On June 8, 2013, about 1900 central daylight time, an Air Tractor AT-301 airplane, N4421S, sustained substantial damage to its right wing and fuselage during a forced landing following a loss of engine power near Farnsworth, Texas. The pilot was not injured. The aircraft was registered to an individual and operated by Terhune Flying Service under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from a private airstrip near the accident site. The pilot reported that he had flown two flights on the morning of the accident, and had to stop flight operations due to adverse weather. When the weather conditions improved he again began flight operations. The accident flight was the first flight after resuming flight operations. He reported that while starting his first spray pass, the engine lost partial power and he was unable to maintain altitude. He attempted to dump his load of chemical but the airplane continued to descend and the pilot attempted to land in a field. During the ensuing forced landing, the right main landing gear collapsed and the airplane came to rest on its nose and right wing. The pilot reported that during the event, the engine continued to run smoothly but at a reduced power output that wasn't sufficient to maintain altitude. Postaccident examination did not reveal any anomalies that would explain the partial loss of engine power. The engine was intact with no external evidence of pre-impact damage. Further examination revealed no damage to pistons, cylinders, or valves and no anomalies were noted with regard to the fuel system. The temperature and dew point recorded at a nearby airport about the time of the accident were 26 degrees Celsius and 15 degrees Celsius, respectively. According to a carburetor icing probability chart, the recorded temperature and dew point were in the range of susceptibility for carburetor icing during glide and cruise power settings.