The glider pilot reported that he was unable to find thermals that would provide suitable lift for continued flight. Subsequently, a field was selected that the pilot assessed was adequate for a successful landing. After overflying the field at 1000 feet AGL the pilot began his approach. The approach and touchdown were uneventful. During the landing roll, as the pilot deployed the speedbrakes and applied braking to the wheel the glider pitched forward and the left and right wings contacted crops that were located on either side of the glider's landing roll. The left wing engaged the crops more fully than the right wing and the glider yawed to the left before coming to rest. As a result of the landing roll, the empennage was substantially damaged. The pilot reported that there were no preimpact mechanical malfunctions or failures with the airframe that would have precluded normal flight.