When Glyphosate Fails – Management Strategies For Resistant Palmer Amaranth

When Glyphosate Fails – Management Strategies For Resistant Palmer Amaranth

In journalism school, professors teach aspiring writers to never use clichés, especially not two in the same sentence. But if the shoe fits, I say wear it.

Glyphosate-resistant Palmer amaranth grows like a weed, and according to emerging research, it could spread like wildfire too. That sentence is no good for college professors across the country, and its truth is even worse for producers throughout the Cotton Belt. Glyphosate-resistant Palmer amaranth ruined a few cotton fields in Georgia last year, and if not properly contained and controlled, it will become a problem elsewhere.


According to Dr. Stanley Culpepper, Extension Agronomist for Weed Science at the University of Georgia – Tifton, producers with glyphosate-resistant Palmer amaranth need to take precautions from preplant to layby. For those growers who haven’t dealt with the problem yet, Culpepper offers a few words of wisdom, another cliché if you will – an ounce of prevention is worth a pound of cure. The reason is simple:

“Palmer amaranth is an extremely competitive weed, and when you think about managing it, the first thing you’ve got to think about is that Palmer is going to grow 1-3 inches a day in good growing conditions. Even in areas of Macon County where we didn’t have good growing conditions last year, it was growing an inch a day,” Culpepper said.

Unfortunately, that’s just one half of the equation. Palmer amaranth produces hundreds of thousands of seeds, and if a tolerant gene emerges, that trait can be reproduced through non-resistant plants. A field with few resistant plants one year can become choked by weeds in just a few seasons. Culpepper explains:

“One of the biggest problems we’ve got is due to the ability of this weed being able to reproduce very rapidly. If you look at average seed production per plant, when it is producing 400,000-600,000 seed per plant, this plant can quickly come in and take over a situation,” he said. “The first field we found resistance in two years ago, 25% of the population was resistant to glyphosate, which meant 75% was sensitive to glyphosate. Within two years, 75% of the population in that field is glyphosate-resistant and 25% is sensitive. That shift occurred in two years and it has got to be because of the ability of reproduction. So that’s how quickly Palmer in general can come in and take over … which in essence, in my opinion, means we go from having a chance to harvest a field to probably not being able to harvest the field at all.”

The Common Denominator

Glyphosate-resistant Palmer amaranth did not randomly. It developed a tolerance and adapted after application and application of glyphosate. If Culpepper could stress anything, he would harp on the fact that producers need to use herbicides that feature different modes of action. Overusing a single weed management tool, especially a broad-spectrum herbicide like glyphosate, can be devastating. At the very least, this single-pronged weed management system got many growers in the situation they are in today.

“If you look at the growers who had problems in 2005, there are several things that are very common. First of all, we were very dependent on a single management tactic. These growers had mostly Roundup Ready cotton, and they sprayed around 3-5 times a year, and that was predominantly the tool they used to manage this pest. Now they certainly were not alone in the single management tactic. Currently, there is over 100 million acres of Round-up Ready technology being grown, and in most of these cropping systems, glyphosate is the predominate weed management tool,” Culpepper said.

In addition to the heavy reliance on glyphosate as a panacea for weed management, Culpepper also said some producers use lower-than-recommended rates, a premise he understands, but the worst possible thing to do in the current situation. “Many of us are guilty of cutting herbicide rates. But I never understand why we want to cut the rate of glyphosate. It is the most effective and economical herbicide in the world, and we will cut the rate to save 25% or 50%,” he said. “I am from a family farm, so I know there are situations when it will pay to cut the rate and add an adjuvant. I am all for promoting adjuvants, but do not cut the rate. That is something that we do not need to do, especially in the situation that we are currently in.”

Is it Really That Bad?

Continuing on the path of monoculture crop systems and single-pronged weed management plans is disastrous. If the most effective and economical herbicide is deemed useless by resistance, it will be a major blow to producers everywhere. That, Culpepper said, is a reality producers need to understand.

“In this situation where we have resistance, and you take a 4X rate of Roundup, and you put it on the Palmer when it is 1 inch, the same Palmer when it is 4 inches and the same Palmer when it is 12 inches (and the palmer survives), you can clearly see we have lost the most effective and economical herbicide in the world,” Culpepper said. “And I think it is arguably the most important herbicide in the world in all production systems in agronomics today and in our state. So we have lost that in these fields, and we are going to continue to lose it as it spreads throughout our state, especially in circumstances where growers are too dependent on glyphosate.”

Grower-induced glyphosate resistance isn’t the only problem. As discussed in the weed management article in the March issue of Cotton Grower, a glyphosate-resistant Palmer amaranth plant can pollinate another plant from at least 200 meters away, and it is very possible that pollination can occur at even farther distances. Not only do producers have to worry about developing resistance on their farms, they also have to contend with it spreading from other areas.

“Obviously, when you have 100 million acres that you are treating with glyphosate, glyphosate, glyphosate, you will, and we did, find resistance. The problem we’ve got now is that it is going to spread, and we have to deal with it,” Culpepper said. “What we really need to do is to delay the arrival or the spread of this resistance. Once we get some number and are able to model this situation, we will be able to tell people we think it will be here in 2 years, here in 3 years, and here in 4 years. But also keep in mind that people who continue to spray glyphosate, glyphosate, glyphosate can create their own biotype in their own area, and it may be completely non-related to pollen movement.”


During UGA research on glyphosate-resistant Palmer amaranth last year, Culpepper’s team experimented with 168 different herbicide systems on 12 acres of small plot research. Of those systems, 155-160 plots had to be mowed under because of Palmer resistance, making it easy to identify possible solutions, but also emphasizing the resilience of the weed. “It makes it easy on what we are going to recommend,” Culpepper said, “because most of the things we tried simply do not work.”

For glyphosate-resistant Palmer amaranth in Roundup Ready cotton, Culpepper recommends Prowl or Treflan PPI combined with Reflex PRE during preplant or preemergence. Postemergence in 1-to 4-leaf cotton, he believes a glyphosate and Dual Magnum mix is effective if no Palmer has emerged, while a glyphosate/Staple mix should be used in Palmer that is 1-2 inches tall. For layby, MSMA combined with one of the following – Direx, Layby Pro, Suprend or Valor – is producers’ best bet. He stresses that vigilance is key to fighting gylphosate-resistant Palmer amaranth, emphasizing that managing resistant Palmer taller than 2 inches in Roundup Ready cotton is even more difficult – if not impossible.

Culpepper realizes his program will cost producers more money, especially when compared to just spraying a few applications of glyphosate. But he stresses that the program protects glyphosate, an important concept that he believes cannot be said enough, and keeps growers from an even worse scenario – mowing down there crop.

“If you look back at this program, first of all, you are going to say that I am costing you money. And I admit that. But the second thing you see is, yes, we are still using Roundup. It is still the most effective tool in 98-99% of our acreage. But what we are doing is surrounding that most important tool with three kinds of alternative herbicide chemistries with different modes of action,” Culpepper said. “This will protect the glyphosate, it will delay or help hold down ALS resistance, and it will also prevent or reduce the likelihood of resistance developing to these chemistries too, because we have three maybe four modes of action in one program. That is a sound program.”

University of Georgia Herbicide Programs for Palmer Amaranth in Cotton

Managing Glyphosate-resistant Palmer Amaranth in Roundup Ready Cotton
Preplant Incorporated (PPI)
or Preemergence (PRE)
(1-4 leaf cotton)
Layby Directed
Prowl or Treflan PPI + Reflex PRE Glyphosate + Dual Magnum (no Palmer emerged) or
Glyphosate + Staple
(1-2″ Palmer)
MSMA + Direx, Layby Pro, Suprend or Valor
Managing Heavy Glyphosate-sensitive Palmer Amaranth in Roundup Ready Cotton
At Plant Postemergence
(1-4 leaf cotton)
Layby Directed
Prowl/Treflan PPI or Prowl PRE
+ Cotoran, Direx, Reflex or
Staple PRE
Glyphosate +
Dual Magnum or Staple
MSMA + Direx, Layby Pro, Suprend or Valor OR Glyphosate + Direx, Layby Pro, Suprend or Valor
Managing Light to Moderate Glyphosate-sensitive Palmer Amaranth in Roundup Ready Cotton
At Plant Postemergence
(1-4 leaf cotton)
Layby Directed
Prowl/Treflan PPI or Prowl Glyphosate + Dual Magnum or Staple MSMA + Direx, Layby Pro, Suprend or Valor OR Glyphosate + Direx, Layby Pro, Suprend or Valor
PRE Prowl/Treflan PPI or Prowl PRE + Cotoran, Direx, Reflex or Staple PRE Glyphosate as needed