Agreed.. They do tend to have wider ribs. I have a Paint Arab- basically the same as yours! I feed her 1 1/4 lb of pelleted rice bran in summer when worked hard as well as a joint supplement called Flex Wafers, and she also has 24/7 pasture and she gets worked hard in summer When we take her places she is feed 24/7 hay in a small mesh hay net- mainly grass and about 1/2 flake of alfalfa or a grass/alfalfa mix with 10-25% alfalfa. In winter she gets 1/2 lb of shredded beet pulp, 1 lb of pelleted rice bran, and a flex wafer, with 24/7 pasture but when it starts raining we start feeding a flake of grass/alalfa mix (15-25% alfalfa). When she starts tearing up pastures (due to them being wet and her running in them) we start putting her in at night and feeding two flakes in a slow feeder net.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown