Did you read pp #1 in my link?
Are you from an open range state or really familiar with open range laws?
Yes. More importantly I read pp #2. The Nevada law defines open range as unenclosed land in 563.355. If a highway right of way is fenced then it can't be considered open range because, well, it isn't open or unenclosed. So the interstate is not considered open range. The question then becomes did Bundy negligently allow his cow into the right-of-way and I assume that would depend on the circumstances. If the state fence was not maintained and had gaps in it that cows could wander through then the negligence would be on the part of the state and not Bundy. But if the fence was in good repair and somehow Bundy's cattle got past it then the liability would be with him.