Corn Planting Dates and Relative Maturity
Corn yields generally are highest for late April and early May planting.   Yields decrease slowly approaching mid May, and faster approaching late May.  Here is a general yield by planting date guideline from the University of Minnesota:

          Planting Date            Potential Yield%
April 25                           100
April 30                             99
May 5                               97
May 10                             94
May 15                             91
May 25                             88
May 30                             83