Poor quality of hair and hoof also is a sign of a protein deficiency. The amino acids link together to form protein. The horse's diet must contain essential amino acids. Those amino acids that contribute to the hair coat include lysine, methionine, and cystine. Methionine, a sulfur-containing amino acid, is found in the hoof naturally.
Fats help the absorption of fat-soluble vitamins, such as vitamin A and B vitamins.
Vitamin A is needed for epithelial cells. A deficiency may be the cause of a dry, lusterless coat. But when you bale hay and put it in the hay mow, the half-life is 60 days. When hay is six months old, all the vitamin A has been oxidized. The horse can store vitamin A in the liver to last a minimum of six months.