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Emerging Solutions

Gil Gullickson 10/25/2010 @ 12:29pm Crops Technology Editor for Successful Farming magazine/Agriculture.com

If you ever doubt how quickly herbicide-resistant weeds multiply, ask Sid Fogg. The story the Heth, Arkansas, farmer tells is typical of many mid-South farmers.

“A couple years ago in 2008, we had a pretty good soybean crop,” he says. After harvest, though, Fogg noticed scattered Palmer amaranth plants across some of his fields. Initially, he figured a sprayer missed some of these pigweed-family members during a postemergence glyphosate herbicide application.

Fogg came back in 2009 with wheat on 400 acres followed by a double crop of soybeans. A timely rain spurred a good early stand of soybeans. Unfortunately, it also triggered a soybean-choking early flush of Palmer amaranth. Fogg applied his normal postemergence chemical application of a full glyphosate rate laced with 6 ounces per acre of Flexstar. The Flexstar addition aimed at controlling weeds like morning glory, waterhemp, and black nightshade that are known to tolerate glyphosate.

However, prolific rainfall pushed a glyphosate-resistant Palmer amaranth explosion that overwhelmed this mix. “By the time we came back again to spray, the pigweeds were too big,” Fogg says. “We had pigweeds that were a foot tall, two feet tall.”

At that height, subsequent postemergence herbicide applications had no effect. Fogg spent about $50 per acre over 400 acres with little weed control triggered by glyphosate-resistant Palmer amaranth. In areas choked by Palmeramaranth, soybean yields dipped from 40 bushels to 20 bushels per acre.

“At that time the price was $9 per bushel, so that is $180 per acre in lost yields on top of the $50 per acre for chemical that didn’t work. In those areas, we had $230 per acre in lost revenue.”

Break Out The Hoes

The mid-South rapidly adopted Roundup Ready technology for corn and cotton when it first appeared in the 1990s. It worked well at first. “Ten years ago, there were no weeds in soybean fields in Arkansas,” says Ford Baldwin, co-owner of Practical Weed Consultants LLC and former University of Arkansas (U of A) Extension weed scientist.

The first chink in glyphosate’s armor appeared in 2003, when glyphosateresistant marestail surfaced. Glyphosateresistant common ragweed popped up in 2004, followed by glyphosate-resistant Palmer amaranth in 2005. Now, glyphosate-resistant Palmer amaranth reigns in the mid-South.

“If you see a Palmer amaranth in your (Arkansas) field, you need to assume it is glyphosate-resistant,” says Jason Norsworthy, U of A weed specialist.

This is compounded by multiplemode resistance, often to glyphosate and ALS herbicides. “There are 16 different herbicide-resistant biotypes of weeds in  Arkansas,” says Norsworthy.

How Resistance Starts

Regardless of location, all herbicide resistant weeds share a similar origin.

“There are resistant genes in every weed population,” says Wes Everman, Michigan State University Extension weed specialist. “It’s just a matter of selection pressure. We look at South vs. North. There are different cropping systems. In the North, as we look at corn and soybeans, there are normally two weed-control applications (annually) and sometimes one.”

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