On August 23, 1994, about 1545 hours eastern daylight time, a Mooney M20B, N778R, a personal flight, lost engine power and was forced landed in a field at Marion, Ohio. Visual meteorologicl conditions existed. The pilot and passenger were not injured The airplane was substantially damaged. The departure point was Marion, Ohio. The local flight was operated under 14 CFR Part 91. After takeoff, the pilot climbed to 3000 feet msl and practiced some manuevers. He returned to Marion and did touch and go landings. He departed Marion and shortly afterward the power loss occurred after he first slowed and then tried to accelerate the airplane. According to the pilot, "I then experienced throttle failure, moved free in and out with no response. I tried for Marion, could not make airport. (I) looked for a suitable landing, lined up with road, gear down and locked, I secured engine for landing. As I touched (down) the left wing caught a field of corn turning aircraft into a ditch. Examination of the airplane revealed the throttle cable had disconnected from the carburetor.