The pilot reported that after passing over power lines, he performed a steep descent toward the field he was spraying. The helicopter had a "high descent rate and excessive nose down attitude" during the spray run entry. The pilot was unable to arrest the descent before the main rotor speed dropped below a safe operating speed. A run-on landing was made in the field at about 30 mph, but the helicopter pitched forward and the main rotor impacted terrain. The main rotor, tailboom, fuselage, and landing skids were damaged. The pilot reported that the accident could have been prevented if he had performed a "less aggressive spray run entry."