A potential blockbuster chemical product from Monsanto in its first year of widespread use has come under scrutiny for damaging soybeans, prompting hundreds of complaints to the state of Minnesota and at least one lawsuit, filed by Arkansas farmers.

The product, an herbicide called dicamba that's used with genetically modified Xtend soybean and cotton seeds, was produced to solve a burgeoning problem: Many weeds have become resistant to Roundup and other popular weedkillers, and growers and crop protection businesses are eager to have a replacement.

The problem, say farmers, is that the new dicamba formulation is vaporizing after it's sprayed and drifting like a fog to injure ­unprotected crops in nearby fields.

"We have a large swath of the state that has reported some level of dicamba damage," said Mike Petefish, president of the Minnesota Soybean Growers Association. "We've got farmers with actual damage to their beans at no fault of their own. They didn't use the product, they didn't buy the product and somehow their fields got damaged."

The Minnesota Department of Agriculture has launched an investigation to determine how the pesticide was used and the scope of the damage, and so far has received more than 200 complaints and reports from farmers.

"This is consuming an immense amount of our time and resources," said Joshua Stamper, director of the department's pesticide and fertilizer management division. The dicamba complaints are double what the department usually fields ­overall in a given year.

The full extent of the damage is unknown because some farmers don't like to report problems, Stamper said, and no one knows how the damage might affect yields this fall.

"There could be really significant economic impacts at a time when we have a down farm economy," he said.

North Dakota, South Dakota and other states also have asked farmers to report problems with the new product, and Arkansas, Missouri and Tennessee have taken steps to restrict dicamba use after being flooded with complaints. A group of farmers from Arkansas filed a class-action lawsuit in federal court against Monsanto Co., and two other major seed and dicamba sellers, DuPont Co. and BASF SE.

Using data from state agriculture departments and university extension experts, University of Missouri weed scientist Kevin Bradley estimated that by Aug. 10, more than 2,200 complaints were filed in more than a dozen states, including Minnesota. There has been damage to 3.1 million acres of soybeans with more expected by the end of summer.

Dicamba has been used for decades, but lost popularity in part because of its tendency to drift or vaporize onto neighboring fields.

But it is on the market again because weeds — like giant and common ragweed and tall waterhemp — have mutated and become resistant to Roundup and other overused herbicides.

Monsanto's solution was to first genetically engineer soybean seeds to tolerate the herbicide dicamba, and then create a new formulation of the herbicide to kill the weeds. The Environmental Protection Agency approved the chemical last November.

When farmers sprayed the new dicamba weedkiller formulation this spring, it worked well on fields planted with the dicamba-resistant soybean seeds, but it also drifted or vaporized and settled on nearby fields injuring other soybeans and other crops. The telltale signs are emerging leaves that become cupped or puckered, or in some cases withered and stunted.

Monsanto officials say the product should not be causing any damage to adjoining fields because the herbicide was reformulated to have minimal tendency to volatilize, or drift.

About 20 million acres of dicamba-resistant soybeans and 5 million acres of dicamba-resistant cotton have been planted this year, said Scott Partridge, Monsanto vice president of strategy, and 95 percent of the growers reported "absolutely no off-target movements" of the herbicide.

But yet hundreds of complaints have been filed with state agriculture departments and with university extension weed specialists. Monsanto published an open letter to growers, sent messages on social media and opened a toll-free phone number to receive feedback about the product, Partridge said, and is investigating every complaint.

While that process is still ongoing, he said, it appears so far that most of the drifting or vaporizing has come from growers spraying in windy conditions or otherwise not following labels. Those instructions also direct farmers to provide wide buffer areas between fields, and to spray low to the ground with special nozzles that minimize drift.

Also a problem, Partridge said, are growers who illegally sprayed older, more volatile dicamba formulations or didn't clean out their tanks thoroughly.

"We're seeing a good deal of off-target movement by causes that are easily fixable," Partridge said.

Monsanto's explanation does not square with the experience of Lawrence Sukalski, who farms 3,500 acres with his family in southern Minnesota and northern Iowa and who planted 1,500 acres of dicamba-tolerant soybeans this year. Two of his neighbors called him to report damage to their soybeans, Sukalski said, even though he followed the herbicide mixing directions, spray pressure requirements and label instructions to the letter.

"I tried to be a steward and I left a 140-foot buffer like the label said, and still it either volatilized or drifted," he said. "It should have been OK, but it wasn't. It's a puzzle."

Sukalski said he hopes the product can be modified to work better because farmers desperately need new technology to control herbicide-resistant weeds. His problems have not damaged any relationships, he said, "but I have heard that there have been some cases where it's neighbor against neighbor."

Jeff Gunsolus, University of Minnesota Extension weed scientist, said the new dicamba technology has greatly reduced volatility, but "reduced volatility does not mean no volatility."

It's possible that some farmers did not follow label directions when applying it, Gunsolus said, but it also seems apparent from multiple reports that many farmers did exactly what was required but still had problems. "This is probably the most complicated label I've seen in my career, and people have run into some issues," he said. "There's lots of frustration and tension out there."

Gunsolus said there's a need for effective weed control, but there's also a need for more farmers to think beyond the next new product and to consider options of cultivating to control weeds, diversifying herbicides, and spraying earlier in the season.

Also of concern, said Gunsolus, is that farmers facing damage from their neighbors may feel compelled next year to purchase the new dicamba-resistant soybean seed as a defensive strategy. "It's an approach that is not respectful of your neighbor's freedom to farm as they choose," he said, and may also affect those growing food-grade beans, organic produce and fresh market crops.

Petefish said that it's important for farmers to record complaints so that regulators can determine whether the product should stay on the market or face tighter restrictions. The dicamba formulations by the three companies received a two-year registration by the EPA, and one-year conditional registration for use in many individual states, including Minnesota.

"It's a big mess to be perfectly honest," Petefish said. "If we do nothing, I can guarantee that a whole bunch of people are going to sue Monsanto, and the state will cancel the product."