The pilot was conducting agricultural spraying operations on a 12 ha cotton crop. He said he had a good knowledge of the field and had sprayed it several times during the season. He was aware of two power lines running across the field and planned to fly under them. One powerline ran diagonally across the direction of the intended spray run. After passing under this wire the pilot felt a vibration through the helicopter. Although it continued to fly in a straight direction, the helicopter would not respond to the pilot's control inputs. It then descended, entered the crop, and slid along the ground for 20 m before coming to rest. The pilot assessed that he had maintained inadequate clearance from the powerline, and that the main rotor blades had brought the powerlines into contact with the tail rotor. The clearance below the wires was reduced by the fully grown cotton. The diagonal orientation of the wires, and reduced ambient lighting may also have impaired the pilot's judgement of the available clearance.