While cruising at 3500 feet, the pilot experienced a partial loss of engine power. Attempts to restore full power failed. He radioed approach control and advised the controller of his problem. He also requested radar vectors to the nearest airport for an emergency landing. Moments later the engine quit. The pilot selected an emergency landing area and established an approach to a large pasture. The right main landing gear broke off at touchdown in the rough/uneven pasture. Examination of the engine revealed a failed engine driven oil pump. The pump had a total of 3620 hours and 1682 hours since last overhauled. Lycoming issued a service bulletin which instructed owners to replace the sintered-iron impellers at 2000 hours of engine service. An FAA AD was also issued which allowed an owner to operate up to 4000 hours before replacing the impellers. The propulsion branch stated that a revised AD would be issued to correct the loopholes which caused this accident.