The pilot of the aerial application flight stated that when the flight returned to the load truck, it still had aerial application product aboard, which he was unaware of. He requested and received an additional 60 gallons of product that was loaded onto the helicopter for the accident flight. After loading the helicopter with the additional product, it now was about 172 lbs over the helicopter’s gross weight. When the helicopter departed from the load truck on the accident flight, the helicopter experienced a decay in rotor rpm and settled into an adjacent corn field about 75 ft from the truck. The helicopter sustained substantial damage to the main and tail rotor blades and tail boom. The pilot stated that he was not properly prepared to jettison the load in the event of an emergency and his power cross checks were not adequate. The pilot stated that there was no mechanical malfunction/failure of the helicopter. An excessive load would have reduced the climb performance capability of the helicopter and resulted in settling.