I just rescued a very underweight 8 yr old Thoroughbred and asked my Vet what I should feed her to put on weight. He suggested Senior XL. I give her that and let her graze and also have a round bail of hay. I feed her a small amount in the morning before work and some more at night when I get home (i work 12 hour shifts). I did this with another horse I rescued and he put his weight back on within 6-8 months. He's 20 yrs old now and looks beautiful. Just a suggestion.