The helicopter pilot reported that, before starting agricultural application flights for the day, he calculated that about 490 lbs of chemical would bring the helicopter's weight near the manufacturer's maximum gross weight limitation. After an agricultural application flight, he landed the helicopter on a platform on top of a chemical reloading truck and reloaded the chemical tank with the precalculated amount. Once reloaded, he maintained a stabilized hover over the truck and transitioned to forward flight. When the helicopter cleared the truck, the "engine [made] a lower sound," and the main rotor low rpm horn activated, and the caution light illuminated. He immediately added power, lowered the collective to restore main rotor rpm, and pitched forward to gain airspeed, but the helicopter would not climb. He then decided to perform a roll-on landing on the adjacent field, but the helicopter landed hard. The helicopter sustained substantial damage to the tail cone, tail rotor blades, and fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation. The pilot added that, after the accident, he recalculated the helicopter's weight and balance. He determined that, during the reloading operation, the chemical tank was not empty but had about 80 lbs of chemical remaining and that the added precalculated amount of chemical exceeded the helicopter's maximum gross weight limitation.