It’s been 14 years since Stanley Culpepper confirmed the country’s first glyphosate-resistant Palmer amaranth population in some cotton fields in Macon County, Georgia.
The University of Georgia Extension weed scientist had been seeing the warning signs of this new beast for several years before the laboratory confirmed his fears, so he’s nearing a major anniversary with the weed.
“We’ve now spent nearly two decades with glyphosate-resistance in this weed,” Culpepper said. “It’s still ubiquitous here. But our weed control programs are doing a pretty good job now.”
The first two stories of this series, The State of The Pigweed explored the spread of herbicide-resistant Palmer amaranth into the Southwest, Mid-Atlantic and North-Central regions of the U.S. This third story heads back to the South, home of the pigweed “veteran states,” where farmers first battled glyphosate-resistant populations.
The war is not won, as farmers and Extension scientists will be quick to note. Many have successfully diversified their weed management tactics and saved farms once overrun by resistant pigweed, but the type and number of herbicide-resistant Palmer populations have continued to grow and spread.
“Our entire weed management program is tuned for herbicide-resistant Palmer amaranth,” Culpepper explained. “Glyphosate, ALS, atrazine — we have multiple resistance in the same populations. So you have to manage every field for resistance, every year.”
But Southern growers are still learning the toughest lesson of all.
“We have to be smart enough to realize that herbicides alone are not sustainable against this type of beast,” Culpepper said.
The Veterans And Their Battles
Most of the veteran states are in the South: Arkansas, Alabama, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina and Tennessee. But the list also includes the southeast corner of Missouri, known as the Bootheel, where Palmer amaranth has driven weed control for many years.
Palmer has long been a problem weed. Resistance to atrazine and ALS inhibitors in pigweed populations date back to the late 1980s and early ’90s. But for nearly a decade after the introduction of Roundup Ready crops, Monsanto and many growers believed glyphosate was indestructible.
Weed scientists warned against overuse of the herbicide, but by 2005, the country had its first official confirmation of glyphosate resistance and the weed took off. Within one year, glyphosate-resistant pigweed was officially confirmed in four additional states: North Carolina, Arkansas, Tennessee and South Carolina. By 2012, all the veteran Southern states had identified and reported it.
These resistant populations quickly dismantled the decade-long practice of relying on glyphosate alone for weed control, University of Tennessee Extension weed scientist Larry Steckel recalled. Even though some other chemicals still had efficacy against the weed, it grew so fast and in such abundance, that farmers fell farther behind each year.
“By 2011, it really blew up,” Steckel said. “That’s when we started disking down fields to try to stop its spread.”
Many farmers resorted to manual removal, in a bid to get persistent weed seedbanks under control, Culpepper recalled. “Just five to six years ago, we were spending millions of dollars a year on handweeding,” he said.
Hard Work, Zero-Tolerance
It took years of hard work, aggressive education, and a zero-tolerance policy on weed seed production for farmers to regain control of many cotton and soybean fields in these Southern states. The hardest step for growers to grasp was, and remains, that the chemical-only approach to weed control is gone and is never coming back, Culpepper said.
“Yes, we need diversity in chemical modes of action, but we can’t win with just herbicides,” he said. “Growers have to use cover crops, handweeding and tillage to get back on top of this.”
Now many Southern scientists routinely recommend an aggressive prescription of crop rotation, chemical rotation, overlapping pre- and post-planting residual herbicides, timely applications of chemicals and hand removal of weed escapes.
Timeliness is perhaps the most important and the most difficult part of the equation, said Louisiana State University weed scientist Daniel Stephenson. Wet weather can derail even the most carefully planned weed control strategy.
“It’s growing 1 inch a day,” he said. “So that pigweed that was 2 inches on Tuesday? By Saturday, you’re dealing with 6-inch plant. And if a Palmer amaranth plant is bigger than 4 inches, pretty much anything you use on it will be a sub-lethal dose.”
More Chemical Failures Ahead
Most pigweed populations in these Southern states have resistance to glyphosate and ALS herbicides. Atrazine resistance appears occasionally, too, and a number of states have pigweed populations that can survive dinitroaniline (DNA) herbicides, such as Prowl and Treflan.
Most recently, however, PPO resistance has surfaced in Arkansas, Mississippi, Alabama, Tennessee, Kentucky and the Missouri Bootheel. Scientists in Louisiana, North Carolina and South Carolina suspect it is already present there, as well.
“We’re very worried about PPO herbicides, because the mid-South and Illinois have a lot of [PPO resistance],” said North Carolina State University Extension weed scientist Charlie Cahoon. “We’re putting a tremendous amount of selection pressure on this class of herbicides in soybeans, cotton, tobacco and peanuts.”
Widespread PPO resistance would be a breaking point for Alabama peanut producers, added Auburn University Extension weed scientist Steve Li. “When we start getting PPO and ALS resistance in the same pigweed, we are almost out of solutions for postemergence control of this weed in our major peanut production region,” he said.
Scientists are increasingly concerned about the longevity of three additional herbicides, too. Glufosinate has long been used as a back-up weapon against glyphosate-resistant Palmer, and auxin herbicides, namely dicamba and 2,4-D, are now being deployed on a growing number of dicamba- and 2,4-D-tolerant cotton and soybean acres.
“I’m surprised we don’t have glufosinate-resistant Palmer yet, because we used a lot of Liberty,” prior to the introduction of dicamba-tolerant crops, said Clemson University Extension weed scientist Mike Marshall. “Now, I’m concerned about dicamba. That’s what many growers have switched over to here. If they’re just using dicamba and glyphosate, they’re really only using one herbicide with activity on this weed.”
Steckel agreed, noting that the increase in PPO resistance has driven a rapid switch to Xtend soybeans and some overreliance on dicamba in Tennessee. “With the broad adaptation of Xtend crops in 2018, many growers have been tempted and, in some cases, a few are utilizing just one herbicide post emergence to manage that weed,” he said.
Even as Palmer amaranth populations check off resistance to more individual classes of herbicides, another threat looms for the Southern states — and all of the U.S.
“We’re pretty confident that we have a handful of Palmer populations in the state that have metabolic resistance, which can lead to resistance against other herbicide groups or modes of action,” said University of Arkansas Extension weed scientist Tom Barber.
Many weeds develop resistance through a specific genetic pathway — such as a change in the target site of the herbicide, which renders the plant less sensitive to the chemical’s effects. But with metabolic resistance, weeds learn to rapidly metabolize, or break down, the herbicide within the plant, thus avoiding injury from it.
A weed that develops metabolic resistance to one herbicide is thus far more likely to be able to survive other herbicides.
“It’s the next big threat, in some Arkansas pigweed populations,” Barber said.
For a map showing the full distribution of Palmer amaranth in the U.S., see this link from University of Wyoming weed scientist Andrew Kniss: https://plantoutofplace.com/…
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org
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