No, the rancher is not. That's what those yellow diamond "entering open range" signs mean. In fact, you hit a head of cattle in open range, you're paying for the ranchers loss.
As an aside, how is a cow supposed to "hit my car" in the first place? They might exceed 20 mph if spooked, otherwise it's like saying that a pedestrian hit your car. The fact that said pedestrian weighs upward of 1200 lbs. does not absolve you, pilot of the 70 mph 4000lb. box of steel.
I watched a bison (juvenile) hit a parked car. On a twisty road we lost a motorcycle driver to a bison that had got out. A bull will attack a car. Cows can panic too.