Invasive pigweed Palmer amaranth, found in New York state, resists common herbicides

The invasive pigweed Palmer amaranth, first found in New York soybean fields in 2019, has been dubbed the ″spotted lanternfly of weeds″ for its ability to spread quickly and wreak havoc on crops. Now a new study sheds light on how formidable an opponent the weed is for New York state growers – and offers a way forward.

The invasive weed Palmer amaranth clogs rows of soybean plants in New York state.

In a paper published in the journal Weed Science on Oct. 28, researchers find that some known Palmer amaranth populations – collected from Steuben, Genesee and Orange counties – have resistance to glyphosate, the leading weed control chemistry used in soybean crops. Researchers also confirmed the mechanism for resistance in the populations and identified classes of herbicides that are still effective.

″We can use this information to let our growers know what to expect, so they can make good decisions about how to manage the species,″ said Lynn Sosnoskie, assistant professor in weed ecology at Cornell AgriTech, part of the College of Agriculture and Life Sciences. ″We want to avoid these population blow-ups that become very difficult to control once they get going.″

Sosnoskie said Palmer amaranth’s resistance is a powerful argument for more research into alternative and integrated weed management strategies, especially as production of new herbicides is decreasing and regulation of herbicides is increasing. She and colleagues at Cornell AgriTech, in collaboration with growers, are working to make alternative strategies more accessible and feasible.

″The use of a single strategy or product puts us in danger of reducing the utility of herbicides, these very valuable tools,″ Sosnoskie said. ″We need to be thinking about how to diversify our system of weed management, and to look closely at how we could use new strategies and technologies in New York, under our production conditions.″

In optimal conditions, Palmer amaranth can grow an inch or more per day, with a single female plant producing hundreds of thousands or even up to a million seeds. Once established, the weeds choke out crops and can clog and impede harvesting equipment. Growers in other areas of the country have reported up to 79% reductions in soybean production and 91% in corn. The weed is native to the southeastern U.S. but has been spreading north as the climate warms. 

Sosnoskie’s team, led by first author and horticulture doctoral student Aleah Butler-Jones, found that the New York state populations are 42 to 67 times more resistant to glyphosate than controls; one of the populations is also likely resistant to atrazine, a leading herbicide for corn. While the researchers found that the weed still responds to other classes of herbicides, they warn that simply switching from one herbicide to another is not a sustainable solution.

″We still have sensitivity to several important groups of herbicides, but we have to be mindful that this sensitivity won’t last forever,″ Sosnoskie said. ″If we don’t use these products responsibly, we could select for resistance to those chemistries, too.″

At the molecular level, the team confirmed the unique mechanism by which the New York state populations resist glyphosate. Instead of the more typical tactic of mutating the protein receptor to which the herbicide binds, herbicide-resistant Palmer amaranth produces an abundance of receptors. This proliferation makes it difficult for the herbicide to fully impede the plant’s chemical processes. Researchers at Colorado State University, who are co-authors of the paper, first documented this mechanism; Butler-Jones confirmed the mechanism in New York’s populations. 

Researchers and growers are experimenting with alternative and integrated strategies for managing weeds, including changes to crop rotations and the addition of cover crops. They are also testing new technologies; Sosnoskie’s lab is currently conducting research on machines that can detect specific weeds and use electricity, lasers or targeted sprays to eradicate them, for example.

The relationship and communication between growers and researchers makes this work possible, Sosnoskie said.

″The best part has been wanting to understand the predicament we’re in with weeds, how much weeds can impact yields and harvest efficiency, and then working with the growers to try something new, something different and exciting,″ she said. 

In collaboration with Cornell Cooperative Extension, Sosnoskie has led outreach to growers about the spread of Palmer amaranth and other pigweed species, spearheading the development of a website to help growers and others identify the weeds.