The aircraft was cruising at 1500 feet above the ground when the engine began to vibrate and the RPM dropped from 2400 to 1900. Even with full power selected the aircraft could not maintain height. The pilot planned a forced landing on to a stretch of trotting track. On final approach he realized that the aircraft was overshooting the planned touchdown point. He used the available engine power and landed in a 250 metre wide paddock adjacent to the trotting track. Braking was not fully effective on the damp grass surface. In the latter part of the roll despite an attempt to veer the aircraft to the left the aircraft penetrated a stock fence at very low speed. The underside of the mid-section of the left wing was damaged as it contacted a fence post which swung the aircraft to the left. Number one cylinder was found to have no compression. About half of the head of the exhaust valve had broken free and been ejected through the exhaust port. The exhaust valve had achieved 603 hours time in service since installation as a new item. There may have been a flaw in the exhaust valve since manufacture. The valve failure was considered to be an isolated occurrence.