Since your vet is coming for Fall shots why not ask what he/she suggests?
My WB was a little ribby coming out of Winter and my vet suggested adding alfalfa hay.
But he is just 17 so I took a Wait & See break and the good Spring grass filled him out just fine.
For your Old Guy maybe make a mush of soaked alfalfa cubes as long as the added calcium & protein won't be a problem.
My feedstore is selling a haylage mixture now - baled but chopped - not sure what all is in it as I have no need, but maybe something for you to look into.
canesisters:
I had a Poison Detector TB too, but your mare takes it to a whole new level!
Hope you find the Magic Potion to get her packing on the pounds
I like my horses a tad on the Chubular side going into Winter.