Experts express weed resistance management

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.”