On June 26, 2013, about 1825, an Air Tractor AT-301, N3168X, sustained substantial damage during a forced landing to a field near Colome, South Dakota, after a loss of engine power. The commercial pilot, the sole occupant, was not injured. The airplane was registered to and operated by Semper Fi Aviation LLC under the provisions of the 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed. The flight originated from the Winner Regional Airport (ICR), Winner, South Dakota, about 1720. The pilot reported that on the start of his third spray pass, he cleared the telephone wires located on the southwest corner of the field, and entered the field flying 95 knots indicated airspeed. The engine instrument gauges indicated 33 inches of manifold air pressure (MAP) at 2,100 rpm. He stated that he "immediately noticed a reduction in power." He attempted to pull back on the control stick to gain altitude, but the airplane's tail and right wing struck the field. During the forced landing rollout, the right landing gear collapsed and the airplane rolled onto its nose. An airplane and powerplant-rated (A&P) mechanic ran the engine while it was still attached to the airframe. The mechanic ran the engine to 32 inches of MAP at 2,100 rpm, and then pulled the power back to 30 inches of MAP. He was unable to run the engine at full power (36 inches of MAP at 2,200 rpm) due to the vibration from the bent propeller and collapsed landing gear. He reported that the engine ran normally without anomalies. No water was found in the fuel system and the weather conditions were not conducive for carburetor icing.