For more than 10 years, weed resistance research has focused on understanding the metabolic capabilities of waterhemp. But farmers face other weeds that are just as problematic in terms of growth rate and resistance to herbicides — such as Palmer amaranth.

Since 1989, cases have proved Palmer amaranth is resistant to herbicides from nine site-of-action groups in the U.S., but not all in the same plant or population yet, says Dean Riechers, U of I weed scientist. The most recent case of resistance to a different site-of-action is a glufosinate-resistant Palmer amaranth population reported in Arkansas in 2020.

Folks at the University of Illinois know this. That’s why they’re in the initial stages of a new project to investigate herbicide resistance and tolerance in Palmer amaranth in the U.S.

“We’ve been focusing on waterhemp because it’s our No. 1 problem weed, but I wouldn’t be surprised if in five years or so Palmer becomes the No. 1 problem weed in Illinois — or at least equally as bad as waterhemp,” Riechers says. What makes Palmer such an aggressive up-and-comer?

“Palmer amaranth is different from waterhemp in part since it originated in northern Mexico and the Southwestern U.S. — basically the Sonoran Desert,” he says. That means it’s extremely drought- and heat-tolerant. However, it’s moved east and north throughout the U.S. over time, including all the way to North Dakota.

North Dakota State University weed specialist Joe Ikley shared on a recent podcast that Palmer amaranth is an emerging problem in North Dakota. Palmer has been identified in 14 counties in North Dakota since 2018 and has been introduced into several cattle operations in the eastern and western parts of the state.

Ikley says some of this movement out of Palmer amaranth’s native regions could be due to human actions — possibly that livestock feeders have imported sunflower screenings as feed that may have contained Palmer amaranth seeds.

That kind of relocation has also happened in Illinois, with Palmer amaranth seeds showing up in imported mulch that had likely been shipped north from Mississippi and other Southern states, Riechers adds. “I think we’re just accidentally moving Palmer amaranth seeds around the country,” he says. “And whenever it gets introduced, it can adapt and become a problem.”

Getting ahead of weed resistance

Riechers is excited about new management strategies like harvest weed seed control and seed destruction machines. However, you can’t rely on that technology alone because Palmer is as crafty as waterhemp in terms of its ability to continuously evolve and develop herbicide resistance.

Since Palmer amaranth is currently resistant to herbicides from nine sites-of-action, farmers are getting close to the point of running out of options to control the weed, he explains. In comparison, waterhemp now is resistant to herbicides from seven sites-of-action.

Palmer amaranth also has an even faster growth rate than waterhemp, Riechers says. When Palmer amaranth reaches about 4 inches tall, the weed is hard to control with any herbicide. Weed scientists describe this lack of herbicide control due to plant height as tolerance. Although the rapid growth rate of Palmer amaranth is likely the main reason behind tolerance, it is possible that other unknown tolerance mechanisms exist in Palmer amaranth.

Weed scientists still have a lot to learn about the resistance and tolerance mechanisms of Palmer amaranth, so Riechers is part of a research team working on a new Palmer focus. The team also is in collaboration with Syngenta scientists from the U.S. and Europe.

Currently, the research team is comparing Palmer amaranth populations from different states and locations within these states, he says. They’ll use the HPPD-inhibitor herbicide Callisto on all populations to see if they are resistant to the herbicide as a starting point.

If the populations are resistant to Callisto, researchers will dig deeper into whether the resistance mechanism is the same in all Palmer amaranth populations or if the mechanisms are different, Riechers adds. They also want to learn what causes Palmer amaranth to be tolerant to any herbicide.

The upshot: Learning how Palmer evolves its resistance could help companies develop the most effective and sustainable herbicides for agriculture to use down the road. And with any luck, that means farmers will have another tool the next time Palmer evolves a new mechanism to combat herbicides.