On August 4, 2001, approximately 1145 Pacific daylight time, a Rockwell S-2R, N4237X collided with the terrain during a forced landing about three miles east of Latah, Washington. The commercial pilot, who was the sole occupant, was not injured, but the aircraft, which was owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 137 agricultural application flight, which had been airborne for about 40 minutes, was being operated in visual meteorological conditions. No flight plan had been filed. There was no activation of an ELT. According to the pilot, he had just completed spraying a field, and was preparing to return to Tekoa, Washington, for refueling and reloading when the aircraft's engine started popping and running rough. Soon thereafter, the power output of the engine dropped to what the pilot thought was about 30 percent of normal. He therefore made a forced landing in a nearby field of mature lentils. Although the initial touchdown was uneventful, during the landing roll, the aircraft went off a slight rise and became momentarily airborne. When the aircraft touched down again, it was nose-low, resulting in the propeller coming in contact with the terrain. As the propeller dug into the soft terrain, the aircraft was thrown sideways, collapsing the main gear and bending the left wing. A post-accident inspection of the engine revealed that the exhaust valve rocker shaft boss on the number six cylinder had cracked. According to the FAA Inspector who monitored the inspection, with the boss cracked, the exhaust valve would not open, and the detonating fuel-air mixture backfired into the intake plenum. No other engine anomalies were discovered, and the age and total time of the cylinder could not be determined.