That's my understanding too. If it's not marked open range, the rancher is required to keep his cattle contained and the drivers are entitled to assume stock won't be wandering on the road.
I have some land designated open range, and I still have a maintained fence near the state highway just for common sense safety to both cattle and cars, but there's no fence on the backside, and it's definitely drive at your own risk on those county roads. Of course, the county roads out this way see maybe one or two old farm trucks a day max, so it's not an issue.