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Volatility Highlights 2018 Dicamba Concerns

Volatility is Prompting Some University Weed Scientists to Recommend Only Dicamba Preemergence Applications in 2018.

Civil War Confederate veterans from Tennessee and Arkansas and parts of Missouri could rightly be called rebels. Today’s farmers and chemical applicators who reside in those regions? Not so much.  

“I hear that all the time, that they are a bunch of rebels, that we don’t know how to spray (dicamba),” says Kevin Bradley, University of Missouri (MU) Extension weed specialist. “I disagree with the mind-set that we are a bunch of rebels.” Bradley was speaking at last month’s Iowa Soybean Association Farmer Research Conference in Des Moines, Iowa.

He noted those areas had more 2017 off-target dicamba damage than other regions. So why the difference?

Well, it’s akin to the line about why bank robbers rob banks — that’s where the money is. Farmers in those areas quickly adopted dicamba-tolerant soybeans that could be sprayed with matching dicamba formulations in 2017 that include:

  • BASF’s Engenia
  • Dupont’s FeXapan Plus Vapor Grip Technology
  • Monsanto’s Xtendimax with Vapor Grip Technology

In 2017, farmers in the southeastern Missouri Bootheel planted 80% of that region’s 300,000 cotton acres with Monsanto’s Roundup Ready Xtend technology, says Bradley. Nearly all these were sprayed with matching dicamba formulations.

Meanwhile, he estimates that farmers planted 65% to 70% of the Bootheel’s 875,000 acres of soybeans to Xtend varieties. Farmers applied dicamba to most of these acres, he says.

“It was one of the greatest areas for adoption of this technology,” he says. “North of I-70, we are not even close to those percentages.”

That concerns Bradley, particularly if Xtend technology adoption increases in 2018 and beyond. Monsanto estimates 2018 Xtend acres to double from 2017 to 50 million acres in 2018. Total U.S. 2018 soybean plantings are expected to hover around 90 million acres.

“I’ve heard that these are just growing pains, that we got a lot of corn injury when Roundup Ready beans were first introduced,” says Bradley. “It is not like that. We have never had a technology that is so sensitive to major crops like dicamba.”

All herbicides can move off-target. It’s just the amount of dicamba that injures off-target plants is much less than other herbicides, he says: 1% of glyphosate’s labeled rate is the lowest observable dose that causes significant crop symptoms,” says Bradley. With dicamba, that figure falls to .005% of the labeled rate. MU tests show that just 1/20,000 of the labeled dicamba rate causes visual injury symptoms.

So What Caused Off-Target Dicamba Movement?

Several factors caused dicamba applied to dicmaba-tolerant soybeans to go off-target in 2017, says Bradley. They include:

  • Physical drift. Whether it was caused by spraying in high winds, using the wrong nozzles, or raising spray booms too high, physical drift played a part in 2017 off-target dicamba movement.
  • Tank contamination. "If you don’t clean out tanks, there will be problems, absolutely,” says Bradley.
  • Temperature inversions. Temperature inversions occur when air patterns flip-flop. During daytime, warm air rises when sunlight hits the ground. When cool air above hits the warm air, wind results. This keys air circulation. As evening approaches, cool air moves to the bottom while warm air migrates to the tip. This stable environment traps any pesticide particles in a suspended air mass that can move little or miles away.

That’s one reason federal regulators mandated that the new dicamba-tolerant formulations labeled for dicamba-tolerant soybeans be applied only between sunrise and sunset. Some states have more stringent guidelines. In Missouri, for example, dicamba formulations labeled for dicamba-tolerant soybeans can only be applied between 7:30 a.m. and 5:30 p.m.

Then There’s Volatility

“I don’t know what percentage falls into this category,” says Bradley. “We are spraying an inherently volatile product. I don’t doubt that companies spent millions and millions of dollars to reduce the volatility (via new formulations), but is that enough? From what we have seen in two seasons, I can’t say it is enough.”

To simulate field dicamba damage from volatility in a research setting, several universities conducted low tunnel experiments. In these trials, scientists place flats with soil treated with dicamba between nondicamba-tolerant soybeans covered by plastic sheeting placed over a dome frame. Forty-eight hours later, scientists removed the flats and evaluate soybeans two to three weeks later for dicamba injury.

Bryan Young, Purdue University weed scientist, found soybean injury in the following categories two weeks after flats were removed.

  • Banvel and Roundup: 37%
  • Clarity and Roundup 16%
  • Engenia and Roundup 21%
  • Xtendimax and Roundup 8%
  • Xtendimax and Roundup and AMS 39%

Bob Scott, University of Arkansas (U of A) Extension weed specialist, found the following soybean injury levels following a low-tunnel experiment.

  • Banvel and Roundup: 46%
  • Clarity and Roundup 19%
  • Engenia and Roundup 18%
  • Xtendimax and Roundup: 22%
  • Xtendimax and Roundup and AMS (Ammonium sulfate): 54%
  • Roundup Xtend 18% (Not commercialized)

The high injury level of mixes containing older chemistry isn’t surprising, such as the Banvel/Roundup and the Clarity/Roundup mixes, says Bradley. Neither are the high levels of damage that resulted from the Xtendimax/Roundup/AMS mixes. That’s why all are not labeled for dicamba-tolerant soybeans.

What’s disconcerting to the university weed scientists, though, is the injury that resulted in the Xtendimax and Engenia mixes. These dicamba formulations have been marketed by manufacturers as low in volatility. In the Purdue trials, the Engenia and Roundup mix injured more soybeans than did the Clarity/Roundup formulation, says Bradley. In the U of A trial, the Xtendimax/Roundup mix injured more soybeans than did the Clarity/Roundup mix.

Meanwhile, MU 2017 air sample studies 96 hours detected dicamba following application with Engenia and Xtendimax.

“Xtendimax and Engina were supposed to be different,” says Bradley. “Something is not right. I don’t think we have it figured out yet.”

What Industry Says

The makers of other dicamba formulations acknowledge off-target movement resulted, but that volatility wasn’t a driving factor. BASF officials say its field reps investigated 787 soybean symptomology claims during the 2017 season, most of which had no impact on yield. However, in a few isolated cases, BASF officials say yield may have been affected where the terminal growth was inhibited.

Main causes include:

  • Incorrect nozzle and/or boom height
  • Wind speed or direction
  • Insufficient buffer
  • Spray system contamination
  • Use of unregistered product
  • Inversion present
  • Spray tank contamination

Monsanto officials say that as of October 26 last year, it had fielded 1,467 applicator inquiries regarding potential dicamba damage and made 1,418 site visits. Of that, 1,222 applicators supplied data for review and climatological evaluation. In 91% of those cases, applicators had self-reported errors from one or more label requirements that could have contributed to off-target movement.

These include factors like:

  • Insufficient buffers
  • Wrong nozzle type
  • Boom height set too high
  • Improper tank mixes
  • Wrong spray pressure

In the majority of cases where injury symptoms resulted, yield damage did not occur, says Scott Partridge, Monsanto vice president of global strategy. “What we saw is when they grew out, the vast majority of fields that showed symptoms of cupping – when you see the white undersides of the leaves – grew out with no yield loss,” he says.

Partridge says he was part of the launch of the first Roundup Ready crops in the 1990s.

“In 1996, we had the first application of over-the-top Roundup Ready soybeans. There was off-target movement as a result of folks learning to use the technology,” he says.

He acknowledges there is more apprehension over dicamba off-target movement than with glyphosate in the mid-1990s. Two differences between then and now, he says.

“In 1996, the introduction (of Roundup Ready soybeans) was on 3 million acres. Last year, dicamba-tolerant corps were planted on 26 million soybean acres and 5 million cotton acres. So, it was almost a ninefold incease in acreage.”

The second difference, says Partridge, is the advent of social media. “In 1996, when farmers talked about what happened on the farm, they sat around the coffee shop table. Now, it is difficult to sort between fact and fiction on social media. It takes it takes on a logic all its own.”

Partridge adds that training and education is how applicators learned how to properly apply glyphosate to glyphosate-tolerant crops. The same strategies apply to dicamba and dicamba-tolerant soybeans, he says.

Can Farmers Comply With the Label and Still Kill Weeds?

Numerous changes have been made for use of dicamba herbicides labeled for dicamba-tolerant soybeans in 2018. Some states like Minnesota also have added restrictions.

The challenge, though, is how to comply with these requirements and still make timely applications, says Bradley. There’s always a tendency to go back and clean up first-application escapes with a subsequent postemergence pass with the same herbicide. Each pass made, though, selects for biotypes that resist the applied herbicide.

Over time, this leads to more and more resistant populations until the herbicide is rendered ineffective.

“We went down that road with Roundup and we got resistance,” says Bradley. On the flipside, the new dicamba formulations remain a valuable tool for controlling herbicide-resistant weeds like marestail and ragweed. That’s why he’s joining Iowa State University weed scientists and recommending using dicamba as a preplant burndown and not spraying it postemergence in June and July. 

“That message is not popular, and a lot of people get mad at me,” he says. “But the fact of the matter is that in June and July for the last two years, we have had major problems with off-target movement. Something is going in on with June and July.”

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