Responding to the first known report of waterhemp showing resistance to HPPD-inhibiting herbicides (such as Callisto, Impact, and Laudis), weed science researchers at the University of Illinois have identified two unique mechanisms in the plant that have allowed the weed to "get around" these herbicides.
Dean Riechers, a U of I Professor of weed physiology, along with other collaborators at the U of I, recently published a paper describing the two distinct metabolic detoxification mechanisms that confer resistance to mesotrione (Callisto) and atrazine (Aatrex) in an Illinois waterhemp population.
"Waterhemp is very diverse, which you can see in the field. There are red plants, green plants, tall, short, bushy—basically a germplasm pool," Richers explains. "If you keep spraying the same herbicide over and over, eventually you're going to find that rare plant that can resist it."
What the U of I researchers found of great concern in this population was the way in which the waterhemp resisted the herbicide—in much the same way that corn naturally resists HPPD-inhibiting herbicides.
"In the pharmaceutical industry, doctors know that you can't keep recommending the same ampicillin prescription. You might have a limited time you can use that, and then you have to use something different," Riechers says. "It mimics corn but also mimics the super bacteria that are resistant to all the antibiotics out there. Weeds are kind of like bacteria in that respect; at least this population is. Whatever active herbicide we throw on it, with the exception of glyphosate, it doesn't work anymore."
The study was prompted in 2009 when a continuous seed corn grower from central Illinois realized the HPPD-inhibiting herbicides he was using were no longer killing waterhemp plants, which by then had grown into a literal mat of weeds across the field.
"It became obvious to the grower that something was wrong, but it probably started years before that," Riechers says.
The grower had been planting continuous seed corn every year, using HPPD-inhibiting herbicides for at least eight years in a row.
"Mesotrione and atrazine are normally two very good herbicides that are safe on corn but still kill waterhemp," Riechers notes.