The pilot was conducting agricultural flight operations in a helicopter using a truck and trailer as a refueling/loading base. Before the takeoff, the helicopter was reloaded with fungicide and fuel while on the truck trailer. The pilot inadvertently departed with the agricultural load hose still connected to the helicopter. The pilot reported that the helicopter yawed suddenly after liftoff, due to the connected load hose, which he perceived as a loss of engine power. The pilot made an immediate hard landing in an adjacent cornfield, during which the main rotor system deflected down and struck the tail. The tailboom and vertical fins were substantially damaged during the hard landing. The pilot reported that there were no preimpact mechanical malfunctions or failures that would have precluded normal operations. The pilot opined that the accident might have been prevented with the use of a checklist, better radio communication with the ground crew, and the use of a breakaway fitting on the load hose.