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Prevention is crucial for weed resistance management

With 1 billion weed seed in a field there is a good chance one of them will be tolerant to some herbicide. Farmers may employ production systems that make resistance more likely. Head off the problem before it emerges.

Herbicide resistance is nothing new to Texas farmers. “It’s been an issue for years,” says Paul Baumann, Texas AgriLife Extension weed specialist.

“We’ve seen resistant ryegrass in Northeast Texas since 1989,” Baumann said during the annual Ag Technology Conference, held on the campus of Texas A&M-Commerce.

“But new issues are developing,” he said. “Weed biotypes are showing up that may look the same as other weeds in the field but have different genetic make-ups and show resistance to Roundup and other herbicides.”

With 1 billion weed seed in a field there is a good chance one of them will be tolerant to some herbicide, he notes.

Farmers also may employ production systems that make resistance more likely. He said the recipe for resistance might include:

  • Using herbicides that act on a single site of activity.
  • Applying the same herbicide multiple times during the season.
  • Using the same herbicide with the same mode of action for several consecutive seasons.
  • Using that herbicide without other weed-control options.
  • Using a stand-alone herbicide and no other product.

“It has happened,” Baumann said. “On the Texas Gulf Coast we’ve identified glyphosate-resistant waterhemp. In 2010, we confirmed glyphosate-resistant waterhemp in several other South Texas counties, and we think it would have exploded in 2011if it had not been so dry.”

He said common waterhemp may be a harbinger. It may be one of the first glyphosate-resistant weed species to show up in Texas, and it’s hard to control. It’s also spreading rapidly. “It was confined to the Gulf Coast 20 years ago,” Baumann said.  

“Is there potential for glyphosate-resistant waterhemp in Texas High Plains cotton?”

West Texas cotton farmers, he said, have done a good job of prevention so far. “Consider: Cotton farmers still make widespread use of soil active herbicides—preplant and pre-emergence products. The added chemistry helps control weeds.

“Also, we typically see lower weed pressure in the High Plains.”

Even so, farmers have reason for concern. “In Terry County, last year folks discovered a population of glyphosate-resistant Palmer Amaranth or pigweed. And that weed is the most common weed species in Texas cropland.”

It’s also the glyphosate-resistant species that has created tremendous problems for farmers in the Southeast.

Baumann recommends Texas farmers head off the problem before it emerges. For cotton, he recommends producers “keep using or go back to preplant and pre-emerge herbicides. Also, switch transgenic technologies—alternate Roundup and Liberty Link, for instance. Rotate chemistry. Use different herbicides and apply alternative post and post-directed herbicides with alternate sites of action, in lieu of or with usual products.”

In corn, Baumann recommends the preplant or pre-emerge herbicides. He recommends using different post products as alternatives or in conjunction with glyphosate.

Rotating grain sorghum also may help. “There are no Roundup Ready-sorghum hybrids being developed,” Baumann said, “so several herbicides are available. Using that alternative chemistry helps break the resistant weed cycle, and since most are not long-lasting, it cuts back on selection pressure.”

Prevention preferred

Bauman said prevention is much preferred to battling established resistant weed populations. “Farmers can employ preventive programs now or they can wait until they see resistance and then act.”

But, considering that one resistant waterhemp plant can shed more than 400,000 seeds, it doesn’t take many to “supply weed seeds for years to come. Also, farmers may get pollen from resistant weeds blowing in from nearby farms.”

Consequently, Baumann encourages farmers to “take a prophylactic approach and prevent resistance by using soil-applied herbicides and other products.”

He said those options don’t mean farmers have to spend a lot of money on early-season weed control. And the investment pays off in more ways than preventing resistant weed populations.

“Early-season weed control limits crop competition,” he said. “If for some reason a farmer can’t make a post-emergence treatment, he could be in trouble without an early-season herbicide program. That’s a killer for yield loss.”

He said remedial weed control technology has given producers a false sense of security. “You can kill weeds that are 10 inches tall, perhaps, but while you’re getting those 10-inch weeds, they’re picking your pocket every day.”

He said a yield drag can occur within the first two to eight weeks of crop growth. “That’s when we need a weed killer out there.”


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