CHICAGO • Monsanto Co. invited dozens of weed scientists to a summit this week to win backing for a controversial herbicide but many have declined, threatening the company's efforts to convince regulators the product is safe to use.

Monsanto faces a barrage of lawsuits over its dicamba herbicide and risks of tighter restrictions on its use, after the chemical drifted away from where it was sprayed this summer and damaged nearby crops unable to tolerate it.

Arkansas and Missouri suffered the most complaints of U.S. states with damage linked to dicamba. Weed scientists from the two states declined to attend the summit on concerns about Monsanto's response to the incident.

The company plans to present data at the summit that it says show user error was behind the damage, contrary to academics' findings that dicamba products can vaporize and move off target under certain conditions in a process known as volatilization.

Missing will be Kevin Bradley, a University of Missouri plant sciences professor who has tracked the number of crop acres nationwide that have been hurt by dicamba sprayings. Bradley said he believed Monsanto was not willing to discuss volatilization.

"I think it's best for me to stay away from that," he said.