The list of herbicide  resistance in waterhemp populations is growing. University of Illinois  (U of I) weed scientists announced today that basic and applied research have confirmed this broadleaf weed species’ resistance to HPPD-inhibiting herbicides commonly used for control of annual broadleaf and grass weed species in corn .
HPPD-inhibiting herbicides (herbicides that inhibit 4-hydroxyphenyl pyruvate dioxygenase) are often foliar-applied and until now have successfully managed waterhemp weed populations common in Illinois and across much of the Midwest.
Several active ingredients from this herbicide family are commercially available, including tembotrione, topramezone and mesotrione. These active ingredients are available either as individual products (such as Laudis, Impact and Callisto) or as components of pre-mixtures.
Initial U of I greenhouse experiments confirmed anecdotal reports from the field. 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.
“Tankmixing 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 the sensitive controls,” says Aaron Hager, U of I Extension weed specialist.
Field research this year confirmed the greenhouse results. Foliar-applied HPPD inhibitors, alone or tankmixed with atrazine , provided poor control of this waterhemp population. Crossing experiments have confirmed that reduced sensitivity to HPPD inhibitors can be transferred to progeny, providing additional evidence that this population is resistant to this herbicide site-of-action family.
“Although waterhemp is indigenous to Illinois, it was not widely recognized as a problem weed species in agronomic crops until it began to spread across the state during the late 1980s and early 1990s,” he says. “Unfortunately, waterhemp populations are continuing to infest Illinois farmland, aided by its ability to develop resistance to herbicides.”
The first announcement of herbicide resistance in Illinois waterhemp occurred in 1997 with ALS-inhibiting herbicides. Since then, triazines, PPO-inhibitors and glyphosate have been added to the list.
“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 and difficult to manage,” he says.
U of I field and laboratory research is under way that will help scientists understand the mechanism(s) that confers HPPD-inhibitor resistance to these plants. Additionally, this ongoing research will allow the U of I to develop effective management recommendations that farmers can implement.
The most recent information about this waterhemp population will be discussed at the 2010 U of I Agronomy Day on August 19.