On July 11, 2000, about 1030 hours Pacific daylight time a Grumman G-164A, N897X, operated by Williams Ag Service, Inc., Biggs, California, experienced a partial loss of engine power while maneuvering during an aerial application flight about 2 miles west of Gridley, California. The pilot made a forced landing on a field road. During rollout the airplane collided with a dirt berm and was substantially damaged. The commercial certificated pilot was not injured. The flight was performed under 14 CFR Part 137, in visual meteorological conditions, and it originated from a nearby field about 1020. The pilot reported to the National Transportation Safety Board investigator that during his third fertilizer-spraying pass over the field he heard a noise. Thereafter, smoke was observed emanating from the R-1340 engine, and the oil pressure dropped. The pilot indicated that as he descended for the emergency landing the intensity of the smoke substantially increased. The operator reported to the Safety Board investigator that he looked at the engine. He noted its No. 2 cylinder exhaust valve head had separated and it broke the cylinder. The airplane's total time was about 12,245 hours. The engine's total time was estimated at 9,300 hours, and it had been operated about 708.8 hours since last receiving a major overhaul.