The year that comes to my mind is 1950.  I was not very old at that time, but remember not being able to get in the field, until about the 10th of May.  This was in NW Iowa.  The whole summer season was cold and wet, and I think this was pretty well throughout the Corn Belt.  The fall harvest was late and corn never did dry down properly.  It was put in cribs very wet.  There may be some old records on the Internet showing corn yields for that year.