A fight over one of the most powerful new weapons against hard-to-kill weeds, developed by agricultural giant Monsanto Co., is spilling into the courts.

Monsanto’s new version of the herbicide called dicamba is part of a more than $1 billion investment that pairs it with new genetically engineered seeds that are resistant to the spray. But some farmers say their nonresistant crops suffered after neighbors’ dicamba drifted onto their land.

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