On June 14, 1996, at 1945 central daylight time, an Air Tractor AT-401, N1004W, registered to and operated by Morrison Aircraft Corporation under Title 14 CFR Part 137, was substantially damaged during takeoff near Cash, Arkansas. The commercial pilot was not injured. Visual meteorological conditions prevailed for the local aerial application flight that was originating at the time of the accident. No flight plan was filed. The pilot reported that he had "been operating the aircraft all day with no problems" and he had "carried 9 loads to the field." The airplane landed for another load and was topped off with 60 gallons of 100LL fuel in preparation for the next flight. The pilot reported that, while on takeoff roll, the engine lost partial power. The pilot said that he estimated the engine power loss to be at about 50 percent. The pilot stated that he "was on a half mile airstrip with no time or field length for a shut down. I began to dump the load and tried to get [the] aircraft off [the] ground." The airplane continued off the departure end of the runway and impacted a dirt bank which separated the landing gear from the aircraft. The airplane came to rest in a rice field, approximately 40 feet from the first impact point. The fuselage and both wings were structurally damaged. The airplane's engine was recently overhauled by an FAA certified repair station. The engine was removed from the airframe and test run. According to the facility that performed the test, the engine "exhibited good oil pressures and was operated through all R.P.M./performance ranges and no abnormal characteristics were observed." See enclosed report for details of the engine examination and test run. The owner's representative stated in a telephone interview with the investigator-in-charge that the power loss was the result of fuel contamination.