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Corn+Soybean Digest

Postemergence Herbicide Tips for Good Weed Weather

Ample soil moisture and warm temperatures are resulting in rapid corn growth as well as rapid weed development. In order to get the most effective results from postemergence herbicides, University of Illinois Extension Weed Specialist Aaron Hager offers a few recommendations.

“Before you apply, scout corn fields to accurately determine the crop’s growth stage,” Hager says. “Adverse environmental conditions can result in corn plants that are physiologically older than their height suggests, so assess the plant’s developmental stage by evaluating leaf/collar number in addition to plant height.”

Look for the maximum corn stage listed on the respective herbicide label and do not apply the product if corn exceeds the labeled stage, he adds. If tankmixing two or more products, follow the most restrictive corn growth stage listed on any of the tankmix component labels.

With the increased occurrence of glyphosate-resistant weed populations and weed species that are inherently less sensitive to glyphosate, tankmixing products with glyphosate to control these challenging species has become a more common practice.

“Tankmixing broadens the spectrum of weeds that can be controlled,” Hager says. “For example, in glyphosate-resistant corn, tank-mixing growth regulators such as dicamba or 2,4-D with glyphosate can improve control of glyphosate-resistant waterhemp and other tough-to-control broadleaf weed species, such as annual morning glory and giant ragweed.”

Warm temperatures and relative humidity can also enhance absorption of postemergence herbicides. Consult product labels when selecting spray additives to include with them, Hager says. “Some labels suggest changing from one type of additive to another when the corn crop is under stressful growing conditions,” he says. “However, attempting to save a trip across the field by applying a postemergence corn herbicide with a liquid nitrogen fertilizer solution such as UAN as the carrier is not advisable. Applying high rates of UAN by itself can cause corn injury, but adding a postemergence herbicide can greatly increase corn injury.”

Effectiveness of postemergence herbicides can be reduced if weeds are stressed before or after the application. Labels recommend avoiding inter-row cultivation within a certain number of days before or after application to avoid reducing effectiveness. While side-dressing corn generally does not disturb as much soil as inter-row cultivation, growth of weeds near to where the applicator knives passed could be slowed for a few days following this type of nitrogen application.

Hager also recommends using caution when applying some herbicide formulations on hot days. “High air temperatures enhance the possibility of volatilization of certain herbicide formulations,” he says. “Vapors are easily moved by air currents and could potentially move out of the treated area and cause injury to sensitive vegetation nearby.”

For more information on applying post-emergence herbicides, read the May 27 edition of The Bulletin, an online publication written by U of I Extension specialists in crop science,at

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