On May 18, 2000, at 1630 hours Pacific daylight time, a Grumman G-164A, N5335, experienced a loss of engine power during aerial application, then nosed over during the subsequent forced landing in a rice field near Biggs, California. The airplane, operated by Williams Ag. Service under 14 CFR Part 137 as an agricultural operation, sustained substantial damage. The commercial pilot was not injured. Visual meteorological conditions existed for the rice seeding operation and no flight plan was filed. The flight originated about 1615 at a private dirt field in Biggs, and was scheduled to terminate there. The pilot reported that this was the sixth flight of the day. He had refueled prior to the accident flight with 60 gallons of fuel, and was carrying a full load of rice seed in the hopper. He stated that the airplane began losing engine power about 15 minutes into the flight, and he was attempting to make a forced landing in an open pasture. Shortly thereafter, the engine lost all power and he landed short of the pasture in a flooded, muddy, rice field. During the landing rollout, the main wheels dug into the mud and the airplane nosed over. A Federal Aviation Administration inspector examined the airplane on May 15, 2000, at the owner's facilities in Biggs. He observed a crack in the number 3 cylinder exhaust valve area below the rocker shaft that continued through the push rod boss area. No further discrepancies were noted.