How to confirm herbicide resistance

You applied an herbicide to kill weeds in the field but some of them survived anyway. Did you just miss those weeds with the sprayer, or have they developed herbicide resistance?

Scott Nolte is a weed specialist at Texas A&M AgriLife Extension. He says one of the first signs of confirming resistance is live weeds that are among or near the same species that are dead.

And, there are several more observations that could offer clues.

"We’re looking for instances where we got good control maybe of other weed species with that herbicide at the same location, but our species in question we didn’t control," says Nolte. "We’re going to be looking for instances where we had good control of that weed species in previous years, or have you started to see maybe a decline in the effectiveness of that herbicide on that weed species over the last couple of years."

 Have you been using the same herbicide repeatedly for the past few years or do you rotate herbicides? Has resistance been confirmed in that species to that herbicide in your area?

If you assume there is a new resistance issue in your field, Nolte says they can do some testing.

"We will plant out seeds of the weed species in question, and then we will also plant out seeds of a known susceptible population, kind of a confirmed sensitive species," he says. "That way, when we apply the herbicide, we can compare the species in question to a known susceptible and we can determine is there a difference in the response."

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