By Tom Polansek

CHICAGO (Reuters) - Minnesota became the latest U.S. state on Tuesday to restrict controversial weed killers made by Monsanto Co and BASF SE that were linked to widespread crop damage, while Arkansas took a step back from imposing new limits.

The United States has faced an agricultural crisis this year caused by new versions of the herbicides, which are based on a chemical known as dicamba. Farmers and weed experts say the products harm crops that cannot resist dicamba because the herbicides evaporate and drift away from where they are applied, a process known as volatilization.

Monsanto and BASF say the products are safe when used properly.

Monsanto is banking on its dicamba-based herbicide and soybean seeds engineered to resist it, called Xtend, to dominate soybean production in the United States, the world’s second-largest exporter.

However, Minnesota will prohibit summertime sprayings of dicamba-based herbicides after June 20 in a bid to prevent a repeat of damage seen across the U.S. farm belt this year, according to the state's agriculture department.

Farmers also will not be able to apply dicamba-based herbicides if temperatures top 85 degrees Fahrenheit because research shows high temperatures increase crop damage from volatilization, according to Minnesota.

"We will be closely monitoring the herbicide's performance with these restrictions in 2018," said Dave Frederickson, Minnesota's agriculture commissioner.

U.S. farmers planted 90 million acres of soybeans this year, and about 4 percent showed signs of damage linked to dicamba, according to University of Missouri data.

Missouri and North Dakota have separately announced deadlines for spraying dicamba-based herbicides next year.

In Arkansas, a legislative panel advised a state plant board to review its proposal to ban the use of dicamba herbicides after April 15, in a win for the agri-chemical companies.

The panel recommended that the board consider scientific-based evidence among other factors to revise the proposal, the state said.

Monsanto believes there is no scientific evidence to support cutoff dates for spraying dicamba herbicides, said Scott Partridge, vice president of global strategy.

"They have essentially hit the pause button here," Partridge said about Arkansas.

Monsanto, which is being acquired by Bayer AG for $63.5 billion, has sued Arkansas to prevent the state from prohibiting sprayings after April 15.

The company and BASF have said that proposal would hurt Arkansas growers by denying them access to products designed to be sprayed on dicamba-resistant soybeans and cotton during the summer growing season.

(Reporting by Tom Polansek; Editing by Cynthia Osterman)