"My mother had two pet cows--now she just has one. Since they are herd animals, is it okay to have just one cow?"
It's okay, but it's not the best situation to put a cow in. I'm glad that it was acknowledged that cows are a herd animal, because that gives the answer right there, right in the question itself.
Cows--cattle, bovines, same deal--are social animals. They thrive best in a social environment, or rather when they have one or more companions as constant company. Preferably of their own kind, but they can do all right if they have sheep or goats as companions.
I've heard of single cows doing fine when they're by themselves, but that's from the outside looking in, from our perspective. See, a cow will certainly be fine physically, but we don't know what's going on inside her head, nor how much she actually misses her herd mate[s]. Mentally, and perhaps emotionally, a cow may be depressed or more stressed because she doesn't feel safe by herself. In this case, she may be pining the loss of her best friend, but in typical bovine-fashion, doesn't show it so readily as maybe a cat or dog (or even human) may do.
That said, I think this person's mother should look to getting a new friend for this cow. The cow will have to adjust, of course, but she'll be happier for it and will feel so much content and secure with her new pal.
The Bovine Practicum Q&A "blog" is an informative, just-for-fun section where I find a variety of questions that are often (and not-so-often) asked by inquisitive people like you, and answer them to the best of my ability. Much of the questions have come from a wide variety of reaches from the Internet.
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