Dicamba Spray Regs Increase Residuals’ Role – In Both Corn And Cotton Belts

Photo: Bob Hartzler

Photo: Bob Hartzler

In the past when consultants and producers talked about the weather, it was mostly about its impact – good or bad – on yields. Do we need a rain or has it rained too much? Is it too hot or too cold?

But these days, weather discussions are increasingly about something entirely different – herbicides, both in terms of activation and potential for spray drift.

It’s a whole new world. Things might go well or terribly wrong with weed control, depending on which way the weather veers.

At times, thing can go right. In 2017, farmers and crop advisors in northeast Arkansas couldn’t have asked for a better outcome, says Chuck Farr of Midsouth Ag Consultants in Crawfordsville.

“Every time we applied a residual herbicide, we got an activating rainfall,” Farr says, still sounding a bit amazed at how well their weed control played out. “We got moisture all year long to keep herbicides going.

Realistically, he knows that fortune may or may not smile. “This year, it won’t take much of a hiccup and we’re going to be in real trouble.”

Spray Bans Reduce Dicamba Options

What’s different about this year? The margin for error in 2018 has thinned down significantly due to numerous spray bans and/or additional restrictions imposed on dicamba in several Midwest and Midsouth states.

For example, the Arkansas State Plant Board banned dicamba sprays after April 15. The ban effectively eliminates the postemergence use of dicamba on Xtend crops across large swaths of the state. That puts even greater emphasis on overlapping residuals – and catching enough moisture to activate the chemistry.

Farr notes that a recent survey among Arkansas consultants indicated that producers in only about 15% of Arkansas might be able to squeeze in an over-the-top-application before the ban date. Most of those crop advisors work in southeast Arkansas, close to the Louisiana border, so they have a bit more time to plant early and gain a stand.

“With the April 15 ban, the only way we might be able to use a dicamba product here in the northeast part of the state would be in a burndown situation,” Farr says. “But I don’t see us being able to get a crop in and making one over-the-top application.”

Residuals, Residuals, Residuals (Did We Mention Residuals?)

Nonetheless, Farr expects Xtend technology to be planted on about 70% of soybean acres in the area this spring, mostly due to their genetics, he assumes. About 30% will be planted to LibertyLink beans, he also estimates.

Those producers who plant Xtend soybeans will be walking a fine line this coming season, Farr emphasizes. The weather will once again play a critical role in how clean the crop starts and stays.

Options are limited in those cases.

“We’re going to spray dirt,” says Farr. “We’re going to spray a residual on top of a residual on top of a residual. If Palmer pigweeds come up, we don’t have anything to control them with — other than a hoe.”

“It’s going to take a program approach,” Farr adds. “We’ll start out clean, burn down with Roundup and 2,4-D or Roundup and some type of dicamba, along with some Valor-containing products. We’ll come behind the planter with Gramoxone and Prefix, and Boundary, Fierce or Zidua. Then whatever is left that we didn’t use behind the planter, we’ll come back over the top and hope for the best. In simple terms, we don’t have anything else to fall back on.”

With LibertyLink, his growers will have a chance to come back over the top with another shot of Liberty, he adds. But the season will run so much smoother if the right amount of rain falls at the right time, regardless of the technology.

“It all goes back to being able to get those residuals activated,” Farr says. “If we miss an activation, we’ll be in trouble.”

The Southeast: 2017 Also Went Well – Now What?

A well-timed rain can also make or break a weed control program for North Carolina crop consultant Billy McLawhorn.

“When we have an extended drought during the spring, Palmer amaranth eats us up,” says McLawhorn, who based that statement on considerable experience. “Fortunately, in 2017 we had frequent enough rainfall in the spring to activate preemergence residual herbicides.”

McLawhorn says North Carolina producers have long been proponents of residual control of weeds and “mixing up chemistry with Roundup Ready crops and with LibertyLink crops. We’ve had to be. This part of the state has been pretty heavy into Palmer amaranth for a long time.”

Unfortunately, weed control costs “have really gotten outrageous, and we pay a premium for some technologies that aren’t working very well,” McLawhorn adds.

Because of complicated crop mixes, the Xtend weed control technology hasn’t made as much headway as the LibertyLink system for soybean producers in the area, McLawhorn notes.

“There’s too much tobacco grown in this area for producers to go wide open with the auxin-based technologies,” he explains. “That’s tempered its use in this part of the Coastal Plains. A fair number of cotton producers have used that technology, but they’ve been really careful about drift. Extension and the companies did a great job of educating growers, too.”

Resistant common ragweed is also starting to encroach on the region, according to McLawhorn. That adds another layer of complication to how he and his farmers will control weeds in the future.

“It’s just cropped up within the last two or three years,” he says. “It may not explode like Palmer amaranth did, but it’s going to be more of an issue. It has resistance to PPO-inhibitors and glyphosate.”

Drift Hits The Midwest, Too

Don’t assume that dicamba drift – and resulting bans – are strictly an issue in cotton country. The 2017 season brought numerous and widespread drift complaints to the Midwest, as well.

“We had entire quarter-section fields where every single plant was affected,” says Dorian Gatchell, an independent agronomist who owns Minnesota Agricultural Services in southwest Minnesota. “I’ve heard everything from a 50% reduction to virtually no yield reduction, depending on when the application was made.”

The Minnesota Department of Agriculture (MDA) reported 253 complaints in 49 Minnesota counties in 2017. University of Minnesota researchers estimate that 265,000 acres across the state were affected to varying degrees.

For 2018, the MDA imposed a ban on spraying dicamba after June 20 and whenever the air temperature of the field exceeds 85 degrees at the time of application – or if the National Weather Service forecasts highs for the day above 85 degrees.

Complications With An Evolving Weed Spectrum

Each growing season seems to bring tougher weeds to southwest Minnesota, Gatchell says. “Because of weed shifts we’ve experienced due to herbicide resistance and our overreliance on glyphosate, it’s been harder and harder to control weeds postemergence,” Gatchell says.

Gatchell notes that waterhemp – the Midwest’s ubiquitous weed pest – has developed resistance to glyphosate and PPO-inhibitors, and likely has gained resistance to ALS herbicides, as well.

One more reason for his concentrated focus – Palmer amaranth was discovered in Minnesota in the fall of 2016 in a field about six miles south of Gatchell’s office in Granite Falls.

“That is a weed that is very concerning, but so far, we’ve only seen it in conservation plantings,” Gatchell notes. “We are still in a lot better shape as far as weed control is concerned than what people report in the South. That said, we have to stay attentive.”