Herbicide-resistant ‘superweeds’ on the rise

Superbugs get the headlines but the threat of superweeds should also sound alarm bells, researchers suggest.

Just as some bacteria are becoming resistant to multiple types of antibiotics, a broadleaf weed common to corn and soybean fields across the US Midwest is resisting an increasing number of common herbicides.

In a recent study by the University of Illinois, Adam Davis and colleagues documented the first instance of a non-grass plant being resistant to a class of herbicides known as Group 15s. 

There are many herbicides on the market in the US, Davis explains, but they all fall into one of 16 classes describing their mode of action (MOA) or specific target in the plant that the chemical attacks. 

Because of both regulations and biological realities, a smaller number of herbicide MOAs can be used on any given crop and the suite of weeds that goes along with it. 

Historically, about nine have been useful for waterhemp (Amaranthus tuberculatus). Now it appears to be resistant to at least seven of them.

“In some areas, we’re one or two MOAs away from completely losing chemical control of waterhemp and other multiple-herbicide-resistant weeds,” says Davis. “And there are no new herbicide MOAs coming out. There haven’t been for 30 years.” 

Co-author Aaron Hager says they don’t want to panic people, “but farmers need to be aware this is real. It continues on with the challenges we’ve warned people about for years.”

The team tested the effectiveness of soil-applied Group 15 herbicides in field trials and then with a dose response test in the greenhouse. In some cases, they could apply a lot more than the labelled dose and still see resistance.

The additional problem is that farmers might not notice poor herbicide performance because waterhemp germinates continuously throughout the season. If a weed pops up mid-season, they won’t always know whether it was exposed to herbicide residue.

“If you think about how you use these products, rarely do they last the entire year,” says Hager. 

“They’re very dependent on environmental conditions to work effectively. It could be too wet or too dry. 

“Generally speaking, you have some weed escape. But many farmers would chalk it up to these weather issues. If you’re not thinking about it, you could very easily overlook resistance.”

Hager suspects the problem is one of metabolic resistance: waterhemp can break down the chemicals before they cause damage. And once we get into the era of metabolic resistance, he adds, our “predictability is virtually zero”. 

“We have no idea what these populations are resistant to until we get them under controlled conditions. It’s just another example of how we need a more integrated system, rather than relying on chemistry only.”

The study’s findings are published in the journal Weed Science.