By John Pocock

Corn and soybean growers who don’t currently use an integrated weed-management program are most likely in a pickle when it comes to potential herbicide-resistant weed problems – and not just with glyphosate, the active ingredient in Roundup, says Mike Owen, Iowa State University Extension weed specialist.

“Glyphosate resistance is a potential issue for most corn and soybean acres in the Midwest,” says Owen. “There are also about 19 other herbicide mechanisms of action, however, for which weeds have evolved resistance. In the Midwest, these mechanisms of action include triazines, ALS inhibitors, and PPO inhibitors. In parts of the U.S., we’ve had herbicide-resistant weeds for over 30 years.”

But when it comes to herbicide resistance in Iowa, the four weeds that researchers and farmers are typically most concerned about are common waterhemp, common lambsquarters, giant ragweed, and marestail/horseweed, says Owen. “Those are the ones that I’d be looking at the closest,” he says. “In particular, some common waterhemp populations are resistant to multiple herbicides: glyphosate, triazine products, ALS inhibitors, and PPO inhibitors.”

Farmers who want to be proactive in their weed-control strategies need to be scouting, emphasizes Owen. “Herbicide resistance in weeds is simply Darwinian evolution in fast-forward,” he says. “So, if you don’t know what’s out in your fields, you can’t make a good judgment on what to use to control them.”

In addition to regularly scouting fields, farmers can employ four other strategies to help minimize the threat from potential herbicide-resistant weeds in corn and soybeans, says Owen. 

1. Start out with a soil-applied, early preplant product.

Use one that gives residual control for the key weeds competing against crop yields that are also present in your fields. “You might want to take a look at some triazine herbicides for use in corn (atrazine, for example) or soybeans (metribuzin, for instance),” says Owen. “It just depends on the specific weeds that you’re trying to control.”