While cruising at 3500 ft MSL, the pilot heard a loud noise (like an explosion) and saw the engine cowling come open. Simultaneously, the engine lost power and would not respond to throttle movement. During a forced landing in a pasture, the aircraft collided with a fence, and at that time, the engine separated from the aircraft. Witnesses, who saw the plane land, stated that the engine was just hanging on and dropping down during the approach. An investigation revealed that one blade of the McCauley, 1A170 propeller had failed and separated in flight. A metallurgical examination revealed the blade had failed from high cycle, low stress fatigue cracking. The crack initiated from an area adjacent to an attachment bolt hole. AD 80-04-05 had previously identified a cracking problem on this model/propeller. It called for a one-time dye penetrant inspection which had been accomplished on the failed propeller. After this accident, AD 82-27-01 was issued to inspect this make and model every 200 hours.