Mastiff dogs are the real culprits in more cases that people would expect. Had been videotaped doing exactly this. A wolf pack will kill the prey, two mastiffs just bite, sit and wait watching the cow die. They can't kill the cow efficiently.

They will respect animals in their "family" herd, but don't have this restriction against cattle from other herds or species that are not defending, specially when not feed accurately by the owners (If they guard sheep they can still see young cows as fair game).

The main difference between wolves and shepherd dogs is that dogs aren't neither afraid to cows or men.