Kansas State University scientists have completed long-term evaluations of a limited number of independent kochia (Kochia scoparia) populations on privately owned land in western Kansas that they have now confirmed to be glyphosate-resistant. These populations have undergone both greenhouse and field testing by Kansas State and Monsanto personnel.
Phil Stahlman, a weed scientist with Kansas State Research and Extension, has listed as many as five glyphosate-resistant kochia populations in western Kansas following lengthy evaluations of greenhouse and field studies. He, along with Kansas State scientists Kassim Al-Khatib, Curtis Thompson and others, 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," Stahlman says.
Adds Thompson: "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 notes 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," Stahlman says.
Monsanto reports it 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, reports 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 2 to 3 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.
Kansas 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's essential that these herbicides have good activity on the targeted species, they say.