Farmers are facing or trying to avoid the nightmare of herbicide resistant weeds and grasses that threaten to upset management practices and alter enterprise choices, chemical selection and tillage systems.

Palmer amaranth, also known as pigweed, is the most commonly discussed or just plain cussed weed across the Sunbelt, but a host of others, nearly 100 separate species, show resistance to one or more classes of herbicides.

Virtually all crop gatherings — from the Beltwide Cotton Conferences to regional expos to county production meetings — now include at least one weed specialist discussing the threat of herbicide resistance and offering recommendations on how to avoid the problem or how to manage it once it establishes itself on a farm.

(See Pigweed marching toward Western agriculture — photo gallery)

Avoidance is the best, least costly, approach, specialists say, but in some areas the mule has left the barn and farmers are left trying to reclaim cropland from heavy weed infestations.

Alan York, retired Extension weed specialist from North Carolina State University and now a consultant, speaking at the recent Bayer CropScience-sponsored Southwest Crop Consultants Conference in Austin, Texas, said some areas in the Southeast have been “overwhelmed.”

Most farmers, York said, are not as prepared for resistance management as they might think. “If they get into a bad problem, they can quickly get overwhelmed.”


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The numbers prove his point. At least 76 weed species have known resistance to some herbicide, and weeds have shown resistance to at least 18 different modes of action. “It’s a big problem.” Glyphosate is the first herbicide that comes to mind in a resistance discussion, but that may not be the worst problem. Weeds and grasses are also resistant to ALS and ACCase inhibitors as well as to glyphosate.

But the glyphosate issue is huge with 99 percent of soybeans planted to Roundup Ready varieties. That’s 90 percent for cotton and 80 percent for corn.

York said observers have identified four weed species with glyphosate resistance in North Carolina. A small stand of glyphosate-resistant common ragweed has been identified. “It was in a small geography but it is spreading. We also have a long history of resistant Italian ryegrass problems in wheat. We’re now seeing glyphosate resistant ryegrass.”

Marestail/horseweed is “a big problem. But Palmer amaranth is the big one. It has been a game changer for us. It’s amazing that one weed species can change everything we do.

“We have to go through a denial phase before we confirm resistance,” York said. A 2005 survey showed less than 20 percent of Palmer amaranth resistant to glyphosate and less than 15 percent resistant to ALS herbicides. “That resistance was relatively isolated and Palmer amaranth was a relatively new weed for North Carolina.”

A 2010 survey showed resistant Palmer amaranth “all over the state and it became our No. 1 concern.”

Growers, York said, “are making progress. It’s not nearly as bad as it used to be. We were having problems with Palmer amaranth before Roundup Ready, and we did a good job of control. But growers selected for resistance by using only Roundup.”

Resistance complicates production. “We’re dealing with it but we’re having to throw the kitchen sink at it.” Control efforts include changing chemistry and management systems. “We’ve gone back to using residual herbicides and direct sprayers,” among other things, York said.

Grower surveys in Georgia, South Carolina and Tennessee also show severe problems with resistant Palmer amaranth. “Farmers in South Carolina are going back to hand weeding. Before resistance showed up only 5 percent were weeding by hand. After resistance, that increased to 50 percent at its peak.”

York said farmers in the Southeast had moved increasingly to no-till production. “To deal with resistant Palmer amaranth, many have moved back to conventional-tillage. They are incorporating herbicides, using more residuals and some are using a bottom plow.

“We see more hooded sprayers and direct spray applications. All those changes have increased production costs. Before resistance, weed control cost about $25 per acre. After resistance, weed control expense is $60 per acre, almost three times as much.