The pilot was herding a bull along a shallow valley and he decided to hover behind it. He entered the hover about 20 feet above the trees from a slight descent and with some right lateral movement. The heading of the helicopter was westerly in a wind from the south at 10 knots. The pilot then initiated a pedal turn into wind. The helicopter stopped turning when nearly into wind and the pilot added almost full left pedal to complete the turn. The helicopter then began to rotate smoothly and rapidly to the right, and the pilot applied full left pedal in an attempt to stop the yaw. As the application of full left pedal had no effect, the pilot closed the throttle but there was no perceptible reduction in the rate of turn. The helicopter rotated through at least 360 degrees before striking the trees and crashing on its side. The on site investigation did not reveal any mechanical defects or anomalies that would have contributed to the loss of tail rotor control.