Minnesota is clamping down on a troublesome pesticide developed by Monsanto that brought a cascade of complaints and damaged an estimated 265,000 acres of soybeans in the state this year. Monsanto is pushing back, questioning how the state came up with the tougher standards and warning that they could be counterproductive.

At stake are millions of dollars in potential sales for the new product, both in Minnesota and nationally. Minnesota ranks third in the nation for soybean production after Illinois and Iowa, and the state's crop was valued at $3.6 billion in 2016.

Monsanto developed a new formulation of an old pesticide, called dicamba, to solve a vexing problem: Many weeds have become resistant to Roundup and other popular weed killers, and growers and crop protection businesses are eager to have a replacement.

So Monsanto engineered soybean and cotton seeds to tolerate the herbicide dicamba, and then created a new formulation of the herbicide called XtendiMax to kill weeds.

The problem is that dicamba is hard to control, and it drifts or volatilizes and spreads to kill soybeans and other crops on nearby fields that are not immune to the herbicide.

Farmers across the country have complained of damage. Minnesota received 250 complaints of damaged soybeans and other crops, said Gregg Regimbal, pesticide manager for the state Department of Agriculture.

"We've never seen numbers like that — never," he said. Normally, there would be about 100 complaints for all pesticide products in a year.

Because of the complaints, Minnesota Department of Agriculture Commissioner Dave Frederickson issued new restrictions earlier this month for using dicamba in 2018. Growers may not use the herbicide after June 20 or on any day when temperatures are forecast to be higher than 85 degrees.

Individual states have authority to issue rules for pesticides that are more restrictive than federal requirements.

Scott Partridge, Monsanto vice president of global strategy, said the Minnesota cutoff date and temperature ceiling are unnecessary and will limit use of the new technology by farmers who need it.

Partridge said that Monsanto has done more than 1,200 tests that have showed that dicamba can be used safely. "We know from the experiences of last season that if our product is applied in accordance with the label, it won't volatilize and move off target beyond the buffer zones and cause either yield reductions or economic loss," he said.

Farmers planted about 21 million acres nationally with dicamba-tolerant soybeans in 2017, and Monsanto expects that amount to nearly double next year.

However, use of dicamba generated more than 2,700 complaints in more than 20 states and affected about 3.6 million acres in 2017, according to reports from state ag departments and state extension weed scientists compiled by the University of Missouri.

The damage can cause leaves to become cupped or puckered, or in some cases withered or stunted. The amount of injury depends upon several variables, so the extent to which yields may have been reduced is unclear.

Partridge said the problems were largely the result of growers who did not have wide enough buffer zones between their fields and neighboring farmland, or did not use special nozzles to spray. Other farmers may have applied the pesticide at the wrong pressure, he said, or too high above the ground, or when it was too windy.

But federal officials were not convinced that growers were completely to blame, and took the unusual step of intervening during the first year of dicamba's two-year initial registration period. In October, the Environmental Protection Agency moved dicamba to the "restricted use" category of pesticides that require more education, training and record-keeping for those who use it. Regulators also reduced the times during the day when applications can occur, and lowered the maximum allowable wind speed for dicamba spraying from 15 to 10 miles per hour.

Minnesota is one of a handful of states that have added to those restrictions. University of Minnesota Extension weed scientist Jeff Gunsolus said the June 20 cutoff date and temperature ceiling are needed to protect vulnerable crops from injury, and they are based on science and the typical life cycle of soybean plants in Minnesota.

The June 20 date was chosen, he said, because 75 percent of the complaints in Minnesota were related to spraying that happened after June 20, which is also the time at summer solstice when soybeans are triggered to move from their vegetative to reproductive (flowering) stage and are more vulnerable to injury. And the temperature was picked because the spray tends to volatilize as the air gets warmer, making it more likely that it will hang in the air and move to injure crops on other fields.

Gunsolus said there's no guarantee that the new state restrictions will prevent all problems in 2018, but they should reduce the probability and likelihood of injury. That's important, he said, because continued use of dicamba without further protection will damage more than crops.

"These sorts of things create issues in communities, neighbors against neighbors," he said. "This is stressful stuff."

A handful of other states have imposed extra restrictions for 2018. North Dakota will not allow it to be sprayed after June 30 or when temperatures are above 85 degrees. Missouri decided last month to prohibit dicamba spraying in 10 counties after June 1 and statewide after July 15. Even tighter restrictions were approved in Arkansas, which had more problems than any other state; Monsanto has challenged those rules in court.

BASF SE and DowDuPont sell similar versions of dicamba-based herbicides that are also affected by the new federal and state rule changes.

Mike Petefish, president of the Minnesota Soybean Growers Association, said that controlling herbicide-resistant weeds is a growing problem in Minnesota, and producers need tools such as the new dicamba formulation. Because of all the complaints with the product, the association formed a task force and recommended that the state impose an 85-degree temperature ceiling and an unspecified cutoff date for application.

Petefish said some farmers may feel the June 20 date is too early, and others may think it's not protective enough, but, he said, it seems to be based on good science. There were clearly significant problems in 2017, he said, so it's a question of keeping the product available to those who want to use it, but also protecting the fields of those who chose not to use it and suffered crop damage.

"It's trying to find that balance, and I think we're pretty close," Petefish said. "Something had to be done."

Tom Meersman • 612-673-7388