Sounding the Alarm

Sounding the Alarm

Since its discovery in 2004, glyphosate resistant Palmer amaranth has kept extension specialists and weed scientists with their hands full. When the resistant strain began popping up elsewhere – first in other counties, then other states, and finally in other regions of the Cotton Belt – those same industry experts were the first to sound the alarms. Four years later, they’ve got a better grasp on managing the problem.

But sounding the alarm is an every day challenge. That’s why Cotton Incorporated dedicated an entire session to the growing problem at their Cotton Management Seminar held in Tunica, MS, in November. The location of the seminar wasn’t just happenstance. Resistant pigweed is beginning to show up in alarming numbers in the Mid-South.


“We see glyphosate resistance as a national issue,” said Dr. Robert Nichols of Cotton Incorporated, while citing the problem’s westward movement from Georgia since 2004. “I would call it an emerging weed in the Mid-South, except our friends in Arkansas might disagree, seeing as how they already have this problem.” He went on to say that all Mississippi counties north of Highway 82 will be increasingly vulnerable to the problem in the near future.

Dr. Alan York of North Carolina State University further illustrated the movement of the weed epidemic. “2004 was the first sighting. We call that the epicenter, in Macon County, Georgia.”

To an expert like York, the weed’s success in such a wide variety of places is academic.

Designer Weed

“If you were going to design a weed, you’d use the blueprint for this one,” York says.

“(It has a) very efficient carbon fixation. It is water use efficient, and has very high temperature tolerance – optimum for photosynthesis at 180 degrees. So when it’s hot and dry, and the crop is struggling to hang on, this thing is happy. It grows very fast – an inch or two a day – and gets very big. Obviously it’s very competitive with the crops.”

In short, the resistant pigweed travels fast, appears quickly, and perhaps most problematically, grows to an unmanageable size almost overnight. At 2 inches tall, the weed can be easily managed. The problem of course, is that it cannot be easily spotted at 2 inches, and often by the time you see it, it has grown too large to kill with conventional chemistry.

“The difference between where it’s manageable and the size where it’s too big to control in a good growing environment might be 72 hours,” says Dr. Larry Steckel with the University of Tennessee. “We’re talking hours to be able to get out there. And as big as a lot of our growers have gotten, that’s going to be a challenge to get across that many acres, over the top, in that time period.”

A Greater Challenge

The general consensus among weed experts is that while Roundup Ready technology made life easier for growers, it also lulled them into a false sense of security. Because of the proliferation of the resistant gene, they say, cotton producers must unlearn some of the habits that came about with Roundup Ready practices.

“We all know that Roundup Ready technology was a good thing. It was idiot proof. But what it did was make our growers into weed sprayers. This resistance is going to make us go back and be weed managers,” says York.

Because of the plant’s ability to get out of hand so quickly, Steckel and other leading weed scientists are advising growers from the Southeast to the Mid-South to take pre-emptive steps against resistant pigweed.

“Even without the glyphosate resistance, we’re making a recommendation for something residual either pre-emergence, or maybe its Valor pre-plant and in that case we’d probably still want to follow it with something pre. We are trying to limit our PPO inhibitors to one application per year. And we are trying to not depend on ALS materials any more than we have to,” says Steckel.