I would have the vet take a second look at her teeth. Senior horses often need more frequent dental care. If she has any problems with her teeth, that would cause her to not chew well, then she'll need to be on soaked feed and soaked hay cube or pellets to make sure she's digesting all of the food properly.
If her teeth are in good condition, then I would have her on free choice quality grass hay or timothy hay, as much as she can eat. For her "feed", I would feed her:
- 2-3 scoops of Alfalfa pellets (using a 3qt feed scoop), with a little water mixed in to soften them up (you only need 1-2 cups of water for this much pellets), split into two feedings if possible
- 1/2 to 1 cup of Rice Bran Oil, or 1-2 cups of stabilized race bran meal.
- 2 scoops a day of Platinum Performance Equine
You won't need any other feed or supplement with the above diet. The only extra thing I might add are some probiotics like Probios powder or Fastrack, to help make sure the horse is digesting all of its food and absorbing the nutrients properly.