Are you sure about her having hay24/7?
I got a mare 300lbs underweight, put her in free choice alfalfa/ oathay, and 3lbs daily of the above mentioned Nutrena Life Design Senior and had her almost where I wanted her within a month. Check out what senior feed your barn offers.
The strategy is what my Snipper eats now, not even two pounds, plus some alfalfa pellets, soaked, and free choice mixed hay, mainly oat and 3-way, some orchard/timothy.
Should she ever lose again I'd put her right back on the senior, maybe together with a little ricebran.