How To Manage Herbicide-Resistant Weeds That Threaten Soybean Yields

Herbicide resistance in weeds costs U.S. soybean producers millions of dollars each year.

Even a loss of five bushels per acre can significantly impact your ROI — particularly with record-high input costs — so it stands to reason that doing whatever is within your control to address resistance is a worthwhile investment.

How Does Herbicide Resistance Impact South Dakota?

SDSU Extension continually updates its collection of South Dakota Pest Management Guides to assist growers, and the topic of herbicide resistance is addressed thoroughly therein.

The two broadleaf weeds showing the most resistance in South Dakota are common waterhemp and kochia, according to SDSU Extension. South Dakota growers have also dealt with resistance in common ragweed, marestail and common lambsquarters to a lesser degree.

Both common waterhemp and kochia have shown widespread resistance to glyphosate and SU herbicides, with the former also presenting resistance to post-emergent PPO inhibitors.

Kochia, meanwhile, creates challenges for growers because they have fewer post-emergence herbicides available in their toolkit.

Despite the chokehold these weeds can put on soybean yield potential, SDSU Extension Weed Science Coordinator Paul O. Johnson believes growers can overcome this challenge with the right integrated weed management program in place.

It Pays to Plan Ahead

According to Johnson, one critical step growers can take is to pay more attention to their pre-emergence herbicide program.

“We talk about the challenges of the POST program, but we can't just be depending on POST or we're gonna have real issues there,” he says. “And so we need to make sure we've got a good PRE program to start with.”

Johnson continues to explain the importance of addressing weeds in the pre-emergence stage.

“With a good PRE program, 90 percent of the weeds are already taken care of. And so, consequently, that post-emergence program only needs to get 10 percent of the weeds,” says Johnson.

In fact, Johnson believes that pre-emergence applications are so important that it might actually pay off to forgo planting a few acres when timing is of concern in favor of spraying for weeds.

“If we've got a whole bunch of acres that are not planted right,” he explains, “then consequently we lose more yield there than maybe not getting one field or two fields planted.”

For many growers, that pre-emergence weed management program begins in the fall.

“One of the things I always like to say is when you're combining, evaluate what you have for weed escapes,” he advises. “That way, you know what's probably gonna be your issues the next year.”

Post-harvest fall herbicide applications can be wise, too, provided there is sufficient moisture to activate the chemistries to kill off weeds.

Rotate. Repeat. Rotate. Repeat.

Johnson is also a staunch advocate for rotating herbicides to keep resistance in check.

One of the main culprits in the rise of herbicide-resistant weeds has been sticking to the same herbicide mode(s) of action year after year. That’s why it’s important to be mindful of herbicide rotations in order to maintain a higher degree of efficacy.

“We don't want just Roundup® on Roundup®,” says Johnson. “We’ve learned that we want to use different chemistries.”

An added benefit of implementing a pre-emergence herbicide program is that it forces growers to use different chemistries. Likewise with crop rotations. “Anything we can do to switch around chemistries is going to help,” says Johnson.

It should be noted that integrating cover crops into the rotation can also support the suppression of weeds in general, as can other soil management practices apart from your herbicide applications.

When executed properly and with sufficient preparation, an integrated weed management program can successfully mitigate the effects of herbicide-resistant weeds.

SDSU Extension Resources

Johnson and his colleagues at SDSU Extension can serve as a great resource in your efforts to combat resistance in your fields. The aforementioned soybean pest management guide is available for free at extension.sdstate.edu, along with an entire library of other topical articles and insights for better production practices.