Pass mare at rescue

Jonni Jump

I work for a rescue trimming horses that come in from all over Ga.
This new mare that was given to them has a layed over cresty next and fat pads on her butt. But is also severely emaciated. 
New owner says she will not eat anything but Sr. Feed. 
I told her to slowly mix in a ration balancer or hay pellets and taper off Sr. Feed. 
What can she feed her to gain weight?
Jonni Jump 
Augusta Ga.
Mirage i.r. friesian/ssh mare 

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