God Bless you for taking her in. Looks like you came in just in time. I will tell you my favorite grain is any type of Ration Balancer. My Equine vet raves about it. We took in an underweight mare and she has put on about 100lbs in two months. our regimen is:
3lbs Ration Balancer twice daily
1/2 lb rice bran daily
timothy mix , 3 flakes a day plus turn out.
6 cups beat pulp
oil dressing - ricebran, wheat germ and cod liver oil
It worked for us and granted we built up to it. But this was a diet put together with a vet. AFTER she was tested for everything under the sun.
Good Luck and keep us posted!