There are a dozen weed species in Illinois known to be resistant to one or more herbicides, but Aaron Hager, weed science professor at the University of Illinois, said there is one that seems to be more adaptable than any other.
"Waterhemp is the one that we focus a tremendous amount of time and effort toward better understanding and characterizing simply because it's the one herbicide resistant weed species now that occurs on more acres in the state and has evolved resistance to more herbicide families than other species that we have," Hager said.
It's important to vary weed control chemistry and tactics so that any given one does not become overused, said Hager.
"Repeated use of the same tactic over and over and over simply allows the resistant population or the resistant biotypes to become dominant members of any particular weed species population, so the resistances that we've seen, for example, in waterhemp have closely evolved in parallel to the types of herbicides that we've used," said Hager.
The biggest worry is the plant that develops resistance to multiple control platforms, said Hager. There are plants in the university greenhouse resistant to four different herbicide families, he said.
"Management becomes much, much more difficult because in each field's case there is more than likely resistance not just to glyphosate, but perhaps to other herbicides that may render both glyphosate and the other herbicides pretty ineffective against the waterhemp in the field," he said.
It's becoming rare that a new active ingredient is introduced to the market, and even if more were coming to the market, said Hager, resistance would likely develop to them too. He sums it up with the observation that "it's going to get worse before it gets better."