At about 1109 central time, an amateur-built Swearingen SX300 was substantially damaged during a forced landing to a field. The pilot and one passenger were seriously injured. The pilot reported that he shut the engine down following the loss of a propeller blade in cruise flight. He then attempted an off-field landing. A post-crash inspection by the FAA revealed a broken shank connecting one of three blades of the composite propeller to the hub.