On August 29, 2012, about 1530 central daylight time, an Air Tractor, Inc. AT-301 airplane, N23960, was substantially damaged after impacting terrain during a forced landing near Ralls, Texas. The pilot was not injured. The airplane was registered to and operated by Beta Ag, Inc., under the provisions of 14 Code of Federal Regulations Part 137, as an agricultural application flight. Day visual meteorological conditions prevailed and no flight plan was filed. The airplane was on a local flight and had departed about 1515 from Crosbyton Municipal Airport, (8F3), Crosbyton, Texas.The pilot reported that the engine sputtered and he then experienced a complete loss of engine power. During the power-off landing to a soft dirt trail in a cotton field the airplane nosed-over, and came to rest inverted. The pilot was able to exit without assistance. There was a substantial amount of fuel on-board, but no fuel spill and no postimpact fire. A postaccident examination of the Pratt and Whitney R-1340 engine, serial number P 326932, revealed there was a fatigue failure of the rocker arm journal casting of the rocker arm cap, which resulted in the complete separation of the exhaust valve rocker arm.