Careless weeds develop resistance to Roundup in West Texas

Reported by: Rebecca Rivers
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Updated: 6/29/2012 7:35 am

Pig weed, or careless weed is an ugly pest that farmers across the U.S. have been fighting for decades. But not until last year did farmers here in our region have an even bigger problem on their hands, and that is weed resistance to herbicides. Dr. Peter Dotray and Dr. Wayne Keeling, both Texas Extension Weed Specialists, gave me a glimpse into the literally growing problem.

"Resistance is certainly not a new thing," Dotray said. "Resistance has been occurring to various pests for over 100 years. So the entomologists have been fighting resistance to insects, pathology folks have been fighting resistance to pathogens. In weed science, some of the first reported cases of resistance occurred around 1950 or 1960."

"Because our moisture is limited, we certainly don't need to be sharing any of it with the weeds, so I think very effective weed control is  more important in a dry climate such as ours so we don't lose production to weeds," Keeling said.

Palmer amaranth, or careless weed, has been showing resistance to Roundup on farms across the south, slowly moving closer and closer to West Texas.

"Careless weed has been developing resistance to glyphosate now for probably about seven or eight years," Dotray said. "Most of that resistance has been occurring in the southeastern part of the United States. It wasn't until last fall when we first found some fields in this particular region that didn't seem to be responding as they normally had been responding."

The specialists at Texas Agri-Life Extension work together with local producers to isolate unaffected careless weed plants.

"Lots of phone calls had been coming in that palmer amaranth was patches and fields were not being controlled by glyphosate, and last year was just so hot and so dry and so windy that a lot of times that's when you'll start to see fields that aren't effectively controlled by herbicides," Dotray said.

Roundup has been such as effective herbicide over the years and consequently has changed some application methods.

"I think there's been a shift in production practices in a lot of grower's operations, from traditionally applying some pre-plant incorporated herbicides, herbicides applied at planting, other herbicides applies in-season, a lot of those products are no longer being used, and growers are effectively controlling weeds with Roundup alone," Dotray said.

Keeling said that producers can be proactive in controlling resistant populations.

"Not just reliance on Roundup," Keeling said. "But using other herbicides either pre-planting incorporated, pre-emerge, post-emerge, with different modes of action that hopefully we can control the largest portion of palmer amaranth before they ever emerge. And then there's less potential for those that emerge being resistant to glyphosate."

Physically eliminating weeds before they reproducer into resistant populations is key.

"The main thing people can do is if they have spots that they haven't killed with Roundup, is use some other means to keep those weeds from going to seed," Keeling said. "Certainly don't want to take an isolated, minor problem and turn it into a field-wide problem in the short run. If that means going out and hoeing spots, or maybe just plowing them up to destroy those resistant weeds, I think that's the most important thing."

Both Dotray and Keeling pointed out that coupling herbicides together can help control pig weed and other difficult weeds.

"More difficult to control weeds like morning glory that are now not being exposed to other herbicides, and primarily Roundup alone are becoming a lot more problematic," Dotray said.

"Anytime you're spraying Roundup, try to put something else with a different mode of action with it to help the Roundup out," Keeling said.

Insect and weed resistance to chemicals is eventually an expected problem and therefore forces producers and scientists to continually evolve their control methodology. However, staying on top of the problem and eliminating pig weeds year round does prove helpful in keeping future generations of resistant weeds from developing.

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