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Volatility From New Formulations Drives Some Dicamba Damage, Say University Weed Scientists

They say volatility is a factor that causes low-volatile dicamba formulations to damage soybeans and other crops.

Kevin Bradley was puzzled. 

Back in 2017, the University of Missouri (MU) Extension weed scientist could not determine why parties involved in that summer’s off-target dicamba dilemma differed in opinions regarding dicamba-tolerant technology. 

To better understand it, Bradley started reading behavioral science journal articles. One such article profiled radiologists who scan lung tissue slides for cancer. Unknown to the radiologists, an attention researcher at Harvard Medical School superimposed a gorilla shaking its fist on a slide. Surprisingly, 83% of the radiologists missed the angry ape that was in plain sight. 

This phenomenon – called inattentional blindness – explains why people miss the obvious because they focus solely on one matter. In this case, radiologists who intensely searched for cancer nodules missed seeing the gorilla.

Kevin-Bradley
Kevin Bradley
Gil Gullickson

WHY DICAMBA VOLATILIZES

Bradley observed back in 2017 that inattentional blindness occurred when it came to dicamba volatility between industry officials and university weed scientists. 

Volatility results when a herbicide converts to a gas. When this happens, the herbicide in a gaseous form can leave the application site and damage plants where it lands. 

Volatility occurs due to the high vapor pressure of dicamba and other Group 4 auxin-based herbicides like dicamba (2,4-D and Tordon are others). This can trigger off-target movement even when an applicator uses appropriate application practices.

Dicamba – like almost all other postemergence herbicides – is a weak acid, explains Bob Hartzler, Iowa State University Extension weed specialist. Chemists refer to them as parent acids. In this state, a hydrogen ion is held to the dicamba molecule due to opposite charges of the two molecules. Most herbicide formulations substitute a salt for a hydrogen ion on the parent acid to boost compatibility with hard water and tank -mix partners. This also reduces dicamba volatility. 

"The degree of (dicamba) volatility reduction depends on the salt, the pH of the spray solution, and the pH of the spray droplet residue,” says William Abraham, Bayer Crop Science director of formulations and delivery technology.

Dimethylamine (DMA) salt is used in older and more volatile Banvel formulations. The diglycolamine (DGA) salt used in Clarity reduces volatility more when compared to Banvel's DMA salt. Further reducing volatility is the BAPMA salt in BASF’s Engenia formulation. 

DGA is still the salt in Bayer's Xtend with VaporGrip technology and DowDuPont’s FeXapan  herbicide Plus VaporGrip herbicide. Accompanying it, though, is the VaporGrip technology that works by reducing the association of hydrogen ions with dicamba. Bayer scientists say this minimizes its volatility potential and helps reduce off-target movement.

Companies manufacturing Engenia, Xtend, and FeXapan started marketing the dicamba formulations advertised as being low in volatility in 2017.

Disassociation

Parent acid disassociation can still occur, says Hartzler.

“Anytime there is free moisture, disassociation can occur,” he says. This can occur anytime in the spraying process – from the time the dicamba strikes water in the spray tank to contacting moisture on the leaves of the crop on which dicamba is sprayed. 

“The best analogy I heard about dicamba was from a chemist,” Hartzler says. “He said dicamba is a lot like nitroglycerine. It is a great explosive, but you never know when it will explode. With dicamba, you never know when it (the formulations) will disassociate and convert to the parent acid. Once that happens, it returns to a highly volatile form.” 

What the industry says about volatility 

Bayer officials say testing for volatility on its XtendiMax product included testing over 1,200 GLP (Good Laboratory Practices) studies over 25 geographies. (The EPA’s GLP compliance monitoring program ensures the quality and integrity of test data submitted to the EPA.) 

Bayer officials say studies showed consistent findings supporting low-volatility claims of XtendiMax between controlled environments and field studies in various geographies.  

Officials for BASF, Engenia’s manufacturer, concur. 

“From a number of field investigations we have had, we identified multiple contributing factors, and we didn't believe volatility is one of those driving factors,” says Chad Asmus, BASF technical market manager. 

An example of this occurred in 2017, when BASF field reps investigated 787 soybean symptomology claims. BASF officials say 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 

To help steward the product, BASF took steps like training 26,000 applicators since December 2017 and reimbursing growrs for buying and installing a hooded spray boom. 

Off-target dicamba damage to soybeans and other crops did occur in 2018. However, it was down from 2017, according to Bayer Crop Science officials. 

In 2017, inquiries regarding off-target dicamba in the Roundup Ready 2 Xtend system tallied 99 inquires per 1 million acres. This year, it decreased to 13 per million acres, and most revolved around weed-control issues, says Brett Begemann, Bayer Crop Science chief operating officer. Xtend soybean acreage is up, though, having doubled from last year’s 25 million acres to this year’s nearly 50 million acres. 

Missing The gorilla 

University weed scientists agree that sound spraying practices can help nix off-target dicamba movement.

“Trust me, if you don’t have all of those things checked off – such as having the right tips, the right boom height, the right wind speed – you will see a problem with dicamba moving off target,” says Bradley.

However, they disagree with industry’s take on volatility, noting it's akin to radiologists missing the gorilla in the laboratory slide. Even with the new formulations, the potential for dicamba volatility still exists, says Aaron Hager, University of Illinois (U of I) Extension weed specialist. 

Hager cites a statement in a 1967 Illinois Customer Spray Operators Training School proceedings: 

Unfortunately, Banvel-D (a dicamba formulation) produces a more severe reaction in soybeans than 2,4-D does, and there is a possibility that volatility may be a serious threat.

“So, what have we learned  about soybeans and dicamba that hasn’t been known for 50 years?” Hager asks. “Volatility can occur when spray solution settles on-site, changes to a vapor, and is then carried off-site by wind.  

"When I see a product that’s labeled as low in volatility, that tells me everything I need to know about the volatility," Hager says. “We know it can happen. The only unknown is the scale of what will happen.”

Temperature Inversions

Temperature inversions can fuel off-target dicamba movement.  

During the daytime, warm air rises when sunlight hits the ground. Meanwhile, cool air comes down, hitting the warm air. When this happens, wind results, which keys air circulation. At night, air patterns flip-flop, with cool air at the bottom and warm air on the top. This creates a stable environment that traps any pesticide particles in a suspended air mass. 

When this air mass encounters a horizontal wind, those particles move someplace. At its worst, the trapped pesticide can land in the middle of a multithousand-dollar-per-acre field of fruits or vegetables.

Inversions are common, occurring anywhere sun hits the surface of the soil, says Mandy Bish, University of Missouri (MU) senior research specialist. MU researchers have found inversions setting up at 5 or 6 p.m., a time when many applicators are still spraying. 

Bish says factors associated with inversions include:

  • Clear night skies 
  • No wind
  • Morning dew or frost 
  • Low-lying fog, which is an indicator of an inversion. 

“If you spray the night before you see the fog, it is not the best indicator,” Bish says. 

Though not foolproof, smoke bombs are one way to detect temperature inversions. Smoke bombs set off at 4 p.m. by MU scientists showed fairly rapid dispersal. Ones set off at 7:30 p.m. lingered approximately 50 seconds, which indicates inversion presence, she says.

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