A deficiency sounds likely, vitamins A & E almost definitely, possibly a lack of quality protein as well. Look into getting a membership with FeedXL.com- it will help you determine how much of each vitamin/mineral your horse needs, and what is being fed in your current diet. Then you can try changing the diet around to better fill the nutritional gaps.
Just as an example, 1-1/2 quarts of pelleted feed is probably around 2 lbs. Decade Complete Diet 13 only has 3600 IU of vitamin A per pound, so ~7200 IU of vitamin A daily. For my horse (1000 lb horse in light/moderate work), FeedXL recommended >30,000 IU vitamin A daily, so 7200 IU just wouldn't cut it.
You might try looking into one of the various ration balancer products- most feed companies make at least one. They're nutrient dense, high in quality protein, and low calorie and designed to be fed at a rate of 1-2 lbs per day. Purina Enrich Plus, Nutrena Empower Balance, and Triple Crown 30% Supplement are some of the nationally distributed ones. From their website, it looks like EQUI-PRO MVP Supplement Pellet might be Poulin's ration balancer (same company that makes Decade Complete Diet 13)