I would give the mare alot more hay then normal (and if she has free for all hay then higher quality is better), preferably alfalfa/mix, if she has teeth proplems there is alfalfa pellets that can be soaked and givin to the mare, hay helps keep a horse warm during the colder months, and should help better with the digestive system.. If the mare is fed with other horses, the horses can steal her food too, but I doubt she is with other horses correct? Make sure the mare is dewormed too. I don't have any suggestions for grain though, but defintly still feed it to the mare.
Make sure the mare is not randomly givin a ton of grain though, slowly increase the grain untill the amount of grain suggested.
Am no expert, but these are simply my suggestions.
Mare may need to visit a vet and dentist just in case. ;)