The engine started to run rough about a minute after takeoff. The 475-hour unrated pilot reported a 500 RPM drop. The pilot applied carburetor heat and switched fuel tanks, but neither resulted in an increase in engine power. The pilot leveled-off at approximately 600 feet above the ground. About 3 or 4 minutes into the flight, a severe engine vibration developed and the pilot elected to shut-down the engine down and attempt to land in a pasture approximately two miles from the airport. During the landing roll the airplane impacted a fence resulting in structural damage to the wings and additional damage to the propeller. A Federal Aviation Administration inspector that traveled to the accident site noted the airplane's engine chip detector light was illuminated and that the propeller was hard to pull through. The reason for the loss or engine power could not be determined.