My 35 year old mare stopped eating hay about 2 years ago.... it is out there for the other horses she just can't chew it anymore so she chooses not to eat it. What we have her on is Purina Senior feed and pelleted hay. We tried the cubes hay but even with it soaked she had a hard time chewing it.
Not sure if you have floated your horses teeth yet.they can get a long one and it hurts them to eat. And grain falls out of there mouth when they eat it..have your vet check her teeth if you havent already.