My ottb lost weight due to a front tendon injury and can't keep her weight on through the winter. ( would also work in the summer ) I have her on two feeds that I mix, they're not too too expensive but very effective. Ottbs can get hot with some foods, so you also wouldn't have to worry about this with this food. One is just a simple purima pellet feed, don't have the bag, but it's just a simple pellet. But the food that works extrenly well it's purina equilibrium trimax, it is sticky due to molasses that's in it, and they love it, fattens them right up and you don't need to worry about a hot horse
also, you can switch out regular bran to rice bran or add rice bran to feeding. Most horses tolerate it well and it adds a lot of calories to a horse's diet. SOME horses DO become "Hotter" while consuming rice bran, other people will swear by it and claim they've never heard such a thing, typically if your horse is a "hot breed" (arabians are hot, warmbloods are not.) then you may be more likely to "See" an increase in energy level resulting in a fresh horse. Maybe your horse wont have any effect except for gaining calories.
Additionally, you can add oil to the feed. Typically about one fluid ounce is a good starting measurement (about a cup). this will add calories, make the horse's coat nice, and *not* add "energy" to your horse.
"Cool Calories 100"
start by feeding one scoop a day for a week, increasing by one scoop each week until you are at 4. Maintain four scoops over the next four weeks and observe weight gain. If gaining weight at a reasonable weight, maintain, if not gaining, increase from four scoops to six...repeat process.
Cool Calories adds "food calories" without giving the horse "energy" to burn...i.e. horse won't act hyper from increased caloric intake.