The pilot stated that he was about to make his third pass on a cornfield when the engine tachometer went to 800 rpm. There was 'no popping, no warning, just low power.' The pilot put the airplane down in the cornfield. On touchdown, the landing gear dug into the mud. Examination of the airplane's engine revealed that the number four cylinder exhaust valve push rod was broken and the number four cylinder intake valve push rod was bent.