On August 14, 2011, about 1230 central daylight time, an Air Tractor AT-301, N73071, sustained substantial damage to its fuselage during a forced landing to a bean field following a partial loss of engine power. The commercial pilot was not injured. The airplane was registered to and operated by Semper Fi Aviation under the provisions of 14 Code of Federal Regulations Part 137, as an aerial application flight. Visual meteorological conditions prevailed for the local flight, which was not operating on a flight plan. The flight originated from the Edgeley Municipal Airport (51D), Edgeley, North Dakota, about 1225. The pilot reported that he performed a normal takeoff in the agricultural airplane, climbed to about 200 feet above ground level, and established level flight. At that time, he reduced engine power to 30 inches of manifold air pressure (MAP) and 2,000 rpm. After several seconds of normal flight he noticed a descent developing and increased engine power to 32 inches MAP but received no response. The pilot added one notch of flaps and dumped the chemical load using the emergency dump handle but the descent continued. The pilot landed the airplane in a bean field. On-scene examination of the airplane after the accident by Federal Aviation Administration (FAA) inspectors did not reveal any preimpact mechanical failures. The engine was separated from the airplane and firewall damage had occurred. There were no outward signs of a mechanical failure of the engine. At the request of the local landowner, the pilot agreed to leave the airplane in the field until after the crops were harvested. There was no further examination of the airplane after removal from the field. The airplane was powered by a supercharged Pratt & Whitney R-1340-AN-1 engine bearing serial number 5684. The engine was rated to produce 600 horsepower for takeoff and 550 horsepower continuously. Maintenance records indicated that the airplane had received an annual inspection that was completed on April 6, 2011. The engine entry on that date noted the all cylinder compressions were 75/80 or better. Additionally, the oil filter was removed and cut open to examine the filter element. No anomalies were noted. The temperature and dew point recorded at the Oakes Municipal Airport, Oakes, North Dakota, about 29 nautical miles east of the accident, were 24 and 17 degrees Celsius. According to FAA Special Airworthiness Information Bulletin (SAIB) CE-09-35, these readings were in a range susceptible to the formation of carburetor ice at glide and cruise power settings. The SAIB further stated that carburetor icing can be detected by a drop in manifold pressure and that there is usually roughness in engine operation. The pilot made no mention of roughness in operation.