On July 6, 1999, at 0830 central daylight time, a Gulfstream-Schweizer G-164B, N8029K, owned/operated by Agrimax LLC, received substantial damage during an in-flight collision with terrain while spraying an agricultural field near Fisher, Minnesota. Visual meteorological conditions prevailed at the time of the accident. The aerial application flight was operating under the provisions of 14 CFR Part 137 and was not on a flight plan. The Airline Transport Pilot reported no injuries. The local flight departed a private airstrip near Fisher, Minnesota, at 0800. According to the pilot's written statement, while on a spray-run a wing tip contacted the wheat crop. The pilot reported that the wheat caused the airplane to turn 180-degrees, flip over once, and come to rest in an upright position. Inspection of the wreckage did not reveal any pre-existing mechanical anomalies with the aircraft's control system or engine.