A probable case of glyphosate-resistant Palmer amaranth (pigweed) has been found in Georgia. The discovery is no surprise: inexpensive glyphosate, an increasing reliance on Roundup Ready crops and the weed’s diverse genetics led many weed scientists to predict such resistance long ago.
“Many of us said with the selection pressure we were putting on pigweed, this would happen,” said Bob Hayes, University of Tennessee weed scientist. “For the amount of pressure, this announcement was right on schedule. I hope this opens a bunch of eyes and shows how important a diversified weed control program can be.
“When this pigweed gets (to the Mid-South), it could set us back. But this is a big enough deal that many smart minds will go to work to find a solution. I think it could dramatically change the cost of production. But I’m not saying the sky’s falling yet.”
Hopes and fears
While no one knows exactly what glyphosate-resistant pigweed would mean to the Mid-South, “it’s fair to say it won’t be good,” said Ken Smith, Arkansas Extension weed specialist. “Pigweed has a history of resistance. It’s a great candidate for it because it has a lot of genetic diversity, it produces a lot of seed, it grows rapidly, it competes well and it germinates over a long period of time. All of those factors mean we’re facing trouble.”
Unlike glyphosate-resistant horseweed (marestail), Palmer amaranth seed isn’t windborne. For that reason, weed specialists don’t anticipate a rapid movement of the pigweed biotype found in Georgia.
“I believe we’ll develop resistance in a local population,” said Andy Kendig, Missouri Extension weed control specialist. “That will probably happen quicker than the Georgia weed will spread to this region.”
The Southeast finding “definitely” changes the playing field, said Arkansas-based Ford Baldwin, a weed scientist and Delta Farm Press contributor. “Anyone who says he’ll eradicate it is kidding himself. It’s good they’re trying but weed resistance doesn’t typically start in a single field and then spread across the country.
“The same mechanism that caused resistance in Georgia is in place across the country in umpteen acres. We’re in a mono-glyphosate culture and this was bound to happen. And it’s bound to keep happening.”
Years ago, Baldwin predicted Palmer amaranth would be the first weed to become glyphosate-resistant. He was wrong — marestail (horseweed) and common ragweed beat pigweed for the dubious honor.
“This is the first serious (glyphosate-resistant weed), though,” said Baldwin. “Common ragweed is easy to control. Marestail isn’t as easy to control, but it can be done. Palmer amaranth, on the other hand, was hard to control even before Roundup Ready came along. What is our alternative if Roundup doesn’t work anymore? That changes the game considerably.
“Resistant pigweed will impact agriculture in Arkansas in many more ways than marestail has. Compared to resistant pigweeds, marestail is a minor inconvenience. Resistant pigweed could be catastrophic.”
Eschewing glyphosate, the most-used herbicide in the world, won’t work, said Kendig. “Just use less glyphosate isn’t a workable solution for growers. A large part of our research program is investigating alternative chemistries. We’ve found if we delete glyphosate from our weed-control programs, new weed problems pop up very quickly. We really need and like the broad spectrum and strong control glyphosate normally provides.
“Our example for the upper Mid-South is horseweed. Over the last few months, we’ve used 2,4-D treatments, Clarity treatments, and Gramoxone treatments on test plots with varying success. Some of the treatments were inadequate as far as overall burndown. We still need glyphosate in the mix.
“Pigweeds are scary. We struggle to control them with conventional chemistry. ALS provided good control for a couple of years. But there’s plenty of ALS-resistant pigweed now, so that’s another problem with this.”
Smith agrees with Kendig. A few years ago, when producers began using Staple on pigweeds in northeast Arkansas, it worked “wonderfully. It killed pigweeds going and coming. Then, all of a sudden, it stopped working as well. Now, we believe half the pigweeds in the state are resistant to ALS herbicides. So when glyphosate resistance becomes a problem here, there’s a 50 percent chance it will already be resistant to ALS herbicides.”
Baldwin, who rarely minces words, said he expects Arkansas will harbor the resistant weed soon. “This is coming, and it’s going to be a massive problem. It’s going to be especially bad on the sandy soils in the northeast. Before Roundup Ready, producers couldn’t control Palmer pigweed. In our research plots, we couldn’t either. When this hits, watch out.”
Looking for solutions
To a man, the specialists said no magic bullet exists to deal with resistant pigweeds. They agree a partial solution will be residual herbicides.
“A preplant residual is probably a good idea but it won’t fit the bill entirely,” said Smith. “A preplant residual will last three weeks at the most. Pigweed germinates all the way through the cropping system. Truth is, putting another herbicide out up front or chucking another herbicide into the tank will help, but it won’t keep us from developing resistance.”
Smith expects more acres will soon be planted in Liberty Link cotton. “That cotton makes use of a different herbicide with a different mode of action. It will be a good resistance management tool. I think producers will eventually rotate Roundup Ready cotton with Liberty Link cotton.”
If resistant pigweed moves into Arkansas in a “big way,” the injury and yield reduction will be “more than Asian soybean rust or boll weevils ever thought about causing,” said Smith. “Unfortunately, if we get into a resistant pigweed with cotton, we have no post-emergence chemistry to go over the top. If pigweed has already germinated, we can’t take it down.
“Pigweed produces 500,000 seed per plant. When you’re facing those kinds of numbers, you can lose a crop. Pigweed can be devastating.”
Smith also predicts the arrival of resistant Palmer amaranth will lead to a reduction in conservation tillage systems.
“We’ve been proud of the con-till systems, and we should be. But I don’t see how resistant pigweed won’t set us back in conservation efforts.”
This belief ties into the rapid increase in farm size since 2000. “That’s happened largely because we can farm more acres under Roundup Ready technology using con-till. If resistant pigweed hits that technology, producers working 6,000-plus acres with a handful of employees are going to be facing some tough choices.”
Hayes said resistant pigweed could cause a move away from conservation tillage. “We’ve seen that happen some in Tennessee due to resistant horseweed. Of course, the current cost of diesel fuel could keep the producers in con-till.
“Not everyone has Palmer amaranth. In Tennessee, it’s predominately on the Delta (western) side. It’s true that in that area, resistant pigweed could have a huge impact. But in the rest of the state, it may have no impact at all.”
Roundup Ready crops have changed the United State’s row-cropping system.
“Farming has changed incredibly because of the Roundup Ready technology,” said Baldwin. “I’m not sure we’re capable of going back and farming like we have in the past. Everything is tied to Roundup Ready now. And that’s fine: Roundup Ready is the best thing that’s happened to us. But this is a huge issue.”
It’s more important then ever before for producers to report any escaped pigweed. Because of weed escapes, Smith has seen more rope-wick applicators used this year than in the previous 10. He assumes the escapes aren’t due to resistance.
“Earlier this year, we had poor application weather and that led to inadequate coverage or larger weeds when they were sprayed. That creates a problem for finding early incidences of resistant weeds. It’s hard to be vigilant when there are escapes everywhere. But please try. If you see any escapes, let someone know.”
“Last fall, the focus was on the threat of Asian soybean rust,” said Baldwin. “But this was always lurking. Let me tell you, when we start getting glyphosate resistance to weeds like Palmer pigweed, it’ll make rust look like a pup. Now, this weed problem isn’t going to blow in overnight like rust. But once you get it, that weed will be in your field every year. It’s going nowhere.”