On August 7, 2014, about 0946 central daylight time, an Enstrom F-28F helicopter, N213BL, was substantially damaged when the pilot executed a precautionary landing in a cornfield after experiencing engine difficulties near Tomah, Wisconsin. The pilot was seriously injured. The helicopter was registered to and operated by Paul Peterson Aviation LLC 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 the loading site about one-half mile from the accident site.The pilot's departure from the loading site on the third application flight of the morning proceeded without incident. However, about one-half mile from the planned application field, the pilot reportedly felt a vibration in the cyclic control and experienced an "irregular" running engine. He immediately elected to execute a precautionary landing to a corn field; the corn crop was approximately 10 feet tall at the time. During the landing, the landing skids collapsed and the tailboom was severed by a main rotor blade. A portion of the tail rotor drive shaft was separated from the airframe. Separation of the drive shaft was consistent with a main rotor strike. The tail rotor transmission separated into two sections; the appearance of the fracture surface was consistent with an overstress failure. The postaccident examination did not reveal any preimpact anomalies with respect to the flight control system and rotor drive system. Examination of the engine revealed that the no. 4 cylinder did not exhibit compression during crankshaft rotation. Further examination of the No. 4 cylinder determined that the intake valve was not seating properly; a gap of approximately 0.015 inch existed when the valve should have been fully seated. Linear scoring were observed on the valve stem. Debris consistent with valve guide material was also recovered on removal of the valve. No other anomalies with respect to the engine were observed. The accident helicopter was purchased by the operator on June 4, 2014. The aircraft was issued a restricted category, special airworthiness certificate for agriculture operations on July 25, 2014. The operator reported a total airframe time of 2,385.1 hours at the time of the accident. The most recent annual inspection was completed in December 2013 at 2,327.4 hours. The helicopter was powered by a 225-horsepower Lycoming HIO-360-F1AD fuel-injected, reciprocating engine. Maintenance records indicated that the engine was overhauled in March 2006. At the time of the accident, the engine had accumulated about 831.2 hours since overhaul, with about 57.7 hours of that time since the annual inspection. The most recent engine maintenance entry was dated August 4, 2014, and consisted of replacement of the spark plugs. The helicopter maintenance records did not contain a record of any unresolved maintenance issues.