Herbicide labels now include a standardized system to inform users of the product’s mechanism of action (MOA). A box labeled ‘Herbicide Group’ is present near the top of the label. The number in the box represents MOA of the active ingredient, based on a system developed by the Weed Science Society of America. Premixes containing more than one mode of action will have multiple numbers listed. An example of the new logo is in the photo gallery to the left.

The intent of this information is to simplify development of herbicide programs that reduce the likelihood of selecting herbicide resistant weeds. In production systems relying largely on herbicides for weed management, using herbicides with different MOAs is the primary means of managing resistance.

Generally, the greater number of MOAs used, the less selection pressure placed on weeds.  However, designing an integrated program is not as simple as randomly adding MOAs. The different MOAs used in the program must have good activity on the important weeds in the field to successfully reduce selection pressure. Following are a few examples where the inclusion of an herbicide in a system relying on glyphosate in Roundup Ready crops would provide little benefit in terms of managing resistance for specific weeds.

  • A Group 2 herbicide would provide little benefit for waterhemp since most waterhemp is resistant to these herbicides.
  • A Group 15 herbicide would provide little benefit for giant ragweed or other large-seeded broadleaves due to its poor activity on these weeds.
  • Tankmixing low rates ( less than 0.75 lb.) of atrazine (Group 5) with glyphosate or other herbicides.

 

The new labeling system eliminates the need for farmers, consultants and suppliers to learn the MOA of all the active ingredients used in Iowa agriculture. However, to use the information properly, users must still know the activity of the individual herbicides on the weeds present in the field to insure that the target weeds are being affected by multiple MOAs.

You can download the Mechanism of Action document from the Weed Science Society of America as a PDF.