My thoughts are first, what is/was causing the mare's underweight condition. If she's simply not been receiving adequate feed for whatever reason, or whether she is an aged mare receiving/not receiving adequate feed, with dental issues. Age of the foal another question, since if the foal's under 6 months what you'll be feeding the mare will be shared with the foal.
The next question I have is how underweight she is; if her bcs is below a 3, I would be cautious with putting her on afalfa except in small amounts which you can increase gradually over a period of weeks. Sometimes too much of a good thing isn't a good thing.
Ideally I think it is safer overall to put an underweight horse on good quality grass which isn't too high in protein, and again, depending upon bcs and other health factors it is always an individual situation.
If she's extremely underweight and considered by you to be a neglect case, my own rule of thumb is to keep the refeeding diet as simple as possible, and not to deworm or vaccinate until the horse is stronger and healthier. Deworming for a very underweight horse is something I think is best for a vet to consult with, esp. with a nursing mare, because a heavy wormload will need to be addressed differently.
A horse will gain 1 bcs point a month with the proper diet, and you'll start seeing progress usually within a week or two :) If the foal is still very young, and you're giving supps to the mare, whatever you're giving may not be appropriate for baby which is something to consider.
Another thing coming to mind; if the mare is aggressive at all with her feed because she's needy for the nourishment, baby might be injured accidentally, so beginning with a simpler diet may be safer starting out.
If you think she and baby are ok to have some mare & foal or senior feed at some point, if the mare is aged the feed may need to be soaked into a mash if mama has any issues with dentition.
As for supplements, my own rule of thumb is to keep it simple initially, and just be patient. If you think she's not so emaciated that she can have free choice grass hay without risking colic or other malfunction, and then some alfalfa and gradual supps, ?
If she's been neglected, she may not have had access to any salt/trace minerals too. Some thoughts I'm throwing out there to you, I hope something helps.
Last edited by DRichmond; 04-30-2012 at 04:01 PM.