On July 21, 2005, about 1130 central daylight time, a single-engine Grumman-Schweizer G-164A, agricultural bi-plane, N8891H, was substantially damaged during a forced landing following a partial loss of engine power, while maneuvering near Tallulah, Louisiana. The private pilot, soul occupant, sustained minor injuries. The airplane was registered to and operated by the pilot. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 137 agricultural flight for which a flight plan was not filed. The local flight originated from the Red Beard Dusting Service Airport (9LS5), near Tallulah, Louisiana, approximately 1100. According to the 2,697-hour pilot, at the completion of an east to west spray pass over a field, the airplane's engine experienced a loss of power and developed an unusual vibration. The pilot turned the airplane to a southerly direction in an attempt to fly back to the originating airport. Realizing that the airplane was not developing sufficient power to return to the airport, the pilot turned the airplane approximately 180-degrees towards a road that he thought would make a suitable landing area. Before the pilot could reach the road, the engine started making "popping" noises, trailing smoke, and experienced a further reduction of power. The pilot then elected to land in a cotton field. During the landing roll, the left main landing gear collapsed and the airplane nosed-over coming to rest in an inverted position. A Federal Aviation Administrator inspector, who responded to the accident site, reported the airplane sustained structural damage during the mishap. The inspector further reported that an examination of the engine revealed the number five-cylinder exhaust valve rocker arm mounting boss was fractured approximately 180 degrees around its circumference. This fracture allowed the exhaust valve to remain closed.