Good Weed Weather Provokes Herbicide Reminders - U of I Extension

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Good Weed Weather Provokes Herbicide Reminders

This article was originally published on May 26, 2010 and expired on June 10, 2010. It is provided here for archival purposes and may contain dated information.

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 post-emergence 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 said. "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 added. If tank-mixing two or more products, follow the most restrictive corn growth stage listed on any of the tank-mix component labels.

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

"Tank-mixing broadens the spectrum of weeds that can be controlled," Hager said. "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 post-emergence herbicides. Consult product labels when selecting spray additives to include with them, Hager said.

"Some labels suggest changing from one type of additive to another when the corn crop is under stressful growing conditions," he said. "However, attempting to save a trip across the field by applying a post-emergence 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 post-emergence herbicide can greatly increase corn injury."

Effectiveness of post-emergence 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 said. "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. It can be found under Ag & Natural Resources on Greene County Extension's website.

University of Illinois Extension provides equal opportunities in programs and employment. Extension programs and materials are research based and strive to meet the needs of people locally. If you need a reasonable accommodation to participate in this program, contact Rick Keim at 217/942-6996.

Source: Aaron Hager, Extension Specialist, Weed Science/IPM,

Pull date: June 10, 2010