THE AIRCRAFT HAD JUST RECENTLY RECEIVED AN ANNUAL INSPECTION. BEFORE TAKING OFF ON A LOCAL FLIGHT, THE FUEL TANKS WERE TOPPED OFF. THE PILOT REPORTED THAT THE PREFLIGHT RUN-UP WAS SATISFACTORY, AND DURING THE TAKEOFF ROLL, THE ENGINE DEVELOPED FULL POWER. HOWEVER, SHORTLY AFTER TAKEOFF, THERE WAS A PARTIAL LOSS OF POWER. REPORTEDLY, THE PILOT CHECKED THE MAGNETOS AND MIXTURE, BUT DID NOT APPLY CARBURETOR HEAT. THE POWER CONTINUED TO DETERIORATE. DURING A FORCED LANDING IN A PASTURE TYPE FIELD, THE RIGHT WING STRUCK THE BASE OF A SILO. AN EXAMINATION OF THE ENGINE AND FUEL SYSTEM REVEALED NO DISCREPANCIES THAT WOULD HAVE RESULTED IN A POWER LOSS. THE TEMPERATURE AND DEW POINT WERE 76 AND 61 DEG. ACCORDING TO ICING PROBABILITY CHARTS, VISIBLE ICING COULD OCCUR IN FLOAT TYPE CARBURETORS AT RATED POWER AND SERIOUS ICING COULD OCCUR AT GLIDE POWER.