Marin County ranchers are complaining that low-level Army helicopter training flights are stampeding their animals. Two ranchers said animals have died and cows have aborted calves because the noisy UH-1 copters speed at 80 m.p.h. five feet off the ground at night. "We're getting a lot of complaints," said Maj. Pat Donaghue, commander of the Army Reserve's 343rd Medical Detachment at Hamilton Air Force Base. About two low-level missions are staged weekly for training helicopter pilots and crews to use sophisticated night-vision goggles. The pilots try their best to cooperate with the ranchers by avoiding herds when possible, but there is no plan to change the training sites, Donaghue said.