The pilot departed for a cattle roundup flight of a herd of cows and a single bull. As he approached the herd, with the intention of driving the cattle into the corral, the bull departed and headed toward an area covered with heavy mesquite brush. The pilot circled the area, and stopped the bull under a large mesquite bush. However, while at a hover the bull made a move to go behind the helicopter and the pilot maneuvered the helicopter aft and in so doing the tail rotor contacted a small mesquite limb approximately 1/2 inch in diameter. The impact resulted in a catastrophic failure of the tail rotor system, which separated from the tailboom. The helicopter began to spin, and the pilot applied correct control inputs and reduced engine power. The helicopter then landed hard in a level attitude, spreading the skids and deforming the lower fuselage structure. The pilot reported no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation.