The dogs CAN'T kill a cow. Get real.
Now, I'm just a dumb farmer from the back wash of pennsylvania, cows and all.
The dogs were in fact and indeed used to herd cattle, and hogs too.
They were easily trained, easy to work with, and could intimidate a 300 pound sow with just a look.
Using them correctly meant that you didn't let them run free.
Now, about rationalization, why don't you try to find a new one?
Dogs can't kill a cow? You are a dumb farmer then. There is a huge difference between a "butcher's" dog and a herd dog.