Vitamin A Important for Growth in Young Horses

Vitamin A, one of only two vitamins not produced in the horse’s body, is best known for its role in vision but also has functions in reproduction, gene expression, differentiation of epithelial cells, embryogenesis, and growth.

Vitamin A is found in abundant quantities in fresh green forages in the form of carotenes, which are converted to vitamin A by enzymes in the intestine. Because this vitamin is fat-soluble, it requires some fat in the diet to be absorbed properly. The small amount of fat typically found in green grass is usually enough to aid in the absorption of this nutrient under normal conditions.

Once forage is cut, there is rapid oxidation of carotenes (up to 85% within the first 24 hours and then about 7% per month during storage), which results in hay being practically devoid of carotenes after extended storage. Horses on hay-only diets show depletion of vitamin A liver stores over a relatively short period of time. The inability of hay to supply adequate vitamin A to the horse is the rationale behind the inclusion of vitamin A in commercial horse feed mixes and supplements.

Vitamin A has a distinct role in growth of the horse. Deficiency adversely affects growth, body weight, rate of gain, and heart girth in young growing equines. Bone remodeling is modulated by vitamin A in the growing animal due to its role in the proper functioning of osteoclasts, the bone cells responsible for resorption of bone. Without sufficient vitamin A, excessive deposition of periosteal bone occurs. The appearance of bones in vitamin A deficiency is actually shorter and thicker than normal. Effects of these bone changes may result in mechanical pressure on certain nerves, such as the optic or auditory nerves, leading to blindness and deafness.

It is possible that some of the systemic effects of vitamin A on growth, as well as the poor growth usually associated with vitamin A deficiency, are related to its effects on growth hormone secretion. Vitamin A takes different functional forms once it is working in the body, one of which is retinoic acid, which has been found to affect growth hormone regulation. Retinoic acid can synergize with either thyroid hormone or glucocorticoids to enhance the activity of the growth hormone gene, and subsequently of growth hormone secretion from cells. Retinoic acid is also essential for embryonic development. Retinoic acid has been implicated in the expression of the genes that determine the sequential development of various parts of a developing fetus and in the development of the vertebrate limb.

For horses grazing sufficient quantities of green pastures, their vitamin A requirement can be met entirely by the carotenes in the forage. In the northern United States and countries, vitamin A supplementation is particularly important because of the short growing season of grasses. Carotenes have not been found to be toxic at any level in the horse because it appears that the enzyme that converts carotenes to vitamin A is found in limited amounts in the body and therefore excessive conversion is not possible.