In some states, you are supposed to put a fence around your cows.
In other states, you are supposed to put a fence around your house. (the cows were there first)

Fencing is expensive but worth it.
Some three board and wire fencing around what you really want to keep is good.

Barbed wire is mostly just to mark the edge of your area.
Cows will not cross it only because it is convenient not to.

I didn't see any markings or tags on those cows. It may be free beef on the hoof.