Glyphosate-Resistant Weeds Take Over The South – for Jul. 29, 2010

In his deep southern drawl, crop consultant Ford Baldwin summed up the weed control disaster facing Arkansas soybean and cotton farmers with a tale of two pickups.

“A typical Arkansas farmer’s pickup four or five years ago would have had a beer cooler, a set of jumper cables, a few tools, maybe a jug of hydraulic oil, and a can of freon to keep the air conditioners charged in the tractors – and that would be it.”

“You look at a farmer’s pickup here this time of year now and it will typically have a bigger beer cooler – and the rest of the back of the truck will be full of hoes.

“That’s typical of what you would have found in my grandfather’s 1954 Chevrolet pickup when he was farming cotton, and there’s something about that picture that’s not right. We’re going backwards.”

Standing tall above the soybeans in the field behind him was the reason why.

Patches of glyphosate-resistant Palmer amaranth waved in the gentle breeze at farmers, agronomists and reporters invited here by Bayer CropScience as part of its “respect the rotation” initiative.


Already twice the height of the planted crop, the plants known locally as “pigweed” were not only thriving in the sweltering heat and humidity, they were on the verge of releasing millions of tiny seeds.

Of the 10 weed species in 20 states to develop resistance to glyphosate since Roundup Ready crops were first introduced in 1997, Palmer amaranth aptly fits the description of “superweed.”

Strong enough to damage farm equipment, it is aggressive and smart too – learning in less than a decade how to overwhelm the product weed control experts claim is, or rather was, the world’s greatest herbicide.

Glyphosate, developed by Monsanto in 1970 and brought to the market as Roundup, has until recently been in a league by itself as a non-selective non-residual herbicide.

Once believed immune to the evolution of resistant weeds, it controlled more than 140 species, and was hailed as the single most important driver in the evolution of conservation tillage.

“You will never see another herbicide as good as this one,” Stephen Powles, director of the Western Australian Herbicide Resistance Initiative at the University of Western Australia in Perth and a leading authority on glyphosate resistance, told about 200 tour participants.


But that’s history for the farmers in the U. S. “glyphosate belt” where it has been virtually the only herbicide applied to one billion acres of soybean, cotton and cornfields over the past decade. So is the industry’s marketing pitch that herbicide-tolerant crops resulted in farmers using fewer herbicides.

Resistant pigweed has spread like a Prairie wildfire since it was first confirmed in 2005, rendering next to useless the herbicide that up until recently, farmers were told was the only one they would ever need.

“In my opinion, the bulk of the value of the glyphosate in the Roundup Ready program is gone in our part of the country,” Baldwin said.

Marestail (horseweed), giant ragweed, johnsongrass, waterhemp and ryegrass have also developed resistance in this region. Researchers predict the next species to evolve resistance will be barnyard grass, a major weed control issue in rice.

Jason Norsworthy, a weed scientist with the University of Arkansas, said the introduction of Roundup Ready crops in the late 1990s made weed control deceptively simple for soybean and cotton farmers.

Farmers previously used a combination of rain-incorporated pre-emergent and post-emergent herbicides to control weeds, but they made a wholesale switch to glyphosate under the Roundup Ready system.

Baldwin said weed scientists, including himself, bought into the illusion too. “I’ve written a weekly column inDelta Farm Presssince 1974, and I can’t tell you how many times I wrote that our research program just doesn’t show any benefit to adding an additional herbicide or tank mix partner,” he said.

“Use Roundup every Monday morning and there’s nothing out there but soybeans and dirt.”


A number of other factors conspired to increase what was already intense selection pressure. Because glyphosate was protected by patent in the U. S. until 2000, farmers initially used less-than-recommended rates on their Roundup Ready crops to save money.

They then began upping rates to address rising tolerance levels. “In Palmer amaranth, we’ve done some work where we took a population that was never exposed to glyphosate and in five generations we had a population that was 2.5 times less sensitive to glyphosate than what we started with,” Norsworthy said. “That’s part of the reason why we are where we are today.”

Application timing also fell by the wayside. Farmers became accustomed to allowing weeds to reach 18 inches high before spraying in an effort to get them all in one swoop, as opposed to using repeated treatments on small flushes.

As reports of resistance grew, Monsanto began in 2008 offering farmers in 13 states rebates on companion herbicides to ward off rising levels of resistance. And it has more recently introduced stewardship extension programs to help farmers cope.

Adding to the problem is the plant’s resistance to ALS herbicide chemistry, which until Liberty Link crops became available last year, left farmers with little choice but to resort to mechanical means of control such as tillage and hoeing crews.


Adjusting to their new weed control reality has required three fundamental shifts in thinking:

The old adage “if it ain’t broke, don’t fix it” doesn’t apply to weed management. “If it’s working, change,” said Powles, noting diversity is key.

Farmers can no longer think about “economic thresholds” in the same way. Weed populations need to be assessed, not based on their potential to harm yield in one year, but for their potential to build the weed seed bank.

The focus of crop management has shifted from annual yield to long-term weed control.

As yet, there are no weeds known to have developed resistance to glufonsinate, which is the active ingredient in Liberty Link and marketed in the U. S. under the brand name Ignite. But researchers are convinced those resistant genes are out there, waiting to be unleashed.

That’s the reason behind Bayer CropScience’s extensive “respect the rotation” extension effort. There is already evidence that farmers with glyphosate-resistant weeds are simply switching from Roundup Ready crops to Liberty Link varieties, placing the same selection pressure on that technology.

Extension agronomists and Bayer officials are advising farmers to use other methods of reducing the weed seed bank before introducing the Liberty Link system to the rotation. They also advise using glufosinate in conjunction with pre-emergent herbicides.


But it’s a hard sell. The desperation of farmers with weed-infested fields, the effectiveness of glufosinate and the unshakeable notion that weed control should be simple is a dangerous combination.

“We’ve got a generation, not just farmers, but a generation of weed scientists, a generation of consultants, of seed dealers who all don’t know anything but Roundup Ready weed control,” Baldwin said.

“I am very concerned that we are going to burn that technology the same way we burned glyphosate,” said Norsworthy. “When you think about resistance, it’s really a numbers game and the numbers are stacked in favour of resistance.”

In other words, the bigger the population of weeds that is repeatedly exposed to glufosinate, the greater the likelihood a resistant gene will gain the upper hand.

“We are putting selection pressure on Ignite on that large pigweed, and we’re not controlling it and we’re getting tremendous seed production. That is basically a prescription for disaster from a resistance standpoint.” [email protected]





“We’ve got a generation, not just farmers, but a generation of weed scientists, a generation of consultants, of seed dealers who all don’t know anything but Roundup Ready weed control.”


About the author

Vice-President of Content

Laura Rance

Laura Rance is vice-president of content for Glacier FarmMedia. She can be reached at [email protected]



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