On May 19, 1995, at 1653 central daylight time, an Air Tractor AT-301, N3655Z, was substantially damaged while maneuvering near Caldwell, Arkansas. The commercial pilot was not injured. The aircraft was being operated by Lindsey Ag-Air Inc., under Title 14 CFR Part 137. The flight originated from McCollum private airstrip near Colt, Arkansas, at 1625. Visual meteorological conditions prevailed for the local aerial application flight and a flight plan was not filed. During an interview, conducted by a Federal Aviation Administration (FAA) inspector, the pilot reported that he had completed the first swath and initiated a climb to reverse direction. While the airplane was in a nose high attitude at about 200 feet above the ground, the engine lost power. The pilot executed a forced landing to a wheat field. During the landing, the aircraft sustained structural damage to the right wing and the empennage. Examination by an FAA inspector revealed a crack in the #4 cylinder rocker box housing on the exhaust side. See enclosed report from FAA inspector.