Is an alfalfa-only diet best for my emaciated senior horse, or grain?

First, be sure to have Boc's teeth checked. At 27 the teeth are probably not in the best condition and you need to then feed accordingly. Horses this age often tend to choke because they cannot chew properly.

I personally would feed an Equine Senior type diet mash and no hay. Only “complete” pelleted feeds which are designed to be fed without hay should be used since many pelleted feeds are only grain substitutes and do not contain the proper mineral balance to be used as the major or sole source of nutrition for the horse. Soak the food for one hour prior to feeding. You could also used soaked beet pulp which is highly digestible and easy to chew once soaked. If Boc's teeth are OK then feeding hay is fine but I would still soak it first.

If she is emaciated and you cannot get her to eat enough calories in order to gain weight then I suggest you add 1/4 to 1/2 cup of oil (corn or vegetable) to the soaked grain. This adds a lot of calories and they like it!

I hope this helps. If I did not answer you question you are welcome to post again or you an always opt to do a private consult where we can chat back and forth and you can ask as many questions as you'd like. Best wishes!