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The Future in Weed Control Will Be Talking 'Mode of Action'The Future in Weed Control Will Be Talking 'Mode of Action'

Call it mode of action or site of action, but either way, it's time to brush up on your herbicide knowledge

Tom Bechman 1

January 23, 2014

2 Min Read

You probably learn the trade names and maybe know the active ingredients of the products you apply. Odds are good that not too many people know the chemical families by heart, and which active ingredients fit in which family. Odds are even higher that many people don't know how each family of herbicides kills the weeds – which sites within the plant, or which modes of action, they use to destroy the weed.

I'll be first and say I don't have either the families or the modes of action memorized. And you may not need to memorize them, but you will want to know how to read tables and charts that tell you which products have which modes of action.


Bill Johnson, Purdue University weed control specialist, told certified crop advisers recently that it's going to be important to know these things as we deal with more resistant weeds. It's important right now, he says, because weeds such as marestail, waterhemp and Palmer amaranth are out there, and some have resistance to more than one family of herbicides already.

Related: Learn About Weed Resistance Up Close and Personal

Working with other land grant colleges, especially Ohio State University, Purdue has developed charts that list the families and modes of action. The modes of action are listed by number. Johnson says most chemical companies are now putting the number of the mode of action of the product on the jug or on the label. You can leave this chore to your chemical supplier if you want, but Johnson says some farmers, especially if they spray themselves, are already studying and asking questions about modes of action.

The goal is to apply several modes of action through the use of burndowns, residual products and perhaps post products during the season, he says. In one example, Johnson demonstrates that if you're after Palmer amaranth and want a product with a different mode of action to spray postemergence, your best bet may be Liberty on LibertyLink soybeans. Choices are limited for post applications on Palmer amaranth and waterhemp. Some of the products that you might use have the same mode of action as residual products you might apply. The more time you use the same mode of action, the greater the odds of building more resistance.

About the Author(s)

Tom Bechman 1

Editor, Indiana Prairie Farm

Tom Bechman is an important cog in the Farm Progress machinery. In addition to serving as editor of Indiana Prairie Farmer, Tom is nationally known for his coverage of Midwest agronomy, conservation, no-till farming, farm management, farm safety, high-tech farming and personal property tax relief. His byline appears monthly in many of the 18 state and regional farm magazines published by Farm Progress.

"I consider it my responsibility and opportunity as a farm magazine editor to supply useful information that will help today's farm families survive and thrive," the veteran editor says.

Tom graduated from Whiteland (Ind.) High School, earned his B.S. in animal science and agricultural education from Purdue University in 1975 and an M.S. in dairy nutrition two years later. He first joined the magazine as a field editor in 1981 after four years as a vocational agriculture teacher.

Tom enjoys interacting with farm families, university specialists and industry leaders, gathering and sifting through loads of information available in agriculture today. "Whenever I find a new idea or a new thought that could either improve someone's life or their income, I consider it a personal challenge to discover how to present it in the most useful form, " he says.

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