The chart below plots the US acreage of Corn harvested vs.production. Green lines plot the production in tons all the way from 1967 to 2009, where as red lines plot the area of corn harvested.
From the chart, it is evident that corn area harvested has remained little changed even as corn production has fluctuated irrespective of acreage. It may be that seasonal ravages, macroeconomic picture etc. can be attributed to the corn output fluctuations.