On June 11, 1999, at 1630 central daylight time, a Grumman American G-164B, N6515, owned and piloted by a commercial pilot, received substantial damage on impact with terrain during a forced landing onto a dirt access road located between a corn field and a wheat field approximately 11 miles northeast of Goodland, Kansas. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 137 aerial application flight was not operating on a flight plan. The pilot reported no injuries. The flight originated Renner Field, Goodland, Kansas, at approximately 1530. The pilot stated that while spraying a corn field, he had exited the field, began to pull up and turn to the east when he noticed a vibration. He stated that he continued the maneuver during which he saw oil from the number eight cylinder leaking onto the exhaust of the engine. He added that at 3/4 through the turn, the engine's rocker arm shaft began to "work its way out". He stated the engine was not developing power, so he elected to land on a north-south access road that was approximately 15 feet in width; the sides of which were elevated approximately 2 feet in height. He reported that during the landing attempt, the aircraft's left wing impacted the wheat, the aircraft rotated 180 degrees and the right landing gear separated from the aircraft. The aircraft continued in a rearward direction into the wheat field. The pilot reported that there was an inch of rainfall the previous day. The rocker arm bolt was fractured at the location of a cotter pin hole.