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Resistant weeds trouble growers

Published on March 9, 2012 3:01AM

Last changed on April 6, 2012 7:29AM

Overuse of Roundup blamed, but problem bigger, expert says


Capital Press

Weeds are becoming resistant to herbicides faster than agri-business companies can create new chemicals to combat them, experts say.

The regular use of popular herbicides exerts "selection pressure" on weeds, driving those with a genetic ability to withstand the chemicals to survive and reproduce.

Meanwhile, manufacturers haven't been keeping pace with new products that weeds have never encountered, said Sarah Ward, a plant science professor at Colorado State University.

"Herbicide research is just not coming up with novel mechanisms of action," she said.

Finding new ways for chemicals to disrupt and kill unwanted plant organisms is difficult and time-consuming, she said.

Meanwhile, the widespread popularity of glyphosate -- a broad-spectrum herbicide that's effective against virtually all weeds -- may have discouraged companies from pursuing new chemicals, Ward said.

Much of the controversy over herbicide resistance in weeds has pertained to genetically engineered Roundup Ready crops that can tolerate glyphosate herbicides, she said.

"They've been fingered as the bad boys in this," Ward said.

While the development of glyphosate-resistant weeds is accelerating in part due to Roundup Ready crops, the problem of herbicide resistance is much broader, she said.

"Every herbicide out there, we're seeing resistance," Ward said. "It's way beyond genetic engineering."

In Oregon's Willamette Valley, for example, new populations of glyphosate-resistant annual ryegrass have been popping up among crops that haven't been genetically engineered.

In 2011, two new populations of glyphosate-resistant annual ryegrass were confirmed in Oregon, bringing the total to eight populations discovered since the early 2000s, said Carol Mallory-Smith, a weed science professor at Oregon State University.

Seven of the resistant weed populations were found in orchards -- primarily hazelnut -- and one was discovered at a Christmas tree farm, she said.

"That doesn't mean there aren't more populations out there," Mallory-Smith said. "Those are the ones we have identified."

For example, a ryegrass population found at a roadside is being evaluated for glyphosate resistance.

It's likely that each population has emerged independently of the others, which could mean the weed is developing several modes of resistance.

"It's not like it started in one place and spread," said Mallory-Smith.

Resistance to glyphosate likely developed in ryegrass due to the plant's prevalence in the region, she said. Because the plant can't self-pollinate, it's also more likely to spread its genes.

The herbicide-resistant weed has been found in orchards, but tolerance to glyphosate can develop anywhere that the chemical is used repeatedly, said Ed Peachey, horticulture professor at OSU.

"Any cropping system where you're spraying Roundup, Roundup, Roundup, you're setting yourself up for resistance," he said.

To prevent the development and spread of weeds resistant to glyphosate and other herbicides, growers should alter their management techniques, said Lee Van Wychen, director of science policy for the Weed Science Society of America.

Using a variety of chemicals and mechanical means of control can forestall resistance by killing weeds that can withstand the herbicide most commonly used by the farmer.

"We're trying to encourage growers to use a multifaceted approach," Van Wychen said.

Convincing farmers to proactively fight herbicide resistance can be challenging if they haven't experienced the problem, said Ward.

Exposure to numerous chemicals can also lead to additional forms of tolerance.

"We're now seeing weeds resistant to more than one herbicide, which I think is going to be the wave of the future," she said.


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