On August 2, 2016, about 0935 mountain daylight time, an Air Tractor, AT 301 airplane, N23923, experienced a partial loss of engine power. The pilot subsequently initiated an off airport landing to a field near Weiser, Idaho. The airline transport pilot was not injured. The airplane was substantially damaged during the landing roll when it collided with a sprinkler system. The airplane was operated by Beck's Flying Service under the provisions of Title 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan had been filed. The local agricultural application flight departed Weiser, Idaho, about 0920. In a report submitted to the National Transportation Safety Board investigator-in-charge, the pilot reported that after departing on the fifth aerial application load of the morning, and while approaching the target field, the cylinder head temperatures (CHTs) rose to over 400o; the airplane had just enough power at 30 inches of manifold pressure (MP) to maintain altitude and airspeed above stall speed. The pilot stated that after making one pass on the field, he observed that the cylinder head temperatures had risen to 414o. The pilot further stated that to stay above gradually rising terrain, he increased engine power to 32 inches MP and half flaps, at which time he noticed that the CHTs had risen to 454o with a full rich mixture. With wings level and about 40 ft above the ground and descending, the pilot dumped part of the load and set his power to 30 inches MP; CHTs remained around 440o. The pilot opined that the airplane would only continue to fly in a slight descent, which prompted him to dump the remainder of the load. With no improved performance, the pilot abandoned the idea of being able to return to the departure airport. However, he was able to maneuver slightly in order to reach an alfalfa field, [avoiding] pear trees before touching ground with the wings in buffet. During the landing [roll], the airplane collided with a wheel line sprinkler system and a tree bordering the field, which resulted in substantial damage to the airplane. A postaccident examination of the engine was performed at the facilities of Anderson Aeromotive, Inc., Grangeville, Idaho. All nine cylinders remained attached to the engine crankcase. A visual examination of the engine revealed that the cylinder head temperature (CHT) and exhaust gas temperature (EGT) probes were connected to the No. 1 cylinder. The front spark plugs were removed from all cylinders with no anomalies noted. The crankshaft was manually rotated, and internal mechanical continuity was established throughout. The cylinders developed compression when the crankshaft was manually rotated. The No. 1 cylinder was removed. When the piston was removed, the lower piston skirt and the oil scraper ring were observed damaged. The inside of the piston had multiple indentations consistent with continued battering. The remaining eight cylinders and pistons were removed, with the similar results noted. Multiple metallic fragments that resembled bearing material were found in the crankcase throughout this process. The chip detector was covered with metallic debris consistent with debris found throughout the internal portion of the engine. The front case, cam ring assembly, and front crankcase assembly were removed with no anomalies noted. The master rod counterweight revealed multiple battering like indentations consistent with the damage observed to the inside of the pistons. Multiple metallic fragments were found in the oil galley. The master rod and link rod assembly was removed from the crankshaft, and the front side of the master rod exhibited two perpendicular dents. The master rod crankpin bearing revealed substantial wear and gouging type damage. The master rod crankpin bearing was thinned or gouged out consistent with the metallic shavings found in the crankcase and oil galley. (Refer to the engine examination report, which is appended to the docket for this accident.) Historical records for the engine were not made available during the investigation.