As the standing-room-only crowd gathers one hot September afternoon, it’s not just the heat they are trying to avoid, it’s weeds — more specifically, herbicide-resistant weeds.

“If I had given this talk 12 months ago, the room would be half empty,” says Aaron Hager, University of Illinois Extension weed science specialist. “But producers are starting to experience weeds that they can’t control, and they are concerned.”

Michael Owen, extension specialist at Iowa State University, who shares the stage with Hager at the seminar, tells the crowd bluntly, “Weeds are the single most economically important pest complex producers have to manage. And weed management isn’t what it used to be.”

Hager and Owen show growers images from areas where resistant weeds have taken over. Palmer amaranth and waterhemp, along with marestail, are resistant to several classes of herbicides and render entire fields unfarmable. Crews of workers are using hoes to chop down resistant Palmer amaranth before it can produce seed. And fields that two years ago only had a “few” weeds are suddenly choked with resistant waterhemp.

“The issue of herbicide-resistant weeds is a crisis to the producers who are living it,” says Arlene Cotie, product manager, communications and trait integrity for Bayer CropScience. “One grower in Arkansas lost $230 an acre and had to hire a chopping crew to cut down Palmer amaranth with stalks so large they couldn’t get their hands around [them].”

And while weeds that have developed resistance to glyphosate have been put under a microscope, experts are quick to point out that glyphosate-resistant weeds are by far not the only concern. Lurking in the shadows are weeds that are resistant to other chemistries, and in some cases multiple chemistries.

“The most important thing to remember is that this is not a problem with the herbicide,” Owen says. “How the herbicide has been used, and the management programs used, have caused this problem.”

In fact, the issue of weeds adapting and changing predates herbicides, and in some cases weeds have changed due to cultural practices. “More than 100 years ago, Charles Darwin described how organisms respond to their environment. They either adapt or die,” Owen says. “Any single thing you do, if it is successful and puts selection pressure on that organism, ultimately that organism will change to survive.”

Producers using a single class of chemistry, year after year, to control weeds turned their fields into petri dishes where the continual use of one mode of action meant that ultimately a few weeds would survive, passing along this resistance to the next generation.

For amaranth species, such as Palmer amaranth and waterhemp, a single plant can produce more than a half million seeds. If these weeds are left to mature, a combine becomes a very large seed spreader during harvest.