The pilot of an Air Tractor AT-402B stated he was making east and west passes above a cornfield, maneuvering under power lines at one end. On his last pass, the vertical stabilizer contacted a power line, shearing the upper section of the stabilizer and the entire rudder. The pilot was able to land the airplane at a nearby airport without further incident. The pilot further stated there were no mechanical failures or malfunctions to the airplane or any of its systems prior to the accident.