The corn market’s rally above $5 a bushel this fall has stirred growing consternation among livestock producers and others dependent on the largest U.S. crop. As the global grain supply outlook tightens, some traders in Chicago are placing bets that prices may double next year. Over the past week, trading firms including JPMorgan Chase & Co. and MF Global Holdings Ltd. bought call option contracts that would pay off if corn rose above $10 or $11 a bushel next spring. The options are traded at Chicago-based CME Group, along with futures based on grain, cattle, hogs and other commodities. It’s...