Isoxadifen-ethyl and Corn

Linda Benedict, Leonard, Billy R., Williams, Billy James  |  3/1/2011 3:41:54 AM

How to mitigate injury from Resolve-Counter interactions

Bill Williams, B. Rogers Leonard and Rakesh Godara

Corn producers who raised corn before the introduction of Roundup Ready varieties are aware of the interaction between sulfonylurea herbicides and soilapplied organophosphate insecticides, which were in use then. Producers often were faced with selecting the best postemergence grass control product, Accent, or the best insecticide, Counter. If Counter was used at planting and johnsongrass was a problem later in the season, Accent could not be used without the risk of severe injury to corn. In Louisiana, corn yield losses associated with injury from Accent following Counter often exceeded yield losses associated with weeds.

Use of Accent for johnsongrass control all but ended after the introduction of Roundup Ready corn, which is resistant to the herbicide glyphosate. The use of Counter for insect pest management also declined as producers adopted Bt corn and new, easier-to-use insecticide seed treatments. Resolve, another sulfonylurea herbicide, has become more popular because it improves annual grass control in Roundup Ready corn. At the same time, interest has increased for using reduced rates of Counter to supplement insecticide seed treatments for improved pest management.

In recent years, some producers have discovered the hard way that, as with Accent, severe corn injury occurs when Resolve is applied following Counter. Even in the absence of Counter, sulfonylurea herbicides can injure corn when environmental conditions favor rapid herbicide uptake or reduce a corn plant’s metabolic activity. As result a safener, isoxadifen-ethyl, was developed to improve crop safety.

Research at the Macon Ridge Research Station has documented the interaction between Resolve and Counter and evaluated the effect of isoxadifen-ethyl on that interaction. Resolve at 1 ounce per acre reduced corn yields 10 to 15 bushels per acre, even in the absence of Counter with no visual signs of injury. Regardless of the Counter rate, a maximum of 13 percent injury was observed from Resolve applied at the growth stage when corn has three fully formed leaves (V3). However, when Resolve was applied at V3, corn height was reduced by 54 percent when Counter was applied at 0.5 pound of active ingredient per acre and 69 percent when Counter was applied at 1 pound of active ingredient per acre at planting. Yield also was reduced, at least equally, when Resolve was applied at V3 following Counter. Applying Resolve at V6 resulted in more injury, greater vigor reductions and greater yield losses than when it was applied at V3.

Applying Resolve with isoxadifenethyl eliminated any negative effects on corn injury or yield when applied at V3 or V6 in the absence of Counter. Isoxadifen- ethyl also reduced visual injury symptoms and vigor reductions when Resolve was applied following Counter. Still, when Resolve followed Counter at any rate, yields were reduced by an average of 10 percent at V3 and and 55 percent at V6.

Isoxadifen-ethyl significantly reduced corn injury from Resolve following applications of Counter. Still, yield losses are likely if Resolve is applied following Counter at rates as low as 0.5 pound per acre. Isoxadifen-ethyl may not eliminate the interaction between Resolve and Counter, but it does enhance corn tolerance to postemergence applications of Resolve.

Bill Williams, Associate Professor and Weed Management Specialist, Scott Research/Extension Center, Winnsboro, La.; B. Rogers Leonard, Jack Hamilton Regents Chair in Cotton Production, Macon Ridge Research Station, Winnsboro, La.; Rakesh Godara, Research Associate, Northeast Research Station, St. Joseph, La.

(This article was published in the winter 2011 issue of Louisiana Agriculture Magazine.)

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