Prior to departure from the field service vehicle assisting with the aerial application flight, the pilot checked his fuel gauges and his personal timer for fuel load estimation. The pilot's plan was to complete a field he had been applying chemical and thought he had enough fuel to complete the short flight. While maneuvering in a slow left turn near the field, the engine experienced a total loss of power. The pilot maneuvered the helicopter to avoid trees and the helicopter experienced a hard forced landing to an adjacent field. The pilot sustained serious injuries and the helicopter sustained substantial damage to the fuselage. Examination of the helicopter by a representative of the operator showed the fuel tanks and fuel lines contained no fuel. The loss of engine power was consistent with fuel exhaustion.