D.F.A. “ought to be doing one or the other” — market milk for farmers or invest in processing, said Peter Carstensen, a former antitrust lawyer with the Department of Justice who tracks the dairy industry. “The conflict of interest is going to result in harm, and the dairy farmer is going to have no good choices.”

Monica Massey, the executive vice president at D.F.A., said in a statement that it was “ridiculous” to claim that the co-op would suppress milk prices. “Our mission is to bring value to our farm families,” Ms. Massey said. “We do this by striving to pay a competitive price to our members and providing a secure home for their milk.”

D.F.A. invests in manufacturing facilities that “bring additional value to our farmer owners,” she added.

But this is not the first time the co-op has faced allegations of anticompetitive behavior. In 2018, D.F.A., which makes billions of dollars in annual revenue, paid $50 million to settle a long-running class-action lawsuit brought by farmers who claimed the co-op had colluded with Dean Foods to lower milk prices. A group of the plaintiffs dissented from that settlement, filing a separate suit against D.F.A. in federal court in Vermont.

That suit alleges that D.F.A. has engaged in a wide range of anticompetitive practices: making deals with other co-ops not to poach one another’s members, sharing milk-pricing information with those rivals to suppress payments to farmers, and signing restrictive supply contracts with processors like Dean Foods and Farmland that made it impossible for farmers outside the co-op to sell to those companies directly.