Weeds - Herbicide resistance
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| Waterhemp seedling. Photo credit: Mike Rankin, University of Wisconsin Extension |
Most weed species contain a tremendous amount of genetic variation that allows them to adapt and survive under a range of conditions. Resistance develops through selection pressure imposed by repeated, often nearly continuous use of a herbicide, or several herbicides that have the same mode of action. Complete reliance on herbicides for weed control can greatly increase the likelihood of herbicide resistant weeds.
Long residual preemergence herbicides, repeated application of postemergence herbicides, or application rates that are too high or too low relative to the amount needed for weed control, will further increase selection pressure. Resistance has developed to several types of herbicides in the North Central region, including ALS-inhibitors, ACCase inhibitors, and photosynthesis inhibitors.
Weeds at risk
Weeds that are most likely to develop resistance are annual weeds with high seed production and high germination rate, especially those that produce more than one generation in a year, or that are extremely sensitive to a particular herbicide.
The weeds known to have developed resistance in the North Central region, and other areas of the country, include:
Cocklebur
Common lambsquarters
Common ragweed
Eastern black nightshade
Giant ragweed
Giant foxtail
Green foxtail
Horseweed
Kochia
Large crabgrass
Shattercane
Smartweed
Smooth pigweed and other pigweed species
Sunflower
Waterhemp
Wild Carrot
Wild Oat
Velvetleaf
Herbicide mode-of-action
The mode of action of a herbicide is the way that particular chemical affects the plant on a cellular level. Weed scientists from 16 North Central Universities have put together a useful chart that places herbicides into groups based on their mode of action. This chart was developed to help you in selecting herbicides 1) to maintain greater diversity in herbicide use and 2) to rotate among herbicides with different sites of action to delay the development of herbicide resistance. Knowing the herbicide mode of action also helps to diagnose herbicide injury.
You can view the chart online in pdf format, or obtain a print copy from the Purdue University Media Store.
View herbicide mode of action chart (1MB pdf)»