Corn ethanol has boomed, now providing more than 13 billion gallons of fuel per year, and consuming 40% of the US corn crop. http://johnhanger.blogspot.com/2012/10/stunning-fact-40-of-corn-and-14-of.html. For that 40% of the corn crop, the nation is getting the equivalent of 10% of its gasoline consumption. That's the trade.
The success of corn ethanol and its displacement of oil is one reason why the nation's oil consumption peaked in 2007 and is now lower than in 1999. It is also one reason why US oil imports have fallen substantially since 2005.