The pilot was engaged in mustering operations and was attempting transition from a low forward speed to an out of ground effect hover in order to turn a breakway beast. As he approached the hover, the helicopter experienced a sudden partial power failure. The pilot maintained the aircraft heading, lowered the collective lever and attempted to gain some forward airspeed. In the attempted landing with partial power, the helicopter struck the ground heavily and its main rotor blades hit a small sapling. The forced landing was made in light downwind conditions. Investigation revealed that the No 2 cylinder exhaust valve had failed in fatigue and the valve seat and retaining cap had failed in overload. Further testing indicated that the exhaust valve had been subjected to higher than normal operating temperatures which would have contributed to the premature failure of the valve.