Even with a full fence, Joey Porter's pit bull and mastiff got out and killed a miniature horse on a nearby property.  Even the animal control officers weren't sure how the dogs got loose.

I can almost bet that Mr. Porter does not have electric fencing along with his regular fencing.  When keeping large dogs, even ones you consider tame, near livestock it is always prudent to have electric fencing.  This is a lesson I learned when our danes killed or maimed three large heifers we had here, on this place.

Now my husband won't allow me to have danes.  I love danes, they are total doofuses.  But before that, they were "boar hounds", with all that implies.

Every dog fence since then has incorporated electric fencing.  It keeps the livestock safe from our neighbors' animals (which have ripped the heads off of goats just for fun) and safe from our own dogs. 

Dogs are predators.  Always will be; people aught to respect them for that.