Well, I discovered it's a bad idea to try to keep more than two brahmin.
I had the idea to have a wee herd, and extended just enough of the wall at County Crossing to make room, complete with troughs. Created five brahmin using Northland Diggers (six total, one was a 'natural'), and then the ridiculous behavior commenced! Suckers would NOT stay in their area, even though it was walled and gated. Aaaaand, the noise...
Tried resetting their AI, no dice. Too bad there is no herd dog, lol. I finally gave up, but didn't want to be empty handed, wanted the meat and milk, etc, so I shot all but two. Well, that made me reload, 'cause the settlers didn't like that, even though they were created and not 'natural'.
So, scrapped them, keeping two. That seemed to work out fine, as I was pretty sure it would, I have two that actually wandered in to Starlight, they're penned in and happy, as are the two now at CC. Oh brother, what a pita. So, if you haven't already tried this, don't!