The helicopter had been transported to a remote area with the skid landing gear secured to the helicopter transport trailer with four heavy duty ratchet type tie-downs. After arrival at the work location the pilot and several ground crewmembers prepared the helicopter for an agricultural application flight. The pilot reported that during his walk-around inspection prior to the flight he had overlooked the right rear tie-down, and he attempted to takeoff with the tie-down still attached to the helicopter. As power was applied the helicopter immediately rolled to the right and impacted terrain. Both main rotor blades were impact separated mid-span and the transmission and skid landing gear were partially separated. The helicopter sustained substantial damage to the main rotor system, transmission, tail rotor, tail boom and fuselage. A postaccident examination found no preimpact mechanical anomalies with the helicopter.