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Superweeds a growing nightmare for agriculture

Nearly 100 separate weed species show resistance to one or more classes of herbicides. Near total reliance on glyphosate was a significant factor in building herbicide resistant pigweed.

Ron Smith 1, Senior Content Director

March 25, 2013

13 Min Read

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.

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.”

 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.

Lack of diversity hurts

Near total reliance on one herbicide and one mode of action was a significant factor in building herbicide resistant stands of Palmer amaranth — and other weed species. “Lack of diversity hurt us,” he said. “We can’t depend on one mode of action.”

North Carolina row crop farmers have had “a tremendous problem with resistance,” York said. “It made us change, and it’s expensive. The Southwest is beginning to see some Palmer amaranth resistance. They should start control efforts early, before it overwhelms them. Early detection is important. Watch for live (Palmer amaranth) plants intermingled with dead ones. That’s a red flag.”

Farmers in the Mid-South states have faced similar problems with first ALS and then glyphosate resistance in cotton, soybeans and corn. Glyphosate resistance in marestail first showed up in west Tennessee in 2001 and then spread to Palmer amaranth and Italian ryegrass.

Producers are finding they can still farm in the face of such resistance, says Tom Eubanks, weed scientist with the Mississippi Agricultural and Forestry Experiment Station based in Stoneville, Miss.

“One of the keys is the use of a residual herbicide,” Eubanks told farmers attending the Iowa Soybean Association’s OnFarm Network Conference in Ames, Iowa. “A second is making sure you apply a postemergence herbicide to Palmer amaranth (pigweed) when the plants are small. If you wait until the pigweeds are more than 2 inches tall, it’s almost impossible to control them.

 “Palmer amaranth has become the driver weed for us in the South,” he said. “One plant that escapes can grow 7- to 8-feet tall and produce 1 million to 1.5 million seed.”

Weed resistance can come as an unwelcome surprise to unsuspecting farmers in Texas, as well, says Paul Baumann, Texas AgriLife Extension weed specialist out of College Station. Baumann told participants in the grain session of the recent Blackland Income Growth (BIG) conference in Waco that they can have resistant weeds and not know it. “It sneaks up on you.”

Using a highly effective herbicide (such as glyphosate) over several years in a row increases the potential to select “those weeds that are not susceptible to the herbicide,” he said. “That eliminates the competition and the resistant weeds flourish.”

Not a new problem

Resistant weeds are not a new problem for Texas farmers. “I’ve been warning farmers to beware of resistant weeds since 2001,” Baumann said. “Using herbicides with a single mode of action multiple times in a season and applying the same herbicide to sequential crops and over several consecutive seasons and without other weed control options,” sets a farm up for herbicide resistant weed populations.

Growers should look for warning signs such as plants that escape herbicide applications and that are surrounded by controlled plants. Also, patches of weeds that persist after herbicide applications may be resistant weeds that grew from plants that went to seed the year before.

Baumann said the problem can spread rapidly. Glyphosate resistant common waterhemp was identified in Wharton County, Texas, in 2005. “We saw more resistance in 2006 and also noticed resistance to other herbicides.” Resistance was also spreading to other counties. In recent years, glyphosate resistant waterhemp has spread from south Texas up into the Blacklands.

Common waterhemp is more difficult to control than the typical pigweed species. “It’s not the same as Palmer amaranth. Some herbicides don’t work as well on waterhemp as they do on Palmer amaranth.” Consequently, proper identification is essential for effective control.

Observers had “a lot of hits” on resistant waterhemp in 2012. “By the time farmers notice they have resistant or uncontrolled weeds, the plants are 12 inches to 16 inches tall,” and harder to manage, Baumann said. “Control could be a timing issue, so it’s important to make sure if it’s resistant.”

That means collecting seed and having them tested. A recent survey collected seed from 20 plants from 17 locations where weeds were not adequately controlled. The seed were grown in a greenhouse and exposed to various rates of glyphosate, from a half-rate to four times the normal rate. Of the 20 plants, 19 had more than 50 percent survival with a half-rate; 17 had more than a 50 percent survival at a full rate; 15 had more than 50 percent survival at two times the normal rate and 8 had more than 50 percent survival at four times the normal application rate. “At that level, we know resistance is an issue,” Baumann said.

The problem is not confined to south and central Texas. In 2011 Texas AgriLife personnel confirmed Palmer amaranth resistance in Terry County, in the High Plains. “Palmer amaranth is the most common weed in that area,” Baumann said.

In 2012, resistant Palmer amaranth was identified in several more High Plains counties. Of 10 suspected sites, resistant Palmer amaranth was confirmed in eight.

Getting worse

Shane Osborne, agronomist at the Oklahoma State University Research and Extension Center in Altus, said during the recent No-till Oklahoma Conference in Norman that folks harbor a lot of misconceptions about herbicide weed resistance but the problem is getting worse.

“Not all weed control failures occur because of resistant weeds,” Osborne said, “and there is a difference between resistance and tolerance. Resistance is an acquired ability of a plant or population to survive an herbicide application. This trait passes from one generation to another.

“Tolerance is an inherent ability of a species to survive and reproduce following an herbicide application, but there is no change over time.”

Some weeds, for instance, are more susceptible to some herbicides than they are to others.

Horseweed has shown some herbicide resistance in Oklahoma for some time, he said. “And we heard questions about potential Palmer amaranth resistance in adjacent regions last year.”

He recommends farmers order herbicide early to make certain they get what they need for the target weeds they’ve identified in their fields. “Make each application count and don’t sacrifice accuracy for speed.”

Ken Metcalf, an independent crop consultant in the Oklahoma Panhandle, also discussed resistant weed issues at the No-till Oklahoma Conference. Northern Oklahoma farmers may have a few different weed problems than farmers in the southern part of the state, he said. Knowing the specific weed population is the first step in developing a control strategy.

The resistant weed populations may differ somewhat across the Sunbelt and herbicide selection will vary according to the weed species, the crop being grown and the management system in place. But the basic tenets of herbicide resistant weed management apply whether the problem occurs in Wilson, N.C., Jackson, Tenn., or Apache, Okla.

The most common denominators include:

• Change to a different mode of action;

• Don’t rely on a single mode of action all season;

• Add residual herbicides to the weed control program, including pre-plant incorporated and pre-emerge herbicides;

• Use full label rates;

• Target small weeds;

• Start clean in the spring, using a burndown and then residuals to take pressure off post emergence products.

Herbicide resistance is also beginning to make inroads in the western states. Weed resistance to an herbicide recently showed its ugly head for the first time in Arizona. Greenhouse assays last fall conducted by University of Arizona weed scientist Bill McCloskey confirmed Palmer amaranth resistance to glyphosate in cotton.

The resistance was found in an 80-acre field rotated with wheat in the Buckeye area. When McCloskey first visited the field, a severe Palmer amaranth infestation had outlasted several applications of glyphosate.

“I am not surprised that glyphosate resistance was found in Arizona,” McCloskey said. “It was only a matter of when.”

Glyphosate is the predominant weed-management strategy used by Arizona cotton growers year after year.

McCloskey expected to find herbicide resistance first in tree crops since glyphosate can be applied up to eight times annually for weed control.


Specialists also have some different recommendations, depending on location, crop choices and management options.

York said direct sprayers and hooded sprayers may help take out herbicide resistant weeds. He already noted hand weeding and some tillage.

“Shifting varieties is also a possibility,” he said. “Include some non-glyphosate tolerant options. We’re seeing more Liberty Link cotton and soybeans.”

But he cautions growers not to abuse that or any other herbicide. “Don’t over-depend on any one product. Use other chemistries.”

Crop rotation also offers resistance management opportunities. Tobacco may work well for some North Carolina growers. “It’s a high value crop that takes a lot of management but will reduce weed pressure.”

Grain sorghum may be an option for Southwest cotton growers. Baumann said Roundup Ready sorghum is not available, “and probably never will be. Sorghum gives us an opportunity to use a different mode of action.”

And Metcalf said canola may be a good alternative to continuous wheat to reduce pressure from cheat and resistant Italian ryegrass. Growers do have the option of planting Roundup Ready canola, however, and should be cautious about overuse.

Baumann said growers should do all they can to “get rid of early-season competition.” Killing weeds early not only takes pressure off post-emergence herbicides, such as glyphosate, but it also gives the crop a better opportunity for early growth. “Farmers may think they can control weeds later,” he said, “but by then, it has already picked your pocket every day it’s allowed to stay in the field.”

Clean fields, he said, are essential. “Controlling 85 percent to 90 percent of weed populations is not enough.

He said hooded sprayers may be useful for post-emergence treatments.

“Always use the full rate of the proper herbicide,” Osborne said. “Lowering rates is the fastest way to get into trouble. Also, don’t forget to control weeds on fence-rows, ditches and turn-rows. Be aware of what you might bring into a field on equipment, in seed or hay. Those are all avenues for herbicide resistant seed.”

Specialists all agree that weed size and application timing are key factors in control success.

And diversifying production practices may become necessary. “Don’t rule out rotation or tillage,” Osborne said. “Can I recommend tillage at a no-till conference?”

Apparently so.  Metcalf also suggested tillage as one possibility. Several other specialists argue that if resistant weed populations become unmanageable in no-till systems, tillage may be necessary until the fields are cleaned up.

They all agree that the best option is to prevent herbicide resistance and avoid the expense, aggravation and the reduced yields that will come with heavy populations of weeds they can’t control.

“Take steps to prevent resistance,” Baumann said. “That means employing a preventive program now. And farmers will get the added advantages of getting rid of weeds early.”

Waiting for resistance to develop before taking action, he said, is asking for trouble, lots of trouble. One waterhemp or pigweed plant may shed as many as 400,000 seeds. If just a few of those survive, the potential for “an overwhelming” infestation is significant.

An ounce of prevention, as the old saw goes…

Glyphosate resistance means Arizona cotton farmers need to change production practices. McCloskey recommends mechanical tillage at pre-plant, in-crop cultivation, and post harvest, plus hand rouging before seed set, and multiple herbicide use with different modes of action.

“The top pre-season pigweed control option I recommend is a dinitroaniline (DNA) herbicide application of Prowl, Treflan, or generics,” McCloskey said. “The best way to apply a DNA herbicide is on the flat with a boom on a disk or field cultivator to incorporate the product.”

Forrest Laws and Cary Blake contributed to this article

About the Author(s)

Ron Smith 1

Senior Content Director, Farm Press/Farm Progress

Ron Smith has spent more than 40 years covering Sunbelt agriculture. Ron began his career in agricultural journalism as an Experiment Station and Extension editor at Clemson University, where he earned a Masters Degree in English in 1975. He served as associate editor for Southeast Farm Press from 1978 through 1989. In 1990, Smith helped launch Southern Turf Management Magazine and served as editor. He also helped launch two other regional Turf and Landscape publications and launched and edited Florida Grove and Vegetable Management for the Farm Press Group. Within two years of launch, the turf magazines were well-respected, award-winning publications. Ron has received numerous awards for writing and photography in both agriculture and landscape journalism. He is past president of The Turf and Ornamental Communicators Association and was chosen as the first media representative to the University of Georgia College of Agriculture Advisory Board. He was named Communicator of the Year for the Metropolitan Atlanta Agricultural Communicators Association. More recently, he was awarded the Norman Borlaug Lifetime Achievement Award by the Texas Plant Protection Association. Smith also worked in public relations, specializing in media relations for agricultural companies. Ron lives with his wife Pat in Johnson City, Tenn. They have two grown children, Stacey and Nick, and three grandsons, Aaron, Hunter and Walker.

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