According to the pilot, he had just departed a gravel road (loading zone) to complete an aerial application to an alfalfa field. Shortly after departure, he executed a right turn and one of the helicopter spray booms contacted the 2 1/2 feet tall alfalfa. The helicopter then began a downward right spin. Subsequently, the spray boom dragged on the alfalfa and "initiated a dynamic rollover." The main rotor blades contacted terrain and the helicopter "cartwheeled" onto its left side. Examination of the helicopter revealed damage to the main rotor blades, tail boom, tail rotor, and fuselage. The pilot recommended the following: "Ensure adequate altitude before initiating turns, and exercise more caution when over tall grass or crops."