Weed scientists at the University of Illinois are currently working with a type of herbicide resistance that has not previously been reported. Basic and applied research is underway with an Illinois waterhemp population that is resistant to herbicides that inhibit 4-hydroxyphenyl pyruvate dioxygenase, generally referred to as HPPD-inhibiting herbicides.

Foliar-applied HPPD inhibitors are commonly used for control of annual broadleaf and grass weed species in corn, says University of Illinois weed specialist Aaron Hager. Several active ingredients from this herbicide family are commercially available, including tembotrione, topramezone and mesotrione. These active ingredients are available either as individual products (Laudis, Impact and Callisto, for example) or as components of pre-mixtures.

Initial greenhouse experiments conducted at the University of Illinois confirmed anecdotal reports from the field, the researcher says.

"Plants grown from field-collected seed and treated with tembotrione, topramezone or mesotrione survived, whereas treated plants from two known sensitive populations used for comparison were completely controlled," says Hager in the most recent edition of The Bulletin. "Tank-mixing atrazine with each HPPD inhibitor improved control of the resistant population over that provided by each HPPD herbicide alone, but survival was still much greater than with the sensitive controls."

Field research conducted in 2010 has confirmed the greenhouse results, Hager adds. Foliar-applied HPPD inhibitors, alone or tank-mixed with atrazine, provided poor control of this waterhemp population. Crossing experiments have confirmed reduced sensitivity to HPPD inhibitors can be transferred to progeny, providing additional evidence that this population is resistant to this herbicide's site-of-action family.

Field and laboratory research is being done that will help researchers understand the mechanism or mechanisms that confer HPPD inhibitor resistance to these plants. They expect the ongoing studies to allow them to develop effective management recommendations that farmers can implement.

Waterhemp is a broadleaf weed species common to many acres across much of the Midwest. Waterhemp was not considered much of a problem species in agronomic crops until it began to spread across Illinois during the late 1980s and early 1990s.

"Waterhemp populations continue to infest additional acres of Illinois farmland, aided by several adaptations that allow the species to thrive in contemporary agronomic cropping systems," Hager says. "One adaptation of particular importance is waterhemp's ability to thwart attempts at control with herbicides."

The initial publication of herbicide resistance in Illinois waterhemp occurred in 1997, when the selection of waterhemp populations resistant to ALS-inhibiting herbicides was chronicled. Since that initial declaration, Illinois waterhemp populations resistant to triazines, PPO inhibitors and glyphosate also have been confirmed.

"While resistance to any one herbicide family can introduce significant management challenges, biotypes with resistance to more than one herbicide family are becomingly increasingly common," Hager adds.