The not-so-good news is that if you have resistant weeds now, you won’t get rid of them. The better news is that by applying herbicides with multiple sites of action each year, you should help curb further weed resistance.

That’s what Pat Tranel says after conducting a three-year study with tall waterhemp. Tranel is the Ainsworth Professor in Crop Sciences at the University of Illinois. He and his staff grew six generations of resistant waterhemp.

“A common practice has been rotating herbicides year to year,” Tranel says. “Common belief held that you could work out of weed resistance issues. We set up an experiment to test the theory.”

Tranel used waterhemp seeds that were resistant to five classes of herbicides: glyphosate, triazines, ALS inhibitors, PPO inhibitors and HPPD herbicides. After weeds grew for six generations without spraying, resistance was virtually unchanged.

There was a slight decrease in ALS resistance, Tranel says, but it was minimal. “At the rate it was decreasing, even if a farmer used an alternative herbicide for nine years, the frequency of resistance to ALS inhibitors would only be cut in half.”

The rotation theory assumes there’s a fitness cost to plants for adding the resistance gene, Tranel explains. For example, plants may not grow as tall or may not produce as many seeds. For the theory to work, fitness cost must be high. Instead, fitness cost turned out to be very low.

“I tell farmers, ‘Once you have resistance, you’re stuck with it,’ ” Tranel says.

That doesn’t mean all is lost. Tranel suggests doing whatever it takes to prevent further weed resistance.

“Do the right things to avoid resistance in the first place,” he says. “That means using multiple herbicides, using a preemergence herbicide and coming back with a postemergence herbicide.

“If you have escapes, it means getting off your tractor and getting rid of them before they set seed. Once they set resistant seed, you’ll have that resistance for life,” he says.

Apply herbicides with different sites of action against target weeds in the same season, Tranel says. “Using three sites of action is better than two, and using four sites of action would be better than three,” he adds. “I like to say, ‘Kill every weed in your field at least twice.’ That may be with two sites of action, one site and tillage, or one site and hand-weeding.”

Jeff Nagel, an agronomist with Ceres Solutions Cooperative based near Lafayette, Ind., is another advocate of multiple sites of action against each target weed each year. 

One useful tool is the Take Action website, iwilltakeaction.com. It contains information about numbers for each class of herbicide, based on site of action.

Many farms are moving to dicamba-tolerant or Liberty platforms. Without proper stewardship, resistance will develop to those herbicides, too, Nagel says.

Glyphosate-resistant weeds in the Midwest include giant ragweed, marestail, waterhemp and Palmer amaranth. The pigweed species are particularly troublesome, Nagel says, as they’re prolific seed producers and germinate throughout the season. Just in sheer numbers of plants per acre, odds of resistance increase.

Management tips

Nagel suggests using a fall-applied herbicide or effective tillage for marestail. Apply a burndown again in the spring, or use effective tillage to control emerged weeds.

He also recommends applying a full rate of a residual herbicide at or near planting with one or more effective sites of action on target weeds. If a weed is resistant to a specific site of action, that doesn’t count as an effective site of action.

Nagel advises applying postemergence herbicides with one or more effective sites of action on 4-inch or smaller weeds. For pigweed species, tank-mix an overlapping residual with the postemergence application — usually a Group 15 herbicide.

Finally, like Tranel, Nagel suggests pulling scattered escapes before harvest.

Resistant weeds can develop this quickly and easily

By Jeff Nagel

Pat Tranel at the University of Illinois determined that getting rid of weed resistance is virtually impossible. You can prevent further weed resistance through dedicated management and use of herbicides with multiple sites of action each year.

It’s much easier to develop herbicide resistance than to control it. Just to prove this, here’s a recipe that would hasten development of resistance for a farm switching to a dicamba platform.

Soybean crop plan

• Don’t apply any herbicides for marestail in the fall. Do use a vertical-tillage tool that doesn’t control fall-emerged marestail.

• Don’t apply a burndown in the spring.

• Don’t use a soil-applied herbicide, or do use a cheap herbicide that contains three sites of action but at too low of a rate to be effective.

• Plant soybeans.

• Only spray XtendiMax, Engenia or FeXapan and glyphosate when soybeans are close to crop canopy and weeds are 8 to 12 inches tall or taller.

• Don’t tank-mix an overlapping herbicide with the postemergence application where pigweeds are a concern.

• Run scattered escaped weeds through the combine.

Corn crop plan

• Don’t apply any herbicides for marestail in the fall. Do use a vertical-tillage tool that doesn’t control fall-emerged marestail.

• Don’t apply a burndown in the spring if you no-till, or don’t use aggressive tillage to control emerged weeds before planting.

• Don’t use a soil-applied herbicide at or near planting.

• Plant corn.

• Only apply a glyphosate-plus-dicamba postemergence herbicide.

• Don’t tank-mix a residual herbicide with postemergence application.

The bottom line is that whether planting Xtend or LibertyLink soybeans, if you primarily rely on the same site-of-action herbicide for postemergence applications, resistance will develop more rapidly.

Nagel is an agronomist with Ceres Solutions Cooperative near Lafayette, Ind.