Kansas State University (K-State) scientists have completed long-term evaluations of a limited number of independent kochia (Kochia scoparia) populations on privately owned land in western Kansas that are now confirmed to be glyphosate-resistant. These populations have undergone both greenhouse and field-testing by K-State and Monsanto personnel.
Kochia, also called fireweed, is a drought-tolerant weed commonly found in cropland, rangeland and pasture, and non-agricultural sites in arid and semi-arid regions of the western U.S. and Canada. Kochia is highly adaptable and grows on many soils including saline and alkaline soils.
Phil Stahlman, weed scientist with K-State Research and Extension, has listed as many as five glyphosate-resistant kochia populations in western Kansas on the International Survey of Herbicide Resistant Weeds Web site following lengthy evaluations of greenhouse and field studies. He, along with K-State scientists Kassim Al-Khatib, Curtis Thompson and other colleagues, including Monsanto scientists, have investigated the sites independently, focusing on the variability of the resistance and difficulties in proving heritability – a trait required for confirmation of resistance.
"This complicates and may increase control costs for those growers who may have a resistance problem, but there are other herbicides that can be used to control kochia," says Stahlman, who is based at K-State’s Agricultural Research Center at Hays, KS.
Thompson, who is based at K-State’s Agronomy Department in Manhattan adds, "If glyphosate-resistant kochia is suspected, the grower should consider a two-pass weed-control program that includes use of residual pre-emergence herbicides that control kochia."
Kochia control can be adversely affected by both growth stage and environmental conditions, with erratic performance fairly common. Initially, the lack of control was thought to be due to factors or circumstances other than resistance. Stahlman noted that some growers learned to manage kochia with glyphosate rates below the recommended rate by using enhanced application techniques.
"We know that herbicide rate is very important in preventing resistance and areas that practiced low use rates were among the first to exhibit lack of control of kochia not due to environmental factors," says Stahlman.
Monsanto is working on a multi-state effort with university scientists in a number of Plains states to continue evaluating standard weed-management recommendations and to learn more about glyphosate resistance in kochia.
Stahlman says there is evidence that a glyphosate-resistant kochia population from Thomas County does not grow as well as a known susceptible population. Thompson, however, reported a glyphosate-resistant kochia population from Stevens County is more aggressive than a nearby susceptible population.
Also, kochia seed viability in the soil, currently estimated at two to three years, is being investigated by a team of university scientists throughout central and northern Great Plains states. Understanding more about the plant and seed characteristics across a wide geographic region will allow greater use of other management tools.
K-State Research and Extension personnel have received reports that there may be other kochia populations in Kansas exhibiting resistance to glyphosate. Stahlman and Thompson advise growers to use appropriate glyphosate rates and other herbicides with a different mode of action in their weed control program where possible, including residual herbicides. It is essential that these herbicides have good activity on the targeted species, they said.
Rick Cole, U.S. weed resistance technology development manager at Monsanto, agrees a program approach is best: "We are working with many university experts to provide growers with the best management practices. To maintain the efficacy of the herbicide and value of the technology, we recommend growers scout fields and utilize additional modes of action that complement the Roundup Ready system to control problem weeds while reducing the likelihood of developing performance issues."
The company and most academics recommend growers adopt best management practices to help growers minimize the risk of developing resistant weeds. Those practices include:
- Start with a clean field by either using a burndown herbicide or tillage to control weeds early.
- Use Roundup Ready technology as the foundation of a total weed-management program.
- Add other herbicides or cultural practices where appropriate as part of the Roundup Ready cropping system.
- Use the right herbicide rate at the right time.
- Control weeds throughout the season and reduce the weed seed bank.