On feeding horses – 5

At 0600 this morning our Nokota horses stood on the porch of their open stable, waiting to greet the first clear, crisp cold subzero (Celsius) morning light, the ground frozen white; a nice change from the latest weeks of rain and muddy pastures. The very surface of their coat was cold, but just underneath my cold hands found heat and warmth. After hay supper the horses went down to graze the pasture, surprisingly maybe, but frozen plants is probably like ice cream to horses. Watching the horses playing around in the emerging sunlight softly embracing the pasture, made me think of the next subject; vitamins.

To have knowledge about vitamins is generally not a serious concern since most of the vitamins are sufficiently synthesized from forage, sunlight and by the horse itself. It does not help us design the right diet, measure up hay rations or choosing the right supplements. However vitamins have a much bigger lesson for us to learn. Vitamins might well be the most important piece of knowledge about horses we will ever get, so hang on.


Vitamins A, D and E are just as essential components as minerals are when it comes to metabolic and biochemical processes in a horses body. The amount needed is, as you already have figured out, depending on a long range of factors, more or less determinable. Young horses, pregnant and lactating mares need more vitamins, but since they also need more proteins we tend to give them the best pastures, more hay and preferably legumes, so without knowing we provide them enough.

Vitamins are basically divided into two groups; water soluble (B,C) and fat soluble (A,D,E,K).

An excess of B and C vitamins are easily flushed out by the urine, so toxicity is unlikely. The vitamins B are a collective name for several like B1 (Thiamin), B2 (Riboflavin), B3 (Niacin), B6, B12, Pantothenic acid. Harvest and storage affects the vitamin content in many ways. Growing forage provide lots of vitamins, but legumes are more so than grasses.

Vitamin B, C and K is sufficiently produced or synthesized by the horse’s digestive system itself, so the content in the food is chiefly irrelevant. However A, D and E may be more of a concern for horses and their owners by a few reasons; one being that the knowledge even within the research community is pretty weak; another reason is that the commercial pressure in vitamin supplements often reaches unhealthy levels, so to speak. Not saying that everything is harmful, but money can be spent on different things in different ways.

The National Research Council NRC is doing a tough job trying to recommend fair levels of vitamins to horses, but the way to set a recommended amount of vitamins is complicated and depends on how active a vitamin is, biochemically. Therefore a strange unit called IU (International units) is used. For example; the need of vitamin E for a lactating mare according to NRC2007 is 2 IU/kg when given 2,5% by bodyweight, well never mind ;).

Vitamin A

This vitamin is also known by its active substance Retinol. It is a long funny molecule that can reshape itself in different ways (stereo isomers). Heat, light and humidity makes its structure change. Fortunately evolution made the most common shape, trans-isomer, the most biochemically active. In plants however, it exists only in a pre-state called Carotenoids, a reddish yellowish colored substance. There are several sub types of this substance, but the one called Beta-Carotene is the most prominent source of Vitamin A. Unfortunately horses are particularly ineffective when it comes to transforming Beta-Carotene into vitamin A compared to other species. Retinol is mainly stored in the liver, but also in fat tissue but at a minor quantity. Research has given that two months of feeding with very low content of vitamin A, may use up all vitamin A stored in the horse liver.

Carotene is sensitive to sunlight so if the forage is dried outdoors it diminishes considerably. It also diminishes over time as the forage is stored. A greenish color in hay is a sign of rich carotene, sun bleached yellow forage is a sign of low carotene content. The same goes for silage, but it is difficult to see the same connection because of the higher water content. Fresh growing grass has a significantly higher content of vitamin A than stored harvested forage, so the lesson here is that horses really need their pasture. Vitamin A is relatively abundant in Timothy, but legumes like clover and alfalfa generally contains almost three times more vitamin A than grasses. Supplementation is hardly ever needed, but a good and familiar winter supplement is of course ordinary carrots (the Nokota horse reading over my shoulder nods in consensus).

Vitamin A has many roles to play. In the eyes, retinol transforms and reacts with a protein called opsin to form rodopsin which works as a light receptor in darkness. A good pasture means good night vision, which can be the difference between life and death for a free roaming wild horse. Vitamin A also has an important part in controlling mitosis. Together with vitamin E it also works as an antioxidant defending the body against infections. Vitamin A deficiency may cause severe problems for brood mares and lactating mares. Reduced growth among foals is another symptom. Ten times more vitamin A than recommended may cause toxicity leading to weak bones and malformations among offspring.

Recommendations: 30 IU/kg bw and twice that for pregnant and lactating mares. (1IU is equal to 0,3 µg all-trans-retinol). In other words good mixed pastures and good hay matters!

Vitamin D

This vitamin exists in two primary shapes; one synthesized in plants called D2 and another synthesized in the body skin called D3, the later is the most active type. The process to form D3 is, simply put; cholesterol in the skin reacts to ultra violet sunlight at wavelength 290-315 nm, the resulting vitamin D3 substance is then transported to the liver. When the horse eats plants containing D2 it is absorbed by the digestive system and then transported to the liver. Vitamin D is then stored in the liver and in the blood, but not as effectively as vitamin A.

The vitamin D content in forage is affected by sunlight; the longer exposure to sunlight the higher vitamin D abundance. Silage contains more carotenoids and tocopherol, but hay contains more ergosterol, which transforms into vitamin D2. Keeping horses outdoors exposed to sunlight all year around is a good way to reassure your horses get lots of vitamin D, though on our 58th latitude or even much further north in Sweden where winters are all darkness, vitamin D deficiency can in theory become a problem. Timothy is known to be richer in vitamin D than other grasses, but legumes like alfalfa is even better.

Vitamin D is vital for the absorption of Calcium and Phosphorus, but also for the formation of bone tissue. Vitamin D deficiency may cause Rakitis among young horses and Osteomalaci among older horses. Toxicity from over supplementing vitamin D2 (over 44 IU/kg bw) may cause loss of weight, cellular damage and inflammations, too high levels of Calcium and low phosphorus in the blood.

Recommendations: 6,6 IU/kg bw. Meaning lots of mixed grazing and lots of sunlight, no blankets.

Vitamin E

This vitamin exists in eight different shapes. Alpha-tokoferol is the most active and far most common type in horses’ bodies. Vitamin E is connected to the process of fat digestion, as vitamin E primarily is stored in horses’ fat tissue. Horses get a little fat during the end of the autumn for a good reason; they store vitamin E in their fat tissue to last past the winter.

Vitamin E is a strong antioxidant and it is important for formation of cell membranes, blood coagulation and DNA biosynthesis. It is also said to have a calming effect on horses. Deficiency may cause muscle and bone problems and diseases like EMDN (equine motor neuron disease) and EDM (equine degenerative myeloencephalopathy).

All kinds of forage, grasses as well as legumes are generally low in vitamin E. It is most abundant in young plants, but it diminishes as the plant grows. It is sensitive to acid, so vitamin E is generally scarce in wet silage with low PH. Excessive exposure to sunlight, moisture and heat have a negative effect on the vitamin E abundance in harvested forage.

Toxicity is generally no concern up to 22,5 mg/kg bw (1000 IU/kg dry substance)

Recommendations: 1 IU/kg bw and twice that for pregnant and lactating mares, which also goes for hard working horses. I repeat lots of grazing on mixed pasture and well made hay.


To wrap it all up when it comes to vitamins; The more you let your horses live like wild free roaming horses do, meaning free choice of grazing out on mixed pasture exposed to sunlight, the better. It is easy, logical and unsurprising, so what’s new about that?


A huge number of horses all over the world are kept locked up in stables and when they eventually do have the privilege to be let out, they are all covered up in well designed, very functional blankets that blocks out all sunlight. It took nature tens of millions of years to perfect the digestive system of a horse to fit in within their ecosystem; the grassland or prairie. The skin and the coat of a horse have likewise taken nature tens of millions of years to perfect. It adapts to seasons and climate, it protects the horse perfectly against any weather and temperature, it reacts biochemically to sunlight producing essential vitamins.

Everything we do to a horse to change its way of life is basically bad for the horse; the wild free roaming horses do not need us, but we need them. Everything we do to change this ecosystem we call earth is basically bad; the earth do not need us, but we need her.

When it comes to big things like the universe and spacecraft design it is easy for us to accept that we must go with the systems, adapt to greater forces like gravity, vacuum and the even more powerful forces working within the atom itself. When it comes to our environment right here where we live, we take for granted that we can change it beyond recognition, without any consequences and without any knowledge about it and I myself is not an exception to that.

Once there were people living in what we call horse cultures, they knew nothing compared to modern humans, even so they managed to live in equilibrium within their ecosystems for many thousand years, which it only took us a century to ruin completely. Why do we do these things?

Human ability to accrete knowledge and to figure things out is amazing and unchallenged, but our ignorance is even greater. Keep this insight in the back of your head when feeding your horses, sometimes the feeling in your guts, given by evolution, is more true than what your neighbor tells you or what you read on the internet 😉

So it turned out that knowledge about horses and vitamins, which may be considered interesting only for the odd and curious, is quite useful to anyone; all knowledge is good knowledge.



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