Populations of Palmer amaranth resistant to glyphosate were first documented in Kansas three years ago. At that time, these populations were limited in range to isolated areas of south central Kansas. Glyphosate resistant Palmer amaranth has gradually expanded the last couple of years, and now appears to be increasing rapidly, especially through the central part of the state.
Several other weeds have also developed glyphosate resistance in Kansas, including common waterhemp, marestail, kochia, common ragweed, and giant ragweed.
Glyphosate resistance can be confirmed with greenhouse and laboratory tests, but at this point that is probably no longer necessary. If a few Palmer amaranth plants or patches of plants survived where glyphosate was applied at the recommended rate with the appropriate adjuvants and spray coverage was good, there is a good chance those plants are resistant.
Figure 1. Surviving Palmer amaranth among dead plants treated with Roundup PowerMax at 32 fl oz/acre when plants were about 6 inches tall. Photo taken at Ashland Bottoms near Manhattan six days after treatment. Plants were confirmed to be resistant by an enzyme assay in the laboratory. Photo by Dallas Peterson, K-State Research and Extension.
Where glyphosate-resistant Palmer amaranth occurs, producers should plan taking the necessary control measures. Another application of glyphosate alone probably will not help. It may be possible to use an alternative to glyphosate as a postemergence treatment in-season this year, although most postemergence alternatives are effective only on smaller plants.
In soybeans the only alternatives would be PPO-inhibiting herbicides such as Cobra, FlexStar, Marvel, or Ultra Blazer. If there are just a few scattered plants, removing them by hand or other mechanical methods before they go to seed may help prevent them from spreading rapidly in the field. If those weeds remain growing in the field and set seed, the action of a combine will spread the seed throughout the field, as well as to other fields.
In fallow, it is best to use glyphosate as a tank-mix with 0.25 to. 0.5 lb ae of 2,4-D or dicamba. Treat fallow as soon as possible because the larger the glyphosate-resistant Palmer amaranth gets, the harder it is to control.
In the future, growers need to consider using a more integrated weed management approach that includes cultural practices and multiple herbicide modes of action, especially preplant and preemergence residual herbicides. Scout fields early after crop emergence and make timely postemergence applications with the appropriate herbicides for control of escaped weeds.
Dallas Peterson, Weed Management Specialist
dpeterso@ksu.edu
Curtis Thompson, Weed Management Specialist
cthompso@ksu.edu
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