The first documented case of resistance to postemergence HPPD-inhibitor herbicides has been confirmed in a waterhemp population in central Illinois.
University of Illinois weed scientist Aaron Hager says the wily weed has already adapted to resist four other classes of herbicide (triazine, PPO inhibitors, glyphosate and ALS inhibitors) in the state, but HPPD-inhibitor resistance has not previously been recorded.
“Although we have isolated cases of four-way resistance, this discovery does not mean all waterhemp is resistant to all these herbicides at the same time,” Hager says. “It does mean that growers need to carefully weigh their herbicide choices and rotate crops and chemistries.”
HPPD herbicides inhibit 4-hydroxyphenylpyruvate dioxygenase. They are commonly used for control of annual broadleaf and grass weed species in corn. Several active ingredients from this family are commercially available, including tembotrione, topramezone and mesotrione, and are available as individual products (Laudis, Impact and Callisto, for example) or as components of premixtures.
Family secrets. Common waterhemp is a member of the pigweed or amaranth family. Like corn and sorghum, it is a C4 plant that is efficient at fixing carbon and is well-adapted to high temperatures and intense sunlight. It can produce 500,000 seeds per plant that germinate throughout the summer.
Waterhemp has male and female plants, increasing genetic diversity and favoring development of resistance. Its native habitat is wet, low-lying areas, but it is quite at home in reduced-
tillage and no-till environments.
Hager says the following combination of conditions might lead you to suspect a resistant weed population:
- The appropriate herbicide rate (plus proper adjuvants) was applied at the appropriate growth stage.
- The environmental conditions during and after application were conducive to good herbicide activity.
- Plants that survived the herbicide application are found next to plants that were controlled.
- The field has had repetitive applications of the same herbicide.