U.S. state attorneys general have joined a federal antitrust probe of the planned merger between DuPont and Dow Chemical Co., according to three people familiar with the matter, heightening risks to a deal that could help reshape the global farm industry. A separate group of state attorneys general are expected to join a probe of Bayer AG's $66 billion plan to buy Monsanto Co., one of the sources said. The involvement of the state attorneys general increases scrutiny of the mega-deals and will complicate what are already expected to be tough and lengthy reviews by U.S. antitrust enforcers. About seven states, including California, have joined the probe of Dow's planned merger with DuPont, according to two people familiar with the matter. It was not yet clear how many states would join the Bayer-Monsanto merger investigation, one source said. The states are concerned that the companies may raise pesticide and herbicide prices for farmers following a merger, and have less incentive to compete to introduce better and cheaper products, two of the sources said.