The pilot reported that the engine started running rough and vibrating. He turned the ignition key to "OFF" and executed a forced landing to a field. The fuselage sustained substantial damage during the forced landing to a rough field. The inspection of the engine revealed that the number one cylinder connecting rod had failed. It exhibited signatures consistent with overheating. The number one cylinder connecting rod bearing was extruded and pieces were found in the oil screen. The bearing exhibited signatures consistent with overheating. About 2 quarts of oil were found in the engine sump with no signs of external oil leakage. The pilot reported that oil level during the preflight was over 4 quarts.