The pilot hovered the helicopter at about 20 feet above ground, in front of cattle, in an attempt to head them off. He became aware that an uncommanded descent had commenced and almost immediately the Low Rotor RPM Warning Horn sounded. He lowered the collective control in an attempt to regain control of the rotor rpm, however, the main rotor blades hit several small trees. A heavy landing resulted during which the tail boom was cut off by the main rotor and the skids were spread. The helicopter remained upright. At the time, the temperature was about 32 degrees Celcius the helicopter was hovering into a 15 knot wind. With the helicopter some 50 kilograms below maximum gross weight and a density altitude of 3000 - 3500 feet, an out of ground effect hover was within the machine's capability. However, the pilot said that he required maximum power to hover the helicopter. It is possible that the wind conditions changed momentarily, unnoticed by the pilot, or that during manoeuvring the power requirement exceeded the maximum available. The machine was flying at too low an altitude to allow a controlled recovery from an over-pitch situation. This accident was not the subject of a formal on scene investigation.