I am on a survival island, so the cows really don't have much to roam. I would keep them in a fenced area, but 'tis not needed. I'm just curious as to whether or not these cows will despawn? I hope not because I need 'em for food.
Cows will not despawn, like all other passive mobs. If you get to far away and unload the land, they will be rendered as not being there, put when you come back, they will still be there. So for your question, your cows are safe.
On vanilla they don't. I'm using Mo' Creatures and all my livestock always despawn. I wish there was a way to prevent them from doing that. I even selected the thing that keeps vanilla mobs from despawning with no luck.