Horses need many vitamins
From time to time, discussions arise about vitamins in horse feeding—their amounts, quality, and appropriate intake. Horses obtain vitamins from their natural diet, but vitamins are also present in complete and complementary feeds, mineral supplements, vitamin supplements, trace element feeds, protein feeds, energy boosters, salt licks, and many other products. In addition, berries, root vegetables, seeds, and herbs fed to horses contain vitamins. Errors in vitamin feeding often result either from excessive use of vitamin supplements or from poor-quality feed. In feeding, particular attention should be paid to the risk of overdosing fat-soluble vitamins when synthetic products are used. Why this is the case will be explained in more detail below.
A horse needs many different nutrients for body structure, vital functions, and movement. Vitamins are one of these nutrients, and each vitamin has important roles in the horse’s body.
Some vitamins may be present in feed in an inactive form and only become active in the body. In plants, the horse’s natural feed, vitamins always occur as precursors. These precursors are converted into vitamins in the horse’s body only if there is a deficiency. Synthetic vitamins, by contrast, are always fully absorbed in the digestive tract, unlike precursors, of which only the required amount is converted into active vitamins.
Vitamin absorption and body stores
Vitamins are divided into water-soluble and fat-soluble vitamins. Fat-soluble vitamins include vitamins A, D, E, and K. Of these, excessive intake of vitamins A and D most commonly causes problems, as they accumulate in the body. In Finland, horses are most often deficient in fat-soluble vitamins, especially vitamin D, which is extremely important for foal bone development. Vitamin E, although fat-soluble, does not cause clear toxicity symptoms even at high doses. When needed, absorption of fat-soluble vitamins can be enhanced by feeding vegetable oils alongside them.
Water-soluble vitamins include B vitamins and vitamin C. These do not accumulate significantly in the body and therefore must be supplied daily. Horses also produce these vitamins themselves via gut microbes. Excess intake of water-soluble vitamins, even from synthetic feeds, usually does not cause adverse effects, as they are not stored in large amounts like fat-soluble vitamins.
Different vitamins behave differently in the body. The behaviour of water-soluble vitamin C is particularly well known. As intake exceeds requirements, absorption from the intestine decreases. For example, about 100 mg of dietary vitamin C is almost fully absorbed, but only half of a tenfold amount is absorbed, and only 10–20% of a hundredfold dose.
For excessive vitamin intake to become a true health risk, amounts must greatly exceed recommended levels. Much more commonly, a vitamin deficiency occurs. Nevertheless, it is safest to supply all vitamins only in moderate amounts corresponding to recommendations.
Natural horse feed contains vitamin precursors
The horse’s most natural feed, fresh grass, contains abundant precursors of vitamins A, D, E, and K. The precursor of vitamin A, for example, is beta-carotene. The efficiency of conversion into active vitamins is linked to the body’s existing vitamin stores. If fat-soluble vitamin stores are already high, fewer vitamins are produced from precursors, and excess precursors are excreted in urine. Therefore, high intake of precursors is not harmful. Overdosing fat-soluble vitamins is mainly associated with the use of industrial vitamin supplements. For example, excessive vitamin A intake is possible when using special vitamin products, not from natural feed alone. Likewise, vitamin D overdose does not occur from feed but from supplements.
Pay special attention to vitamins during the indoor feeding season
During the indoor feeding season and for non-grazing horses, vitamin intake requires closer attention. Supplementation is usually necessary for heavily worked horses, broodmares, and foals. Hard work depletes vitamin stores faster than they can be replenished from basic feeds. Digestive disorders, antibiotic treatments, stress, and high-concentrate diets also increase vitamin requirements. As a rule of thumb, horses grazing at least 2–3 hours per day have the lowest need for vitamin supplements, while competition horses, broodmares, and growing foals during the indoor season have the highest needs.
For pregnant mares, adequate intake of vitamins A, D, and E is important, as well as B vitamins. For competition horses, vitamin E is particularly essential for muscle function, energy production, and oxygen utilisation. It protects the horse’s body from exercise-related stress, promotes recovery, and strengthens immunity. Competition horses experience many stressful situations that can impair the utilisation of water-soluble vitamins. Horses require B vitamins for energy production, nervous system function, metabolism, and muscle maintenance. Green plants are rich in folate, a B vitamin known as folic acid in industrial supplements. Folate is needed for blood cell formation and cell division and is especially important for pregnant animals.
Vitamin C promotes iron absorption from feed and has been shown to improve competitive performance. Horses also need vitamin C to maintain immune defence and as a building block for cells. Vitamin C is poorly absorbed from the large intestine, making endogenous production an important source. In adult horses, vitamin C deficiency is most often seen when gut function is impaired or the horse is stressed. Rose hip is an excellent vitamin C supplement for horses. Vitamin C from rose hip has been shown to be better utilised than synthetic ascorbic acid. Studies have demonstrated that feeding rose hip has anti-inflammatory and antioxidant effects in horses, likely due to its vitamin C content. Research has also shown that rose hip improves performance and recovery (Winther, Kharazmi, Hansen, Falk-Ronne 2010). In ageing horses, dental, intestinal, or liver function may be compromised, making supplementation of B and C vitamins necessary.
Embrace diversity
When a horse’s diet includes a variety of vegetables, root crops, berries, herbs, grains, and seeds, a broad intake of vitamins and other nutrients is ensured. Plant pigments, although not vitamins, also benefit health by acting as protective antioxidants. In addition to vitamins, plant-based foods contain many other health-promoting compounds that are not yet fully understood. These include carotenoids, polyphenols, flavonoids, and phenolic acids, which may have antioxidant or hormone-like effects.
A synthetic product containing all vitamins and essential minerals is always fundamentally poorer in composition than a diverse natural diet. Excess vitamin intake is possible only through excessive use of synthetic vitamin supplements, never through natural feed.
