The pilot reported that, while enroute, the airplane's engine began to run rough and lose power. He immediately turned toward the nearest airport and slowed the airplane to 100 knots. He added that he pushed the mixture to full rich, which caused a short burst of power that lasted just a few seconds. He then pumped the throttle and applied carburetor heat, but then realized he did not have enough altitude to make the airport and began to look for an alternate landing site. He further added, that after locating a ridge top field, he slowed the airplane to 75 knots and pulled one notch of flaps and trimmed for landing. As he lined the airplane up with the field he pulled full flaps at about 300 ft above the ground. The touchdown was smooth and the airplane continued the landing roll in wheat that was approximately 3 ft tall. Subsequently, the right wing struck an industrial irrigation nozzle and the right wing separated from the fuselage. The airplane sustained substantial damage to the right wing and stabilator. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The pilot added that, during the engine failure, he did not attempt to change fuel tanks from the empty right wing fuel tank, and as a recommendation to make the fuel selector check the first part of the restart procedure. The Federal Aviation Administration inspector reported that, while on scene, he was able to run the engine. He added, the left tank was approximately ½ full of fuel. He switched the fuel selector to the left tank and started the engine normally. The oil pressure was fine; fuel pressure normal on both the electric pump and engine driven pump; and the magnetos checked fine.