On June 26, 1996, at 0900 central daylight time, an Air Tractor AT-301, N8883S, sustained substantial damage during a forced landing following a loss of engine power, near Ness City, Kansas. The pilot reported no injuries. The 14 CFR Part 137 flight was operating in visual meteorological conditions. No flight plan was on file. The flight departed from a private strip on the local flight near Ness City, Kansas. The pilot stated that that during his third spray run the engine began to vibrate. He said that he initially reduced power, but then tried to advance the throttle again. He then found that the vibration was continuing. He elected to execute a forced landing; however, he overflew the intended field and touched down in a wheat field. The touchdown was in mud and the airplane nosed over. Subsequent to the accident an examination was conducted to determine the source of the vibration. A counterweight bolt on the propeller was found broken. The pilot was unable to account for the reason the bolt broke and did not have any record of the time in service for that component.