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Use multiple sites of action in herbicide programUse multiple sites of action in herbicide program

ISU specialists recommend choosing herbicides according to sites of action needed to control particular weeds.

Rod Swoboda

December 13, 2018

4 Min Read
sprayer in field
FIRST STEP: Choosing the right herbicide and applying it at effective rates is the first step in developing a long-term weed management system.

In 2016, Wallaces Farmer teamed with Iowa State University Extension weed management specialists, who wrote a four-part series on what farmers should consider when planning a weed management strategy. ISU’s Bob Hartzler and Meaghan Anderson have now updated that series of articles on the ISU Integrated Crop Management website.

Following is the first article. We will post the rest of the updated articles on the Wallaces Farmer website, one article each week. The article below, written by Hartzler and Anderson, looks at using multiple, effective herbicide sites of action for the weed species affecting your fields.

Choosing the right herbicide and applying it at effective rates is the first step in developing a long-term weed management system. Below is what else Hartzler and Anderson have to say.

Resistance to herbicides developing
With the stagnant development of new herbicides and weeds seemingly evolving herbicide resistance faster than ever before, it's important to maximize the usefulness of every herbicide application. A new herbicide site of action (or herbicide group number) for use in corn and soybean production has not been discovered since the early 1980s.

According to Dr. Ian Heap with weedscience.org, since the 1980s, the confirmed number of unique cases of herbicides developing resistance to weeds globally is increasing at a rate of about 12 discoveries per year.

To delay the development of weed resistance problems as long as possible and assure herbicides retain their value, you must:

Use multiple herbicide sites of action that target problem weeds. This is more complicated than simply designing a program with multiple sites of action.

Assure the different sites of action are effective against the problem weeds.

Make sure the herbicides are used at effective rates and applied at the right time to provide maximum control.

Realize that herbicides put significant selection pressure on weed populations, making herbicide resistance development an inevitable outcome of using herbicides. Thus, a more diverse system of fighting weeds is necessary for long-term success, including non-herbicidal options as much as possible.

The first step in planning a herbicide program and battling herbicide resistance is to make sure your program includes multiple sites of action, or herbicide groups. The herbicide site of action is the specific molecule that the herbicide binds to; this binding disrupts a biological process (the mode of action) and results in death or injury of the plant.

The site of action is a specific subset of the herbicide mode of action. The mode of action is the biological process that is affected by the herbicide, e.g. photosynthesis, amino acid synthesis. Important herbicide sites of action and their corresponding herbicide group numbers are listed in Table 1.


Herbicide groups are a relatively new way of determining the site of action of the myriad herbicides on the market. Each site of action has been assigned a number, and most herbicide labels prominently display this group number on the product label.

If a product includes two different sites of action, the label will have two different group numbers listed. Keeping track of the herbicide group numbers is the simplest way for farmers to keep track of the different sites of action they’re using in their herbicide program.

Herbicide labels online
If you don’t have your herbicide labels on hand, one handy website to look up pesticide labels is cdms.net. After clicking on the label database section, you can look up the product by brand (trade) name or manufacturer. When you find the product you are looking for, click on the link called “specimen label” to bring up the product label.

You can then pull up a PDF copy of the label and can even use the CTRL+F function to bring up a search bar and find a particular word or phrase you’d like to find. Look at the herbicide group numbers listed on the first page of the Authority First herbicide label. Most, but not all, herbicide labels will display the group numbers in this way.

Other resources
Here are some additional resources on developing effective herbicide programs:

Herbicide Guide for Iowa Corn and Soybean Production lets you look up herbicide group numbers for products with only one active ingredient, or group numbers for premixes with multiple herbicide group numbers.

Take Action on Weeds website includes a page for looking up herbicide group numbers by either trade name or active ingredient. If you know your herbicide trade name, just type it in to find the group numbers that product contains.

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