On August 13, 1993, at approximately 1115 central daylight time, a Grumman-Schweizer G-164A, N7993, was substantially damaged when it nosed over after encountering rough terrain during a forced landing near Garwood, Texas. The forced landing was executed following a partial power loss after takeoff. The airplane was on an aerial application flight and was flown by a commercial pilot. There was no flight plan filed and visual meteorological conditions prevailed. The pilot, the sole occupant, was not injured. According to the pilot, the engine began backfiring and running "very rough" with an associated power loss shortly after takeoff from the private ag strip. He stated he immediately began dumping the load and turning back toward the airstrip. He further stated that switching the magnetos and increasing throttle did not alleviate the power loss and he was unable to sustain level flight. He elected to execute a forced landing in a rice field, as it was the only option available. He said the airplane touched down in a three point attitude; however, it nosed over during the landing roll. The reason for the power loss was not determined due to the operator having disposed of the engine prior to a determination being made as to the cause.