The pilot reported that during cruise flight the helicopter's engine had a total loss of power and he made an emergency landing in a wheat field. He said the main rotor blades contacted the tail boom and sheared off the tail rotor during the landing. The helicopter sustained substantial damage to the tail rotor, tail rotor drive shaft, and tail boom. The pilot reported no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. The pilot reported using a placard as a reference for a fuel tank sight gauge. A postaccident examination by an FAA Aviation Safety Inspector revealed approximately three and one-half gallons of fuel in the right fuel tank and approximately one and one-half gallons of fuel in the left fuel tank. This make and model of helicopter has a capacity of 61 gallons of fuel of which 57.5 gallons of fuel is useable and three and one-half gallons of fuel are unusable. The fuel lines to the engine are located in the rear of the fuel tanks. A pitch attitude change such as a banking turn or nose down attitude, could have caused a disruption in fuel flow to the engine with the low fuel levels that were found in the fuel tanks.