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Vitamins and how horses get them.

Vitamins are divided into two groups:
• Those that are fat soluble, A, D, E and K;
• And those that are water soluble, the B group of vitamins and Vitamin C.

The water soluble vitamins are not stored so a constant supply is needed, whereas the fat soluble vitamins can be stored in fatty tissues, and will be released back into the system when the fat is used. Nature has things worked out! These fat soluble vitamins are mainly available in larger amounts during the summer, when horses gain weight, thus enabling storage to take place. Over winter, as the fat stores are used up, so these vitamins become available once more.

Vitamin A is important for vision and in the immune systems. A deficiency will cause night blindness, susceptibility to infection and may affect fertility. Over doses of Vitamin A cause fragile bones and developmental bone disease, but there is no upper safe limit to carotene. Herbivores such as horses do not need to take in Vitamin A, as they make it themselves from carotene on the lining of the small intestine. When artificially supplemented as Vitamin A, as opposed to carotene, the vitamin is sourced from animal sources such as fish oil.

The actual requirements of horses for Vitamin A have not been clearly defined. In fact, if the horse has sufficient carotene, it does not need any Vitamin A. It is assumed that the minimum requirement is 30 IU per kg of bodyweight, so a 500 kg horse will need 15,000 – 30,000 IU per day. 1 mg of carotene can yield 400 IU of Vitamin A. Forages contain 30 – 385 mg of carotene per kg of dry matter.(Equivalent to 12,000 – 154,000 IU of Vitamin A) Mature grass hay will be at the lower end of the range and actively growing pasture, at the top end of the range. Using the lowest figure, 1kg will yield 30 mg of carotene which in turn could make 12,000 IU of Vitamin A. So long as a horse has some sort of forage it is unlikely that it will be Vitamin A deficient. Cereals and straws are poor sources of carotene, with only 2-4 IU of Vitamin activity per kg. 1kg of, for instance, flash dried lucerne, that might have 200 mg of carotene, will yield 80,000 IU of Vitamin A.

This explains why feed manufacturers feel compelled to add Vitamin A to compound feeds and why it is totally unnecessary when green forages form even a tiny amount of the diet. The only horses likely to need Vitamin A will be those with no access to grazing, forage provided as straw and supplemented with straight grains, bran etc.

Vitamin A is highly toxic, causes fragile bones, and orthopaedic disease in young stock. Toxicity from carotene has not been reported.

Vitamin B is actually a whole range of related micro nutrients. Deficiencies are rare as the B Vitamins are made by the micro organisms of the hind gut. A forage based diet free from preservatives (which are anti microbial) will ensure adequate levels. Foals do not have an effective gut flora until they are 7.5 months so weaning earlier than this may induce deficiencies, first expressed as depressed appetite. Anxiety and any other cause of scouring will deplete B vitamins.
Thiamin (Vit B1) is involved in metabolism. Brewers yeast is an excellent source (95.2 mg/kg). Eating bracken induces a deficiency but horses on normal diets do not require additional thiamine.
Riboflavin (Vit B2) is found in lucerne as well as being made in the gut. Deficiency in horse has not been seen.
Niacin (Vit B3) has roles in metabolism, processing DNA and mobilising calcium. It covers nicotinic acid and nicotinamide. It is present in cereals but is unavailable. It is in lucerne, grass and is also produced by microbes. Deficiency has not been seen in horses.
Biotin (Vit B2 complex; was Vit H) is essential for cell proliferation. Lucerne is a good source, as well as microbial fermentation. It has been thought to be essential for good hooves, but evidence for the benefits of supplementation are varied and inconclusive; poor hoof quality may be due to insufficient protein and calcium. Lucerne will correct this as well as supplying biotin.
Folate including folic acid, is needed where cells are rapidly growing or turning over. It is found in fresh and preserved green food, but insufficient in cereals. Exact requirements are not known but will be higher for instance in gestation. Deficiency in horses has not been identified.`
Vit B12 is made in the hind gut and also requires cobalt, but horses can thrive when the Co is so low that cattle die. Foals get it from colostrum and store it in the liver. Deficiency has not been reported.
Pantothenic acid is widely distributed in feeds; no deficiency or toxicity has been recorded.
Vit B6 is also widely available; no requirement has been established nor any deficiency or toxicity recorded.

Vitamin C (ascorbic acid) is an anti oxidant and is also involved in the synthesis of collagen, for instance. Horses synthesise it from glucose in the liver. Deficiencies have not been confirmed but may be associated with lowered resistance to infections. There is no established requirement for Vitamin C. Although levels in horse feeds have not been established, fresh forage is likely to be a prolific source.

Vitamin D is made by the action of sunlight on a form of cholesterol. In plants it forms as D2 and in animals, as D3. It is important in the absorption of calcium and the movement of calcium into and out of bones. It is found in hay, lucerne and fallen leaves. Grass left to mature will have Vit D2 in the dead leaves. Fat reserves will supply Vit D. when mobilised in the winter. Deficiency has not been reported in horses kept with some exposure to sunlight. Excessive D3 (above 44 IU/kg BW/d., which is a total of 22,000 /d for a 500 kg horse)) is toxic and will cause calcium to be deposited in soft tissues such as tendons or even heart muscle. When added to feeds, Vit D3 (animal source) is used, at levels of up to 4,400 IU/Kg of feed.

Vitamin E is important as an anti oxidant and for healthy muscle function. Selenium may be required with it. Deficiency causes white muscle disease. Vitamin E is found in fresh and flash dried forages at levels of up to 100 IU/kg. Grains have around 20-30% of the forage value. Natural Vitamin E is nearly 50% more effective than artificial. Requirements have been re-estimated from 105 IU/d for a 500 kg horse to 500 IU/d. This is a huge increase and may suggest that it is now over-estimated. Lactating mares may need 1000 IU/d. As an oil soluble vitamin, it can be stored in body fat and utilised when fat is used, when the natural diet is diminished. However, feeding high oil in the form of, say vegetable oil, can remove this important vitamin. Horses fed primarily on grass and preserved green forages will have sufficient Vitamin E. Oil fed in whole grain form, such as milled, cooked linseed is safer as it is not oxidised and so will not deplete Vit E. When added to rations Vitamin E is sourced from sheep carcass fat or sheep wool fat.

Vitamin K is another vitamin synthesised by gut microbes. It is also present in forages, but is in very low concentrations in cereals. Its most important function is in blood clotting. Deficiency can be induced with warfarin, used as a rodenticide and in the treatment of e.g. navicular disease. Otherwise, deficiency is not recorded in horses and nor have requirements been established, so forage and microbes must meet requirements. Early (under 7.5 months) weaned foals may be at risk of deficiency, especially if fed grain rather than good quality forage.

Vitamins added to feeds are usually in an artificial or synthetic form, or in a form not normally ingested by herbivores. When the horse is appropriately managed, with plenty of turn out and fed a forage based diet, vitamin supplementation is not needed. When high levels of compound feed are used, such as for competing horses, and an additional balancer is used, perhaps also with a vitamin supplement, the risk of over dosing on the toxic vitamins (Vit A, Vit D3) is real and high. Many event horses, for instance, are at risk of toxicosis from over supplementation.

In order to protect vitamins from degradation, they may be combined with preservatives such as BHT, coated with fats, or incorporated into a gelatine mix to form a spray-dried powder or beadlets. Gelatine is derived from pigs trotters and other animal remains.

The natural diet of horses has always and will continue to provide what the horse needs. The more cereals and their by products are used, the more depleted the diet becomes and the greater the need for supplementation. There can be no recommended level of any vitamin in a compound feed as no account is taken of the balance of the diet. Hay, grazing and lucerne are all valuable sources of essential nutrients and should never be underestimated or under appreciated.

© Jane van Lennep / Simple System Ltd
May 2010.