On June 23, 2004, at 0945 central daylight time, an Air Tractor AT-401, N1013N, piloted by a commercial pilot, sustained substantial damage during a forced landing near Britton, South Dakota. Visual meteorological conditions prevailed at the time of the accident. The aerial application flight was operating under the provisions of 14 CFR Part 137 without a flight plan. The pilot was not injured. The local flight departed Britton Municipal Airport (BTN), Britton, South Dakota, at 0900. The pilot reported that the engine began running rough after he completed spraying and was returning to BTN. The pilot stated he verified the operation of both magnetos, but the engine continued to run rough. The pilot reported vehicular traffic prevented a landing on a nearby road, so he attempted to fly the airplane back to BTN. The pilot stated he eventually had to perform a forced landing into a soft field, during which the airplane nosed over. A Federal Aviation Administration (FAA) inspector examined the engine and found the shaft mount for the number eight cylinder exhaust rocker-arm cracked and separated, consistent with damage caused by a stuck exhaust valve.