On July 13, 2000, about 0715 hours Pacific daylight time, an Air Tractor AT-401, N4508Y, operated by Farmers Aerial Service, Inc., Blythe, California, experienced a partial loss of engine power during an aerial application flight near Ripley, California. Unable to maintain altitude, the pilot made a forced landing in a cotton field. During rollout, the airplane collided with an irrigation canal soil embankment and nosed over. The airplane was substantially damaged, and the commercial certificated pilot was not injured. Visual meteorological conditions prevailed during the flight that was performed under 14 CFR Part 137. The flight originated from Blythe about 0700. Under the supervision of a Federal Aviation Administration airworthiness inspector, the engine was partially torn down and examined. The inspector reported finding the connecting rod associated with the No. 7 cylinder severed at its midpoint location. Also, the rod had separated from the crankshaft. The inspector indicated that the rod's separation from the crank precipitated its midpoint break. The connecting rod pieces were severely peened and mechanically damaged. No other evidence of any preexisting failures or malfunctions was noted. The engine's total time was reported at 5,267 hours. It had been operated for 475 hours since receiving its last major overhaul.