My horse gets four flakes of coastal a day, and about 2-3 lbs (estimate, I use a scoop and a quarter instead of weighing) of grain a day, but it isn't a "fortified" grain. It has rolled oats, barley, corn, flax seed I think, and vegetable oil. The woman I purchased my horse from had him and all of her horses on this grain (I buy it from her), and I decided to keep him on this diet because there have been no complications with it. He also has a salt lick and electrolytes in his feed.
Anyway, he keeps weight on and his coat is extremely healthy looking, and he is very energetic. So I was wondering 1) if he needs a multivitamin, and 2) if it would be alright to put him on a muscle builder. He shed his extra fat pretty easily, but he hasn't muscled out very noticeably and I don't know if it's due to a lack of vitamins or if he needs a little boost.