When I first had this thin skinned horse delivered, I started feeding him Triple Crown Senior mixed with Triple Crown 30% supplement (about 5 pounds per day of a 1-1 mixture). In early spring, I’ve moved him strictly to Triple Crown Lite and have been supplementing with Tri-Amino (inexpensive and contains the tall poles in the tent in terms of what’s needed for protein synthesis), SmartDark & Handsome (double doses; which has the biotin), SmartMuscle Stamina (which also has medium chain tryglycerides–coconut oil; and DMG, which seems to address other “phantom” issues). Early spring I also fed mega doses of Omega Horseshine for two months, and then switched to Rice Bran Oil (1/2 cup per day). (I not only had thin skin and ugly coat, but he struggled keeping muscle.) I also feed joint supplements that have collagen, which may be helping the thin skin issue, too.
I like Triple Crown Feeds, and have been feeding their products to my horses for 15 years. I feed the Senior to some of my horses, but my problem child gets the Lite, as it is more substantially supplemented, has reduced amounts of soy products (no soybean meal or soybean oil), and no molasses. This way I can improve the quality of fats. But if you’re feeding 5+ pounds of the Senior, then you’re probably giving sufficient vitamins, and maybe only need to try adding the Tri Amino.
If you’re feeding fewer than 5 pounds you might need more multi-vitamins (which I like the old version SmartVites), and adding the Tri Amino and the SmartMuscle Stamina with the biotin you’re already feeding should really help with the skin issue, but might also help with his training (and recovery).