The pilot was mustering cattle when the helicopter yawed to the right. Believing there was a tail rotor failure, the pilot turned to the left towards a clearing to land as soon as possible. The landing was heavy, and the main rotors severed the tail boom. Investigation revealed that the initial yaw to the right had occurred when the main rotor blades struck trees. The pilot did not realise during the short time interval between the decision to land and the landing itself, that if there had been a tail rotor failure he would not have been able to maintain normal control when positioning for landing.