Roundup-resistant weeds an evolving challenge

Water hemp in a field of soybeans. (Photo by Bob Hartzler)
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October 10, 2012 - 6:30am

Farmers and weeds are in a constant competition. But when the herbicide called Roundup came along it gave farmers a clear edge. Now, after years of exposure, weeds are beginning to catch up and farmers are using more chemicals to try to stay ahead.

Genetically engineered Roundup Ready crops came on the scene in the mid-1990s. The seeds, which were able to tolerate Monsanto’s Roundup herbicide, were adopted quickly by farmers. By 2011, more than 90 percent of soybeans and cotton, and more than 70 percent of corn were planted with Roundup Ready seeds.

“It was so simple and forgiving and flexible and there weren’t a lot of risks involved with it,” said Chuck Benbrook, a researcher with the Center for Sustaining Agriculture and Natural Resources at Washington State University.

In a new study, Benbrook determined farmers used more herbicide in recent years as weeds started to adapt to Roundup, or the generic version called glyphosate.

Photo by Bob Hartzler

Marestail is called horseweed by some farmers. Because of its slender leaves, it is hard to apply a lethal dose of herbicide, which allows the plant to build resistance.

Photo by Bob Hartzler

Giant ragweed is a familiar nemesis for farmers, and a growing threat as a Roundup-resistant weed.

Photo by Bob Hartzler

Water hemp in this soybean field was not killed by Roundup.


“The so-called herbicide tolerant Roundup Ready corn, soybeans, and cotton (have) driven up herbicide use by about 527 million pounds over this 16 year period,” Benbrook said.

Benbrook said that is a total increase of about 7 percent from 1996-2011. Some of the largest annual increases came in just the last few years as more farmers began to battle Roundup-resistant weeds.

 “Certainly in last four or five years it has really gotten to be a serious problem for farmers in southeast where many of them don’t have now any effective herbicides to deal with glyphosate-resistant palmer amaranth,” Benbrook said.

Monsanto did not respond to a call seeking reaction to the study.

The problem farmers face is similar to the overuse of antibiotics, and doctors’ race against drug-resistant infections. Benbrook said farmers relied too heavily on Roundup and now the effect is beginning to wear off for farmers like Ron Pavelka near Hastings in central Nebraska.

Talking from the cab of his combine, Pavelka said most of his corn and soybeans are Roundup Ready and have been since the seeds became available.

In the beginning, before there were Roundup Ready soybeans, weed control in soybeans was always an issue for producers,” said Pavelka, who is also on the Nebraska Soybean Board. “There were a lot of different options but nothing that worked as well as a Roundup application.”

But these days, Pavelka said Roundup is not working as well on weeds as it used to. Like many farmers Pavelka has added other herbicides to the mix to finish the job. He also started spraying fields in the fall to help keep weeds down. Still, a few slip through the cracks.

“In our area here the number one issue would be with a weed called marestail,” Pavelka said. “It kind of has a really slender leaf so it’s an issue getting the proper amount of herbicide on that plant. And it’s just a tough woody old thing.”

Across the Midwest, Bob Hartzler has been watching a handful of weeds form pockets of resistance.

“For Iowa and most of our surrounding states, water hemp is the number one weed,” said Hartzler, an agronomist and weed specialist at Iowa State University. “It’s a native species. Giant ragweed is another native species that’s adapting to corn and soybeans. It’s the number one problem in states like Ohio, Indiana.”

To Hartzler, the problem in the Midwest is serious, but not equal to the disaster some southern cotton farmers have experienced.

“I don’t think we’ll have the scenario where we’ll see entire fields being abandoned and bringing in crews to hand weed our fields,” Hartzler said. “But we are going to see increases in the amount of herbicide used, and we’re going to see increases in tillage which contributes to erosion.”

Chuck Benbrook believes farmers can manage resistant weeds, for now, by going back to some of the chemicals they used before Roundup came along. Monsanto and Dow are working on new seed varieties which can better tolerate 2,4-D or dicamba, two of the oldest but most effective herbicides used by farmers.

However, Benbrook said, older herbicides have drawbacks. Many are more toxic than Roundup and more likely to drift in the wind which can potentially damage neighboring fields.

And Bob Hartzler said using more herbicides to handle weeds does not address a fundamental problem.

Plants like water hemp, marestail, and giant ragweed have become very good at adapting to whichever herbicides farmers choose. Hartzler said conventional farming plays to their comfort zone.

“I like to say you couldn’t sit down and design a better system for weeds to adapt to,” Hartzler said. “We’re relying almost entirely on herbicides. We’re growing two crops with the same life cycle. And weeds just thrive under that system.”

Weeds will continue to adapt. The question is what farmers will do to stay one step ahead.



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