On April 3, 1998, at 0910 central standard time, an Air Tractor AT-401, N1008T, operated by Kroeplin Ag Service, was substantially damaged when the airplane veered off the runway during the takeoff roll and went through a ditch. The commercial pilot was not injured. The CFR Part 137 flight was departing a grass airstrip near Highmore, South Dakota, on a local flight. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot reported that during the takeoff roll he noticed the airplane was not accelerating normally. He reported that he checked the manifold pressure and RPM's, and that the RPM's were low. He checked the propeller control at full forward. The pilot reported that he veered to the right to avoid a highway at the end of the runway. The airplane went through a fence and into a pasture, breaking the left landing gear as it slid to a stop. The airplane was inspected. The engine was started and run to approximately 1,800 RPM and the operation appeared normal. A small amount of water was drained from the right fuel cell.