On November 6, 2017, about 1310 central standard time, a Robinson Helicopter R44 helicopter, N7085M, was substantially damaged when it impacted powerlines and then terrain 4.5 miles south of Larchwood, Iowa. The commercial pilot and passenger sustained serious injuries. The aerial observation flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no Federal Aviation Administration (FAA) flight plan had been filed for the flight. The local flight departed about 1300.According to the passenger and witnesses on the ground, the helicopter was assisting in locating and herding 300 cattle that were loose in the area. The helicopter was assisting the ground team to push the cattle towards an open gate directly beneath powerlines. Witnesses on the ground observed the helicopter hovering and as it started to turn, the "tail" of the helicopter hit the powerlines. The helicopter descended to the ground and came to rest on its right side in a corn field. Several witnesses and the passenger stated that the helicopter looked and sounded normal prior to the impact. A video of the accident taken by another witness on the ground showed the helicopter maneuvering near the powerlines, with the nose of the helicopter facing away from the powerlines. The tail of the helicopter impacted the powerlines, followed by the main rotor blades. The helicopter rotated once before impacting the ground. According to a FAA inspector who responded to the accident, the tailboom separated from the fuselage and both main rotors were substantially damaged. An examination of the helicopter, engine, and remaining systems revealed no mechanical anomalies that would have precluded normal operations. An official weather observation taken 18 miles northwest of the accident site reported the weather as wind 060° at 7 knots, visibility 1.5 miles in light snow, overcast skies at 2,300 ft above ground level, temperature -1° C, dewpoint temperature -4°C, and an altimeter of 30.26 inches of mercury. The witnesses on the ground at the accident site characterized the weather as overcast, windy, unlimited visibility, and no precipitation. At the writing of this report, the pilot had no recollection of the accident or the events that led up to the accident.