Source: Stu Ellis, University of Illinois

You may have some problem weeds in a field that are showing some resistance to your typical herbicide program. So being a smart fellow, you spray them with a different herbicide and find that it has little or no impact on the weeds. You begin to panic at the idea that you are cultivating weeds that cannot be controlled by herbicides. That is probably not the case, but you have to be one step smarter than your weeds, and here is how to do it.

All winter you will be barraged with magazine and television ads for different types of herbicides until your eyes glaze over. With the old stand-bys, the new chemistry on the market, and the new blends of tank mixes, it is hard to watch the budget and still maintain an effective weed control program. You are just like every other Cornbelt farmer.

However, the weed scientists have responded to your confusion with a new system of categorizing herbicides to make it easier to control problem weeds. You will see a new feature on herbicide labels with a number that ranges from 1 to 28. This concept of the Weed Science Society of America divides the different herbicide chemistries into their different mechanism of action. For example, if an herbicide inhibits a specific enzyme in the weed, it will have a number assigned to it. If another herbicide inhibits a different enzyme or growth process in the weed, it will have a different number. Consequently, if that first herbicide does not control your problem weeds, you can try an herbicide with another number. Before this system, you might have been trying two or more herbicides that had the same mechanism of action, and that is why the second or third choices were not giving you any response.

The 28 different choices offered by the Weed Science Association are quite scientific, and without an organic chemistry degree, it may be difficult to understand how they are organized. That is where the numbering system becomes important. If your old faithful herbicide carries a number 2, which is an ALS inhibitor like Accent, and does not seem to give acceptable control, you might shift to an herbicide with a different number, 14, for example, which is a PPO inhibitor like Reflex. This is where your chemical dealer will provide some assistance.

Iowa State University weed scientist Bob Hartzler provides a short list with some of the more popular herbicides. In his latest fact sheet, Hartzler provides some samples of how you will begin to use the numbers to guide your selection decisions, rather than having to learn complex chemical names. He says:

  • A Group 2 herbicide would provide little benefit for waterhemp since most waterhemp is resistant to these herbicides. (Accent and Pursuit)

  • A Group 15 herbicide would provide little benefit for giant ragweed or other large-seeded broadleaves due to its poor activity on these weeds. (Dual and Outlook)

Weed scientists have developed a new mechanism for identifying how herbicides work on weeds that will rely on a numbering system from 1 to 28. Depending on how the chemistry of the herbicide attacks the weeds, each herbicide will carry one of those numbers. Users of the herbicides will be able to better understand how to select herbicide alternatives that may have good results, when an initial herbicide was unable to provide satisfactory control.