Waterhemp just got uglier. The first documented case of resistance to post-emergence HPPD inhibitor herbicides is being confirmed today, July 19.
A waterhemp population in a central Illinois seed corn production field has been shown to withstand foliar applications of the popular herbicide. University of Illinois weed scientist Aaron Hager says the wily weed has already adapted to resist four other classes of herbicides (triazine, PPO inhibitors, glyphosate, ALS inhibitors) in the state, but HPPD-inhibitor resistance has never previously been recorded.
HPPD-herbicides inhibit 4-hydroxyphenyl pyruvate dioxygenase. They are commonly used for control of annual broadleaf and grass weed species in corn. Several active ingredients from this herbicide family are commercially available, including tembotrione, topramezone and mesotrione. These active ingredients are available as individual products (Laudis, Impact and Callisto, for example) or as components of premixtures.
“Anecdotal reports from the field in 2009 indicated there might be a problem,” says Hager. “Plants grown from seed collected from this field were treated with in the greenhouse with tembotrione, topramezone, or mesotrione and survived. Other waterhemp plants known to be sensitive and used for comparison were completely controlled.” Hager adds that tankmixing atrazine with each HPPD inhibitor improved control of the resistance population, but survival was still much greater than the sensitive controls.
“Field research conducted in 2010 has confirmed the greenhouse results,” he adds. “Foliar applied HPPD inhibitors, alone or tankmixed 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 this population is resistant to this herbicide site-of-action family. University of Illinois weed scientists Dean Riechers and Pat Tranel have been participating in the investigation.
Syngenta field studies showed that pre-emergence applications of HPPD inhibitors Lumax® and Lexar® (containing mesotrione, S-metolachlor and atrazine) provided effective control of this waterhemp population.
“This particular field was used for seed corn production, and for seven consecutive years there was over-reliance on certain post-emergence HPPD-inhibitor herbicides to control key weeds,” said Chuck Foresman, manager of weed resistance strategies for Syngenta. “Hybrid seed corn production systems often preclude the use of important broad-spectrum herbicides like glufosinate or glyphosate. The lack of diversity in both herbicide modes of action and crop rotation in this field led to the development of resistant waterhemp.”
Common waterhemp is a member of the pigweed or amaranth family—which explains some of its stubborn ways. Like corn and sorghum, it is a C4 plant that is very efficient at fixing carbon and well-adapted to high temperatures and intense sunlight. It’s capable of producing 500,000 seeds per plant that tend to germinate throughout the summer. Waterhemp has separate male and female plants and cross-pollination between plants increases the genetic diversity of a population and favors development of resistance. The native habitat of waterhemp is wet, low-lying areas, but it is quite at home in reduced tillage and no-till environments.
One reason this announcement is so troubling is HPPD inhibitors and glufosinate-based herbicides had been left as the remaining mode of action herbicides left to control the problem. Several companies are working diligently to bring HPPD resistant technology to the market.
“This discovery does not mean you should not use HPPD-inhibiting chemistry,” Hager says. “What it does mean is you need to rotate or diversity the types of herbicides used.
There are herbicide-resistant populations in every field. If you use the same product over and over, you will eventually select for that population.”
Preserving the technology means rotating modes of action and crops, using full herbicide rates, tank mixing herbicides with multiple modes of action, as well as appropriate cultural practices like tillage and cover crops to reduce selection pressure.
“What is most important right now is keeping your eyes open to know what you have in the field,” Hager adds. “If you see a small weed patch, I’d like to see it removed by hand before it goes to seed. If the plants are scattered through the field, you need to start a plan of attack for next year.”