The pilot reported that while in a turn to reverse direction after completing a downwind spray run, "I noticed my RPMs getting low. As I was coming back down into the field I was rolling on throttle but could not get RPMs where I wanted them. I was hoping for the wind to help me, but there wasn't much wind at the time." The pilot stated that he pushed the cyclic forward to get airspeed, followed by pumping the collective to try to get power back. "I did this all the way until I hit the ground." The pilot reported that the helicopter impacted the ground in a flat, level attitude at about 40 miles per hour before skidding 30 feet and coming to a stop in an upright position.