On June 29, 1997, at 1301 central daylight time (cdt), a Cessna A188B, N4485Q, was substantially damaged following a partial loss of engine power and subsequent forced landing in a swamp shortly after takeoff from McGregor Airport, McGregor, Minnesota. The airline transport rated pilot reported no injuries. The 14 CFR Part 137 aerial application flight was not operating on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The flight departed McGregor Airport, McGregor, Minnesota, at 1300 cdt. According to the pilot's written statement, it was the fourth flight of the day and during takeoff the airplane did not accelerate. The takeoff was not aborted due to insufficient runway remaining. The pilot lifted off into a shallow climb to facilitate acceleration. The pilot said, "...aircraft performance was not adequate to clear trees on the end of the runway... ." A Federal Aviation Administration (FAA) inspector represented the NTSB during the on-scene investigation. The Inspector's visual inspection did not reveal any mechanical problems that would have resulted in a partial power loss. The pilot did not perform an emergency hopper dump of the 1200 pounds of herbicide. During the course of the accident investigation, it was determined that the pilot did not possess an Agricultural Operator Certificate in accordance with 14 CFR 137.11 and that there was no documentation to show that he received training on aerial operations in accordance with 14 CFR 137.19(e). This was the pilot's second similar accident with in the last two weeks.