The vast majority of corn grown in the US is for cattle feed. And if we're talking about energy efficiency, that's pretty much as bad as it can get. Not to mention that if we worry about feeding the planet, we should immediately stop growing cattle, except possibly where they can graze on land too marginal for agriculture. Growing a heavily fossil-fuel based crop like corn to feed cattle only makes sense because a) most of the costs of the fossil fuel are externalized, and b) no one actually cares about how many people we can feed, because the people that can pay want beef.