Battle Continues Against Palmer Amaranth

Clint Thompson Georgia, Pest/Pest Control

UGA’s Stanley Culpepper gives a talk about pigweed research at the Sunbelt Expo. (UGA CAES photo)

What is the most challenging pest facing Georgia farmers? According to a University of Georgia (UGA) Extension survey, it’s not an insect or disease. It’s a weed, and that weed is named Palmer amaranth.

UGA Extension weed scientist Stanley Culpepper issued the survey to gauge the severity of Palmer amaranth compared to other annual pests. The results were staggering. Of more than 1,700 farmers surveyed, Palmer amaranth overwhelmingly was listed as the toughest pest; worse than white flies, stink bugs and nematodes.

“The take home message is Palmer amaranth is more impactful than the other nine pests. This is pests, not weeds,” Culpepper said.

Resistance Issues

Culpepper confirmed resistance to post-applied PPO herbicides in a couple of populations in Georgia. It was discovered to be resistant to glyphosate in 2005.

What’s the next weed to replace it? The answer is simple.

“If you’ve ever heard me talk, the next weed to replace it is still Palmer amaranth. It’s just going to have a different resistance added into its toolbox,” Culpepper said. “You’ve got glyphosate resistance. You’ve got ALS resistance.

“I think you have to have the mentality, I already have a resistant plant. Whatever that product is, if I don’t make good decisions, I’m going to have resistance on my farm, potentially in three years.”

Dan Westberg, BASF Regional Technical Service Manager, added, “The next Palmer amaranth I think we need to watch out for is that one with the metabolic resistance. If they all weren’t already resistant to it, you’ve just rendered a whole set of sites of action ineffective.”

What is Palmer amaranth?

If not managed correctly, Palmer amaranth, also known as “pigweed,” can quickly overtake a field. It can reach heights of 7 to 10 feet, rob cotton plants of needed water and nutrients and snag cotton equipment moving through a field.

Growers must implement sound diversified management programs including the rotation of herbicide chemistries. As cotton growers begin planting this year’s crop, they need to make certain no Palmer amaranth is present at the time of planting any crop. Use cover crops, tillage or burndown.

Two pre-emergence herbicides with active ingredients effective on Palmer amaranth are recommended, as are sequential post-emergence applications with a layby hooded or direct application system.

“If you look at that population we have here in Georgia, we can’t kill it with Roundup; we can’t kill it with our ALS herbicides, such as Staple. We can’t kill it now with the post-emergence PPO herbicides. It looks like we’re not going to be able to kill it with the residual Valor and Reflex,” Culpepper said. “Think about farming that way. It sends a great message to our growers, and I think they’re taking it very seriously, and we’ve got the best growers on the planet. But that diversified sound program is not only great for managing this pest, reducing selection pressure to other herbicides, it’s just an all-around sound approach to long-term sustainability.”

About the Author
Clint Thompson

Clint Thompson

Multimedia Journalist for AgNet Media Inc.