A new study of biotech seeds in the United States alleges that industry leader Monsanto Co. engages in “heavy-handed investigations” and “ruthless prosecutions” of farmers who use the company’s seeds without paying for them. The Center for Food Safety, a Washington-based advocacy group and frequent critic of the biotech industry, said a nationwide review found that Monsanto has filed 90 lawsuits targeting 147 farmers and 39 small businesses since 1997. Some of the lawsuits were filed against farmers who planted biotech seeds without paying Monsanto, as required in their contracts with the company. But some others were sued simply because biotech crops had drifted into their fields and sprung up amid their crops, the study alleged.