers decide on herbicide products, Legleiter said they should consider weed species present, weed height and crop growth stage.

“The majority of conventional post-applied corn herbicides are effective on select weed species and only at certain weed heights,” he said. “Typically, a combination of products or a prepackage of active ingredients is needed to achieve control of all weed species present.”

Herbicide labels are a valuable resource for growers because they list which weed species the products control and recommended application heights.

Another resource is the Indiana and Ohio Weed Control Guide, published by Purdue Extension and Ohio State University Extension. The guide is available for free download from Purdue Extension’s The Education Store and searching for product code WS-15-W.

In addition to product choice and application timing as it relates to weed height, Johnson said it’s extremely important for growers to consider the growth stage and height of corn at the time of herbicide application to prevent injury.

“The type and amount of injury from an application beyond the labeled window is dependent upon the herbicide, other environmental stresses and the exact timing of application,” he said.

Injury symptoms can include ear pinching, ear bottlenecking, internode stacking, onion leafing, rat tailing, brace root malformation and green snap.

  • Johnson and Legleiter also offered a list of helpful tips for avoiding herbicide crop injury:
  • Don’t use contact herbicides right before rain or when there is heavy dew in order to avoid washing the herbicide down into the corn whorl.
  • Don’t apply growth regulator herbicides after several nights with temperatures 45 degrees Fahrenheit or cooler.
  • Don’t use UAN solutions as the herbicide carrier when applying atrazine premixes to spike-stage corn.
  • Don’t mix growth regulator herbicides with chloroacetamide herbicides and apply postemergence.
  • Don’t apply ALS inhibitors past the V6 growth stage in corn.
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