From the FWIW Department:

You couldn't have a bellwether (or its bovine equivalent) for cows, because the cow leader does not lead in a progression of the herd. Cows divide into roughly three strata, with A cows having the highest status, B cows the next, and C cows the last. In a procession of cows, the B cows go first, followed by the A cows, and then the C cows, so that the most important and hence valued cows are protected in the center of the procession.

Whether the cows would attempt to form a circle around Chief A cow as she was dragged off by the butcher, I dunno. [wandering-off-to-look-up-Temple-Grandin emoticon] Then again, I dunno why anybody would be slaughtering cows anyway. [realizing-I-shouldda-quit-before-I-started emoticon]