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New tools for managing resistant weeds

Resistnat weed control
Controlling herbicide resistant weeds is a complex process incluidng "starting clean" at planting time.
Managing herbicide resistant weeds takes a multi-pronged approach, including new technology, some old-school practices and a lot of persistence.

Herbicide-resistant weeds are no longer a problem that could cause trouble for Texas farmers sometime in the future — rather, “Weed resistance is real and a problem across Texas, the country, and the world,” says Pete Dotray, professor of weed science at Texas Tech University and a Texas AgriLife Extension specialist.

Pest resistance, including resistant weeds, isn’t new Dotray said at a recent Ag Technology Conference on the Texas A&M-Commerce campus. More than a hundred years ago — in 1908 –resistance to an insecticide was first noted; fungicide resistance was identified in 1940; and weed resistance to herbicides was confirmed in 1957, when spreading dayflower was identified as resistant to 2, 4-D.

And from there, things only got worse, he says. “Globally, we know of hundreds of specific cases of weeds that are resistant to herbicides, displaying “an inherited ability to survive a previously effective herbicide application through biological changes in the resistant plant.”

Those changes may include:

  • An altered site of action
  • Changes in metabolism
  • Enhanced expression of site of action
  • Reduced uptake of the herbicide
  • Reduced translocation
  • Compartmentalization



“It’s complicated,” Dotray says. “Herbicide resistance includes many internal factors that allow weeds to resist once-effective herbicides.”

Documentation shows 251 weed species resistant to 161 herbicides, affecting 90 different crops. Of those resistant weeds, 36 are resistant to Roundup. Palmer amaranth (pigweed), one of the biggest problems for glyphosate (Roundup)-tolerant crop production, has emerged as one of West Texas cotton farmers’ biggest weed management challenges.

Those farmers will have new options this year, Dotray says, with approval of new technology and new herbicide combinations that are effective against glyphosate-resistant pigweed. XtendFlex and Enlist, using new formulations of dicamba and 2,-4-D on cotton plants tolerant of those chemistries, will offer significant opportunities for controlling pigweed.

But he emphasizes that new varieties tolerant to dicamba are not tolerant to 2, 4-D, and those tolerant to 2, 4-D are not tolerant to dicamba. He also cautions producers not to overuse either. “If you overuse dicamba or 2,-4-D technology, we will not have them available for very long.”

He notes that some weeds already have confirmed resistance to this class of herbicides, and warns that over-reliance on them — as has been the case with glyphosate following development of Roundup Ready varieties — will select for resistant weed survival.



He says Texas currently lists seven weeds as resistant to one herbicide or another. “Palmer amaranth resistance showed up in 2011, and we are concerned over other weeds with potential for resistance.”

An important factor in the current resistance problem is a change in weed control programs.

“Before Roundup Ready, weed control was more diverse. Farmers used yellow herbicides, at-planting applications, post-directed applications, cultivation, non-selective herbicides applied under hoods, lay-by treatments — a lot of options for controlling weeds.”

The convenience of Roundup Ready technology brought changes on a lot of farms, Dotray says, and over-reliance on one herbicide “selected for resistance in the weed population.” Palmer amaranth was identified in just a few West Texas fields in 2011, but “in 2012 and 2013 we found more, and more the next few years. Now, we believe pretty much every cotton field in West Texas has a resistant weed population.

“We still have a lot of clean fields,” he says, and “following best management practices may result in weed-free cotton, even in fields with significant resistant weed populations.” Fields side-by-side may show completely different levels of weed control. Why?  He offers several reasons: “They started clean; they used DNA herbicides and pre-emergence materials chosen for the weeds present in the field; they made timely postemergence applications; and they made sure to get thorough coverage. Also, they managed escapes, and used multiple modes of action for different crops in adjacent fields.”



Activation of the applied herbicides is also important, Dotray emphasizes, and targeting small weeds for postemergence applications is crucial.

Late season control is also important, and producers should prevent weeds from re-emerging and producing seeds that add to the resistant population for the following season. Rotation to crops that have alternate herbicides available, with different modes of action and different weed species targets, is also beneficial.

Recent weed surveys are helping gather information on how growers are managing weeds, what they are seeing, and how they are responding, he says. The data show that 65 percent of growers are using trifluralin preplant incorporated, and 36 percent are using pendimethalin. “We are seeing an increased awareness of what weed resistance is, and we’re gaining better knowledge of what to do and what’s available to control these weeds.”

Glyphosate-resistant Palmer amaranth is more prevalent in West Texas than in any other part of the state, Dotray says. Their studies continue as he and other weed scientists look for better ways to manage weed problems. “We are learning more about the biology of the weed — seed production, for instance. Pigweed seed numbers may be as high as 600,000 per plant. Even late emerging weeds can produce seed quickly.”



He has recently begun a seed viability study to determine how long a Palmer amaranth seed can persist in the soil. “It’s a big seed, so we bury it in the soil and dig it up later to test viability. The goal is to determine how many months that seed will persist and how long a producer would need to follow an intense weed management program to effectively reduce the resistant weed population.”

Farmers will have more new products coming, Dotray says, including Brake herbicide, an old aquatic weed product in a new mix — fluridone and fluometuron. Inzen, available for sorghum, will help manage resistant weeds in a rotation crop. XtendFlex and Enlist technology are now available. Engenia has just been labeled.

“These options provide more opportunities to control resistant weeds, using products in a new way,” he says. “They increase the availability of different modes of action.



“But, beware the risk of overuse,” he repeats. “If we are not careful, we will be in a similar situation to what we’re seeing with Roundup.

“Potential misuse is another concern. We have to watch how these products are applied, including ground speed, weather conditions, spray nozzle pressure, and wind speed/direction. We have to manage off-target applications — non-tolerant crops, organic production, and apiaries. We have to be extremely careful how we use this new technology.”

A new Texas AgriLife/Texas Plant Protection Association program and mobile app, Flag the Technology, offers a useful tool for identifying off-target sites and preventing application errors, Dotray says. Growers also may have “one opportunity to reset the clock” by going back to old-school technology —deep breaking the land. “That can break the cycle of resistant weed seed production.” But, he cautions about overdoing deep tillage. “If you deep break, and then do it again two years later, you will bring buried seed back up.”

Managing herbicide-resistant weeds offers a complex challenge to cotton farmers — in Texas and across the cotton belt, Dotray says. But with new tools, employing some old-school technology, and a commitment to start clean every spring, producers can keep fields clean and reduce the in-field populations of resistant seed.

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