On December 16, 2004, at 1620 central standard time, a Bell UH-1, N51AG, registered to and operated by AG Rotors, collided with trees during a spraying operation five miles west of Rockford, Alabama. The agricultural flight was operated under the provisions of Title 14 Part 137 with no flight plan filed. Visual meteorological conditions prevailed at the time of the accident. The pilot was not injured and the helicopter sustained substantial damage. The flight departed from a field site located in the vicinity of Rockford, Alabama, on December 16, 2004, at 1600. According to the pilot, during a turn while applying fertilizer over pine trees he heard a "loud bang"; at the same time he lost tail rotor authority. The pilot attempted to level the helicopter and settled into the trees. Post accident examination reveal the helicopter was upright and on its skids. Flight control continuity was established with the cyclic and the collective controls. Examination of the tail rotor blades revealed leading edge damage on both blades. Examination of the helicopter revealed the lower fuselage was buckled and the tail rotor rear housing of the tail rotor gearbox transmission was broken and cracked. The tail rotor assembly was examined and revealed that the fracture features on the case assembly revealed an overload event. The wear patterns on the teeth of the bevel gears within the gearbox showed that they were forced apart. There was no evident of any pre-exiting condition on the components examined that would suggest a failure before the tail rotor strike.