She should get 1.5 to 2% of her body weight in feed per day.Mostly Hay...
Most people feed in at least to feedings. So if she is 1000lbs she needs to be eating 15 to 20 lbs of food per day.
If she is old, try adding a senior ration. You can wet that to, if she has trouble chewing.
Sorry edit: 15 to 20 pounds if she is 1000 pounds and up it if need be.