How the System Works

Here is one example of a classification scheme which uses mode of action followed by site of action.

If we work our way back up through the classification system we see that the product Assure II is absorbed foliarly, its’ common name is quazilofop-p, the chemical family for Assure II is the aryloxyphenoxypropionates. This herbicides site of action is the ACCase enzyme and that Lipid Synthesis Inhibition is the mode of action for Assure II.

Here is another example that uses mode of action followed by chemical family.

In this example, we can find out that the herbicide Permit has the active ingredient halosulfuron, belongs to the sulfonylurea chemical family and works by amino acid synthesis inhibition via the acetolactate synthase enzyme or ALS. Depending on the source of the classification scheme the hierarchy may be denoted using roman numerals, letters, and numbers or some combination of all three or no markings whatsoever. The destinations are not all that important and once you grasps the meanings of the terms and how they fit into the system then you will be able to better understand herbicide classification.

Once you have worked through the mode of action lessons and have a better understanding of how herbicides work then the classification system can help you with such areas as proper herbicide selection, diagnosing of herbicide injury and herbicide resistance management. Below are some examples of how understanding classification can assist you in weed management using herbicides.

Let’s say you are growing conventional soybeans and you have a problem with green foxtail in your field. In the past, you have used Herbicide Y to control grass, however the local Co-op is out of Herbicide Y and only has Herbicide Z available. Because you now understand mode of action, you know that Herbicide Y and Z have the same mode of action, site of absorption and are in the same chemical family. Herbicide Z is similar to Herbicide Y and control green foxtail.

Now let’s say that you are an agronomist at the local co-op. Farmer Bill calls you to come look at his corn field. He is complaining that the herbicide you sprayed this year has injured his corn. The corn is yellowing, has purple venation, and is stunted. Your co-op applied Herbicides 201 and 301 before the corn emerged. Because you understand mode of action you know that 201’s mode of action injury symptoms would be buggy whipping and wrapping of the corn. You also know that 301’s mode of action injury symptoms would be whitening of plant tissue. Neither of these symptoms are present. You suspect some type of another injury. Upon further investigation you find out that the farmer applied Herbicide 901 to his beans last year and he has had very little rainfall and the symptoms are worse in the sandier parts of the field. Herbicide 901 would have higher risks for carryover in dry areas of the field with low organic matter.

Now let’s say that you are a crops extension educator. A farmer calls and is concerned about weed control in his soybeans. This year he is switching to Herbicide G resistant soybeans from conventional soybeans. He has a problem with Herbicide P resistant waterhemp due to repeated application of Herbicide P. He has read through his state weed control guide and discovered that both Herbicide G and P both have the same mode of action. He now worries that Herbicide G will not control his Herbicide P resistant waterhemp. Being equipped with your newly acquired knowledge of herbicide mode of action, you can ease farmer Bill’s concerns by telling him that while Herbicide G and P have the same mode of action, they do have different sites of action. Herbicide P is an ALS inhibitor and Herbicide G inhibits 5-enolpyruvvyl shikimate 3 phosphate (EPSP) synthase. Therefore the Herbicide P resistant waterhemp will be susceptible to the Herbicide G.

One of the major questions you may have is where can I find a herbicides mode of action when all I have is a trade name? Unfortunately this information is not provided on the herbicide label. The first person to contact is your state weed specialist to determine any resources they might have relating to herbicide mode of action. Many times your state will have some type of weed control guide that will include this information. For example, the University of Nebraska annually produces the “Guide for Weed Management”. Within this guide is information on herbicide classification and in addition a herbicide dictionary is provide that lists active ingredients, uses, and mode of action. Other resources are listed in below.

• Herbicide Handbook. Weed Science Society of America.
• Herbicide Mode of Action. 2013. Kansas State University.
• Herbicide Mode of Action and Injury Symptoms by the University of Minnesota - Extension Service