I feed blue seal sentinel feeds as they are so easily digestible. I like purina feeds but my 4 year old TB choked on the pellets and horses can’t choke on the extruded pellets of sentinel feeds. you’ll need something with a little more protein as your are trying to build muscle. A horse in our barn broke his jaw last year and literally lost so much weight he was skin and bones. we put him on sentinel safe start. and he did great. he his now back at work. I also have had good results from Gleam and Gain. My TB was kind of scrawny when I got her and Gleam and Gain helped get weight on her without making her hot. also make sure you have her teeth checked.