What else can i do to make her gain Weight?

my miniature horse is about 20 years old. she has heaves. but she is alot better now that she is on medicine. but she cant have hay and she wont eat her dengy. Her weight has started to go down but she eats gr but with winter coming up im not sure what to do. i don't know if she will eat her dengy or not. she is on about two cups of grain. 1 1/2 cups safe choice and 1/2 cup of Triple crown senior. i don't know if i should up her grain, she is on a steroid i don't want her to colic or to founder. what can i do to?