New Information on Herbicide Labels

December 21, 2010
ICM News

By Bob Hartzler, Department of Agronomy

Herbicide labels now include a standardized system to inform users of the product's site of action (SOA). A box labeled 'Herbicide Group' is present near the top of the label. The number in the box represents SOA 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. Following is an example of the new logo.  Site of action also is referred to as mechanism of action.

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 SOAs is the primary means of managing resistance. 

Generally, the greater number of SOAs used, the less selection pressure placed on weeds.  However, designing an integrated program is not as simple as randomly adding SOAs. The different SOAs 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.
• Tank-mixing low rates ( less than 0.75 lbs) of atrazine (Group 5) with glyphosate or other herbicides.

The new labeling system eliminates the need for farmers, consultants and suppliers to learn the SOA 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 SOAs.

Use this link to reach the Weed Science Society of America Mechanism of Action document.


Bob Hartzler is a professor of agronomy with extension, teaching and research responsibilities.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on December 21, 2010. The information contained within may not be the most current and accurate depending on when it is accessed.