Widespread crop damage from dicamba herbicide fuels controversy

This year farmers across the U.S. Midwest planted soybeans genetically engineered to tolerate the herbicide dicamba. The seeds were so popular they were used on an area about the size of South Carolina. For many growers it was a huge relief to finally have a tool that kills several fierce weeds that have become resistant to the herbicide glyphosate, also known as Roundup.

But other farmers are not happy; in fact, they are downright angry. They opted to plant other varieties of soybeans, which are sensitive to dicamba. After their neighbors began spraying dicamba, these farmers discovered that their own soybean fields displayed the classic signs of dicamba-induced damage, including wrinkled and cupped leaves and stunted growth.

State agriculture agencies—particularly in Arkansas, Illinois, Missouri, Ohio, and Tennessee—are now investigating thousands of reports of purported dicamba injury. And herbicide companies are at odds with university weed scientists over why the damage is happening.

A venerable pesticide in use since the 1960s, dicamba is known to be volatile. So farmers that choose to plant dicamba-tolerant soybeans are required to use one of three new low-volatility formulations of the herbicide and closely follow application directions.

But clearly things went wrong this year. University and agricultural extension experts contacted by C&EN say they have visited hundreds of fields across their states in search of answers.

They allege that the new dicamba formulations remain volatile enough to cause at least some of the damage they are seeing. That would mean that the herbicide can evaporate, travel in the air, and cause off-target damage even when applicators follow label instructions. Herbicide makers don’t agree and say their customers didn’t follow the rules for using dicamba. Misusing pesticides is illegal and can result in fines of up to $25,000 depending on the state.

It would be bad news for everyone if the new dicamba formulations are damaging soybeans due to volatilization. Herbicide companies would be liable for dicamba-injured fields. Growers of dicamba-tolerant crops would be powerless to prevent damage to their neighbors’ fields when they spray. And growers that do not want to plant dicamba-tolerant seeds would be defenseless against damage from movement of dicamba.

In an ironic twist, the debacle may result in higher sales of the modified seeds, which are made only by Monsanto, as farmers may buy the seeds solely to protect themselves.

“University weed scientists believe there is volatility occurring to some extent with these formulations,” says Kevin Bradley, a professor of plant sciences at the University of Missouri. One sign of volatility, he adds, is that whole fields planted with nonresistant seeds show dicamba damage. “We’ve seen a lot of that.”

The state of Missouri has seen 257 cases of suspected dicamba damage, according to Bradley. The number includes more than 100 cases that occurred after the state’s agriculture department issued additional restrictions on the pesticide’s use.

Tennessee’s Department of Agriculture has also tightened guidelines for dicamba applicators in response to reports of injured crops. Larry Steckel, a row crop weed specialist and plant science professor at the University of Tennessee, has seen the damage firsthand.

“I’ve been in hundreds of fields—30,000-plus acres easily,” Steckel says. “The reports kind of died down in late July, but in the last five or six days we got more calls.”

Tennessee farmers plant soybeans from May to July, Steckel explains, and dicamba-tolerant crops planted in July likely triggered the new complaints.

“This is large-scale movement. I’ve never seen anything like this, and I have done this work for 30 years,” he says. Like Bradley, Steckel is concerned that the products’ volatility may be to blame. “When you get three or four farmers spraying thousands of acres and it moves and shows the wide-ranging damages we’ve seen, that suggests volatility.”

Mark Loux, a professor of horticulture and crop science at the Ohio State University, agrees. “Yes, it is volatilizing for sure,” he says. As evidence Loux cites both the timing of the movement—up to several days after application—as well as the distance dicamba is traveling.

But Scott Partridge, Monsanto’s vice president of global strategy, takes issue with that assessment. Greenhouse and field trials show that “if the label is followed, the product will not move far, including through volatilization,” he says.

Plant scientists point out that it can take 14 days or longer after dicamba is applied before damage occurs. That makes it difficult to link a specific application of dicamba to a case of wrinkled soybeans.

Dicamba is a synthetic version of the plant growth hormone auxin. When sprayed on a weed, dicamba doesn’t kill on contact. Instead it works systemically, inside the plant’s tissues. Once it is absorbed by the plant, dicamba causes new leaves or flowers to grow uncontrollably, first deforming, and then killing, the plant.

Because dicamba, an organic acid, is known to be volatile, three big agrichemical companies—BASF, DuPont, and Monsanto—developed new formulas intended for use with Monsanto’s new soybean seed.

Monsanto’s formulation, called XtendiMax with VaporGrip technology, adds a diglycolamine salt to reduce volatility. The further addition of the proprietary VaporGrip additive corrals loose hydrogen ions that could cause the dicamba acid to re-form in the applicator’s mixing tank.

Monsanto reports that VaporGrip reduces relative dicamba volatility by an additional 90% over a version that contains only the salt. But the firm does not provide absolute figures of how much dicamba can enter the gas phase. DuPont makes a similar dicamba product called FeXapan.

Monsanto is conducting its own investigation into instances of off-target damage from dicamba. “We’ve communicated with all of our customers and ask any customers that have any concerns about what has happened in the field to contact us,” Partridge says. “And we are visiting every single field.”

Partridge says his team has seen a number of different causes for the damage. “While we can talk about the observations we’ve made, we can’t calculate the relative percent that is due to any error, incident, or anomaly.” In some cases, he says, applicators use older formulations of dicamba—which is illegal—on their crops. They may use mixing tanks or spray equipment contaminated with other pesticides, or improperly mix the products in the field.

And some farmers select the wrong equipment to apply the new formulations, Partridge says. That could include attaching the wrong spray nozzle, positioning spray equipment too high, or using improper pressure. And some applicators are not observing required buffer zones. Any of these shortcomings can cause droplets of dicamba to drift to neighboring fields.

There are also important weather considerations, Partridge adds. Dicamba should not be applied if the wind speed is greater than 16 km per hour or paradoxically, less than 5 km per hour.

Very still air, particularly in the evening, night, or early morning, can signal a temperature inversion, meaning that a layer of stable, cooler air is floating just above the ground. In an inversion, droplets of dicamba will hang suspended until the air mass moves again, taking the chemical with it.

Agriculture experts at BASF say they have flagged the same issues in their visits to farmers who are using the company’s Engenia brand herbicide. Engenia is a N,N-bis-(3-aminopropyl)methylamine salt of dicamba. It was the only formulation of dicamba that the state of Arkansas approved for use on soybeans.

As of mid-August, Arkansas had reported 876 dicamba complaints, the most of any state. It has banned the use of any dicamba products for the rest of this year’s growing season.

Both Monsanto and BASF emphasize the need for grower and applicator education. BASF has also given away 600,000 spray nozzles. On a July conference call, Gary Smith, BASF’s Midwest regional technical manager, reported that more than 15,000 individuals have been trained. “Where we were able to do face-to-face training, we saw fewer complaints,” he said.

But university scientists say it is not fair to blame farmers. For one thing, they point out that even the most conscientious farmers will have difficulty following the unusually stringent restrictions.

And if they are able to comply, volatility can still undermine those efforts. “When you say ‘low volatility’ five times fast you think there are no issues with volatility, but that is not correct,” says Aaron G. Hager, a professor of crop sciences at the University of Illinois, Urbana-Champaign. “Soy is so sensitive to very small amounts of dicamba. It is an amount like the spray when you open a can of Coke—but spread over an acre.”

That sensitivity makes risky any use of dicamba near nontolerant soybeans, as this year’s results prove, Steckel claims. Steckel and Bradley both say the new formulations should have undergone more intensive testing in the years before commercialization, and that independent university and extension researchers should have had access to the products to perform volatility tests.

BASF tells C&EN it conducted trials to study volatility and did work with some university scientists. However, Monsanto did not engage independent volatility testing on the product it commercialized.

Bradley and other researchers are now making up for lost time. So far Bradley has completed a half-dozen experiments using an air sampler to quantify the amount of dicamba that remains in the air after it is applied. His preliminary results show it can linger in the air column for 24 hours or longer.

Tom Mueller, a University of Tennessee plant sciences professor, has also detected dicamba volatilization, according to data posted on his department’s blog. Researchers say larger field trials are needed to find out if the amount of volatile dicamba is biologically significant.

“There is a still lot to learn, and I don’t know we’ll get the answers quickly enough, because farmers are now making decisions about what seeds they will use next year,” Bradley says. “They can’t stand another year of the same thing.”