Tens of thousands of soybean and cotton farmers across the country are taking free but mandatory training in how to properly use a weed killer blamed for drifting and damaging crops in neighboring fields.

The U.S. Environmental Protection Agency required the training and other restrictions last fall in a deal with three major agribusiness companies — Monsanto, BASF and DuPont. All three make special formulations of dicamba for use on new soybean and cotton varieties that are genetically engineered to resist the herbicide, using seed technology commercialized by Monsanto. The products are increasingly popular because they give farmers a new weapon against aggressive weeds such as pigweed that have become resistant to other herbicides such as glyphosate, also known as Roundup.

Farmers have used dicamba on a smaller scale for decades. Its tendency to vaporize and drift led the three companies to develop less-volatile formulations for dicamba-tolerant crops, which came into widespread use last year. But farmers who planted older, non-resistant varieties and didn't use dicamba soon began reporting damage to their crops and blamed nearby farms that did use it.

"It takes focus and time to learn to apply a new product. ... Training and education is critical," said Scott Partridge, Monsanto's vice president for global strategy.

The in-person training sessions are kicking into high gear this month and in March. Monsanto is confident that the training will sharply reduce drift problems this season, Partridge said. Over 91 percent of "off-target applications" last season were a result of farmers not following the label instructions, he said. In Georgia, where training was already mandatory, he said, the state received no complaints of dicamba drift last year.

Monsanto held its first of several sessions in Minnesota on Monday. The company expects to hold several thousand nationwide eventually, Partridge said. BASF and DuPont are making similar pushes across farm country. The manufacturers are conducting the sessions in 26 states, while government agencies in seven others hold similar trainings.

The trainings cover everything from choosing the right spray nozzles, sprayer heights, proper pressures, spray rig speeds, wind speeds and other weather conditions, and best practices for cleaning equipment. They last only about 1½ hours, but he said that's sufficient to drive home the key points because Monsanto also provides a technical support phone number and other tools. For farmers who don't have the proper nozzles, Monsanto plans to hand out over 1 million, free of charge. It will also roll out a smartphone app to give farmers real-time weather conditions for their fields.

Nearly 26 million acres were planted in dicamba-tolerant varieties last year, including over 20 million acres of soybeans. Monsanto expects the number of dicamba-tolerant soybean acres will likely double this year, Partridge said, based on the demand the company is seeing from growers. Tests by both Monsanto and independent academic researchers show a 5.7 bushel-per-acre yield increase compared with another popular weed control system for soybeans, he said.

"We're excited about it and want to do everything we can to make sure that folks have the best experience possible in 2018," Partridge said.

The new federal restrictions, which made dicamba a "restricted use pesticide," limit its use to days when winds are under 10 mph and include new record-keeping requirements. But some states have imposed additional restrictions.

Arkansas had the most crop-damage complaints in the country last year at nearly 1,000 and adopted the toughest rules. The state banned dicamba in most cases from April 16-Oct. 31, which essentially rules out using it on soybeans. Monsanto has sued to block that ban from taking effect. Minnesota, which received 253 complaints, set a June 20 cutoff date and prohibited applications on days when temperatures exceed 85 degrees. North Dakota cuts off applications at June 30 or the crop's first bloom phase, whichever comes first.

Mike Petefish, who farms around 5,000 acres near Claremont in Dodge County of southern Minnesota, said he expects the training sessions will be popular. Farmers generally accept the new restrictions, he said.

"I know farmers are really concerned about keeping the product," said Petefish, also president of the Minnesota Soybean Growers Association. "I know that for some who have serious herbicide resistance problems with weeds, there really isn't any other product available."

Gregg Regimbal, a pesticide manager with the Minnesota Department of Agriculture, said the training material "certainly looks thorough to me." Training offered in one state will be accepted in the other states that allow the companies to conduct it, he said.

Monsanto and other manufacturers are being sued by farmers who say their crops were damaged by the herbicide last year. Many of those cases have been consolidated in federal court in St. Louis, Missouri. An Arkansas jury in December convicted a man of second-degree murder in the shooting of a farmer who accused him of using dicamba and damaging his crops.