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.
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.