Is it true that large pastures do not benefit from the 2nd herdsman?

I've seen no great evidence that multiple herdsmen help enough to offset the loss of a laborer. If you feel strongly about employing a 2nd herdsman, they'll serve you better with their own pasture.

• Slaughter: If your pasture has more than the target number of animals and your town is under quota for food, an animal will be slaughtered. While I can see that multiple herdsmen will speed this process in the short term, I don't see a long term advantage to slaughtering your animals quickly. In fact, given that my town usually has more than enough meat and the animals will last through the winter better than a bean plant, I'd want more hustle out of my farmers than my herdsmen.
• Clean up: Collecting the meat of the slaughtered and any by-products are labor-queue tasks. As such they will be performed by full time laborers or any other near by citizen (likely a herdsman) that enters labor-queue mode. I try to keep at least 10% of my population as general labor. I'd rather have a Laborer than a 2nd herdsman.
• Neglect: Having a single active herdsman will prevent animals from dying of neglect (or is that actually old age?). It is possible that disease or remarkably distant housing could keep your single herdsman off the job long enough to be a problem. Housing is an easy problem to fix, and I'll take my chances that I might loose an animal or two on those very rare occasions my herdsman is sick for an extended period.
• Growth hormones?!?: I've seen no evidence that animals grow faster when coddled.