As suspected drift from dicamba took a toll on farmers the past two growing seasons, Monsanto — the Creve Coeur-based agribusiness company that helped give the herbicide newfound prominence with its introduction of dicamba-tolerant crop varieties — publicly urged growers not to spray illegal kinds of the product while new formulations supposedly less prone to drift waited for regulatory approval.

But a class-action lawsuit filed Wednesday in federal court in St. Louis accuses company sales representatives of secretly giving farmers assurances that using unauthorized or “off-label” spray varieties would be all right.

“This was Monsanto’s real plan: publicly appear as if it were complying, while allowing its seed representatives to tell farmers the opposite in person,” the suit alleges, based on farmer testimony. “Their sales pitch: assure purchasers that off-label and illegal uses of dicamba would ‘be just fine.’”