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Vitamins are organic compounds that are essential for all living things. Vitamins have diverse roles within the horse’s body, affecting energy metabolism, growth and repair, muscle function, neurological health, immune function and much more.
Vitamins function as cofactors for metabolic reactions, including the breakdown of sugars and fats for energy, hormone synthesis, nerve impulse transmission, muscle contraction and more.
Your horse’s current vitamin requirements depend on age, exercise level, reproductive status (pregnant and lactating) and health status.
Whether your horse is meeting their requirement for a given vitamin depends primarily on what is in their feeding program, but can also be affected by gut health and endogenous (internal) production.
Other factors such as genetics (breed), nutrient interactions and health status can also play a role in how well the body absorbs and uses vitamins.
Forages can vary greatly in vitamin levels depending on the type of forage and when/how it is harvested and stored. As an example, fresh pasture is generally a good source of Vitamin E, but this vitamin degrades quickly after hay and grasses are cut.
Most horses should receive a vitamin and mineral premix to prevent deficiencies and ensure optimal health. Hoof quality, skin and coat condition, energy levels, mood and resistance to illness are all improved when vitamin and mineral requirements are met.
To find out whether your horse is meeting their vitamin requirements, submit your horse’s diet for analysis by our equine nutritionists.
Vitamins for Horses
The vitamin requirements for horses have been established by the National Research Council through an extensive review of scientific literature. These requirements are published in the Nutrient Requirements of Horses, last updated in 2007. [1]
The vitamins that you horse needs in their diet can be divided into two categories:
Fat-soluble vitamins:
The fat-soluble vitamins include:
- Vitamin A
- Vitamin D
- Vitamin E
- Vitamin K
These vitamins form solutions when mixed with fats (lipids) and are absorbed and transported in the body similar to fats.
In general, the requirements for these vitamins have been more carefully researched than the water-soluble vitamins.
Fat-soluble vitamins are stored in the body which can result in toxicity issues with excess consumption.
Water-soluble vitamins:
The water-soluble vitamins include:
- Vitamin B1 (Thiamin)
- Vitamin B2 (Riboflavin)
- Vitamin B3 (Niacin)
- Vitamin B5 (Pantothenic acid)
- Vitamin B6 (Pyridoxine)
- Vitamin B7 (Biotin)
- Vitamin B9 (Folate)
- Vitamin B12 (Cobalamin)
- Vitamin C
Vitamin C is made in the liver and typically does not need to be supplied in the horse’s diet to avoid deficiency. However, there may be cases in which supplementation provides an added benefit.
The horse’s hindgut produces significant levels of B-vitamins that the horse can absorb. Requirements for these vitamins are typically met by fibre-digesting microbes.
However, if hindgut function is impaired or nutrient absorption is poor, supplementation may be necessary. Furthermore, supplementation beyond the minimum requirement may support optimal health, such as feeding 20 mg per day of biotin to promote hoof health.
Water-soluble vitamins are not stored in the body and excess amounts are excreted in urine.
Fat Soluble Vitamins
Vitamin A
Vitamin A is most well-known for its role in night vision and eye health. It is also important for reproductive health in mares and for maintaining immune function.
Vitamin A is made from the precursor beta-carotene which is found in high levels in fresh forage and converted to vitamin A (retinol) in the intestine. Vitamin A is stored in the liver and can be used to meet the needs of the horse when beta-carotene intake is low.
Horses grazing on green pasture are likely meeting their vitamin A requirement.
Levels of beta-carotene quickly decline in stored hay. Therefore, vitamin A deficiency is more likely in the wintertime when access to fresh pasture is limited. [2]
Roles:
- Acts as an antioxidant to neutralize free radicals before they can damage cells
- Helps cells of the immune system act more efficiently to protect against infection
- Supports reproductive health in mares and reduces embryonic mortality
Requirements:[1]
Beta-carotene requirements for mature horses at maintenance are based on intake levels required to prevent night blindness. This is the main sign of vitamin A deficiency.
There is also an additional allowance to maximize tissue storage based on research in other animals.
Maintenance: 30 IU vitamin A / kg bodyweight (BW) (15,000 IU for a 500 kg mature horse)
Growth: 45 IU vitamin A / kg BW
Pregnancy and Lactation: 60 IU vitamin A / kg BW (30,000 IU for a 500 kg mature horse).
Some research suggests supplementation with vitamin A and vitamin E can improve reproductive status in mares including more serviced heats and a greater number of live foals.
Work: 45 IU vitamin A / kg BW (22,500 IU for a 500 kg mature horse).
Vitamin A requirements for working horses are not well defined. The requirement is based on assumptions that it would be between maintenance and gestation/lactation.
Sources:
Fresh, green pasture is the best source of beta-carotene, which is converted to vitamin A in the gut.
Supplementation in the form of retinyl-palmitate might be warranted if there is minimal access to fresh pasture and/or if there is greater demand (growth or pregnancy/lactation).
Deficiency Symptoms:
Night blindness is the classical vitamin A deficiency symptom found in most species, including horses. This requires a chronic intake of very low beta-carotene diets.
Less severe deficiency can impair growth and hematopoiesis (formation of blood cells).
Respiratory infections in weanlings have been associated with low vitamin A status. [3]
Vitamin A deficiency has been linked to reproductive failure in several species but requires more research in horses.
Toxicity Symptoms:
Vitamin A toxicity can result in fragile bones, hyperostosis (abnormal calcification of soft tissues), poor skin condition, and malformation of the growing fetus. It might also be involved in developmental orthopedic disease in growing horses. [1]
Upper Tolerable Limit:
16,000 IU/kg of dry matter intake, or approximately 160,000 IU per day for a 500 kg horse. [1]
Vitamin D
Vitamin D exists in two forms for horses: vitamin D2 (ergocalciferol) is produced by fungi that grow on plant material or vitamin D3 (cholecalciferol) that is supplemented in the diet or synthesized in skin.
Vitamin D3 is made in skin when it is exposed to sunlight (ultraviolet light B) which removes 7-dehyrocholestrol from cell membranes in skin. This is converted to vitamin D3 in the liver and further converted to its active form, calcitriol, in the kidney.
Although there are seasonal fluctuations in vitamin D levels, wearing blankets does not appear to significantly affect vitamin D levels in horses. [4]
For most mammals and birds, vitamin D is important for maintaining calcium and phosphorus balance in the body. It acts on the intestine to increase calcium absorption, on the kidney to support calcium resorption, and at the bone to influence calcium and phosphorus mobilization.
In comparison to other animals, horses have much lower levels of vitamin D and its metabolites in blood and higher circulating levels of calcium. [4]
Horses do not appear to rely on vitamin D for calcium absorption in the gut, unlike other animals. They also secrete more calcium in urine than other species.
More research is needed to understand what role, if any, vitamin D plays in maintaining calcium homeostasis in horses. [5]
Roles:
The role of vitamin D in horses is not well-known. It appears to be less impor