I have had good success with adding Chia seeds and coolcalories as well as good quality hay–grass and alfalfa mix. The chia seeds are a good source of omega 3s, protein and can help with digestion, the cool calories is a really good fat source and is easy to add and most horses really like it. I would also wet her food so it is easier to eat. The good quality hay with alfalfa provides protein and is easier to digest. I have found that with my horse the ribby hay belly thing comes from a lower quality hay. The protein from the chia and alfalfa along with the fat helps build a topline, one without the other doesn’t work as well-at least for my horse.