Fat soluble Vitamins are those vitamins that are stored to some extent in the horse. In general they are Vitamins A, E, D and K. Vitamin A is important for vision, reproduction, growth, skin and intestinal health. There is some level of Vitamin A in freshly cut hay but it deteriorates very rapidly. You can see by the body functions that are controlled by Vitamin A that it is very important and needs to be supplemented daily.