Corn Yields Have Increased Six Times Since 1940
The chart above displays annual U.S. corn yields (bushels per acre) back to 1866 (USDA data here). After remaining flat between 1866 and 1939 at about 26 bushels per acre, corn yields started increasing dramatically in the 1940s due to the introduction of hybrid seeds, and the widespread use of nitrogen fertilizers and herbicides (source). By 2009, average corn yields had increased by more than six times to a record high 165 bushels per acre, before falling last year to 153 bushels per acre.
According to the Corn Farmers Coalition:
HT: Lee Coppock
Update: The chart below shows that real, inflation-adjusted corn prices have trended downward over time as corn yields have increased, from $16 per bushel in 1948 (2010 dollars) to about $4 per bushel in 2010.