What Vitamins Do Horses Need?

What are vitamins for horses?

Vitamins are organic compounds that are needed in very small quantities to sustain the health and life of the horse’s body. Vitamins help the horse’s body carry out the functions essential to life, such as:

• Production of skin, hooves, bone and muscle
• Strengthening of the immune system
• Maintenance and repair of cells, tissues and organs
• Correct functioning of the nervous system and brain
• Converting food into energy

Most of the vitamins a horse requires are naturally supplied by forage, including grass, hay and haylage, when these are of good quality and form the majority of the diet. This is consistent with guidance from the National Research Council, which assumes a forage-based diet as the foundation of equine nutrition [1].

However, if your horse is on restricted summer and spring grazing, or it is winter, or you routinely soak the hay your horse eats, you will want to know which vitamins you must supplement for horse health.

What vitamins do horses need?

Vitamins for horses can be divided into two groups: fat-soluble and water-soluble. You can also divide them into those you need to worry about and those you don’t.

Forageplus only stock the vitamins your horse might be deficient in if on a forage-based diet.

We don’t believe in adding vitamins to our supplements unless your horse truly needs them because they are not in the greatest proportion of the diet being eaten. The greatest proportion of the diet will always be grass, hay or haylage.

Horses’ vitamins from green and growing grass

If horses have access to abundant fresh, green pasture, they are unlikely to be deficient in most vitamins. Fresh forage is a rich natural source of fat-soluble vitamins (A, D, E and K), as well as vitamin C, and forms the basis of the vitamin supply assumed in National Research Council feeding models [1].

However, individual requirements can vary depending on factors such as workload, health status, stress levels, and environmental conditions. Horses in higher levels of work, those under physiological stress, or those with underlying health challenges may have increased vitamin requirements beyond what pasture alone provides.

For horses with compromised hoof quality, there is research evidence to suggest that additional supplementation of certain B vitamins, particularly biotin, may be beneficial. Although B vitamins are synthesised by hindgut microbes under normal conditions, this synthesis may not always meet increased demands.

Stress, whether from exercise, illness, or metabolic strain, is thought to increase the body’s requirement for B vitamins due to their central role in energy metabolism, tissue repair, and keratin formation. In these situations, targeted supplementation can be a useful support alongside a forage-based diet.

Fat-Soluble Vitamins

Fat-soluble vitamins are essential for a horse’s overall health and well-being. These vitamins are stored in the horse’s body and liver, playing crucial roles in various bodily functions. The four fat-soluble vitamins are:

• Vitamin A: Vital for maintaining healthy skin, a shiny coat, and good vision. It also supports the immune system.
• Vitamin D: Crucial for calcium absorption, which is necessary for strong bones and teeth. Horses can synthesise sufficient amounts with adequate exposure to sunlight.
• Vitamin E: Acts as a powerful antioxidant, protecting cells from damage and supporting muscle function and immune health.
• Vitamin K: Necessary for blood clotting and bone health, ensuring that wounds heal properly and bones remain strong.

These vitamins are found in fresh green forages, hay, and other straights.

Ensuring your horse receives adequate amounts of these vitamins through their diet or supplementation is essential for maintaining optimum health.

B vitamins for horses

The B vitamins are water-soluble, which means that when you soak your hay, they will be lowered as part of the rinsing and soaking process and thrown away with the water you have soaked the hay in.

Among the B vitamins, folic acid is essential for supporting various aspects of horse health, such as immune function and energy production.

It is important to supplement additional B vitamins to horses that eat soaked and rinsed hay as the greatest proportion of their diet.

If your horse has poor hoof quality, you can add extra B vitamins, although the horse can synthesise these itself. Research has shown that supplementing biotin at levels of around 15–20 mg per day can improve hoof quality over time in horses with poor hoof horn [2].

Alfalfa, however, is high in biotin, especially when fed fresh. Baled alfalfa has a level of around 0.24 mg/kg

Vitamin B12 is synthesised by microbes in the horse’s hindgut, and deficiency has not been documented under normal feeding conditions [1]. No B12 deficiency in horses has ever been documented, so this is another vitamin you need not worry about.

However, horses on long-term ulcer medications may need supplementation, as research in humans has shown malabsorption due to the action of stomach acid-suppressing drugs.

Vitamin C for horses

Vitamin C is a water-soluble vitamin, meaning it does not linger in the horse’s body for extended periods. The body cannot store it, and it is quickly excreted through urine. As a result, water-soluble vitamins require more frequent replenishment compared to fat-soluble vitamins.

Horses can synthesise vitamin C in their bodies to prevent deficiency, but the role of dietary vitamin C is poorly understood, and there is little research to suggest optimum levels.

Research does show that heavy exercise and illness can lead to low vitamin C blood levels, and to maintain and support health in horses, extra supplementation can be beneficial for these horses.

The NRC 2007 records that there is poor information on the levels of vitamin C in common horse feeds, but research suggests that a horse on green pasture will receive 20 grams per day, whilst a horse on a hay-based diet will receive less than 1 gram.

Horses on green and growing grass, therefore, will be exposed to high levels of vitamin C, perhaps the source of ‘Dr Green’.

When you are rinsing hay to lower sugars for a laminitic horse, you should not supplement extra vitamin C because it can increase iron uptake. 

Many laminitics are iron overloaded, so we do not advise the feeding of extra vitamin C to horses with laminitis.

Vitamin C can enhance iron absorption in other species, and given that iron intake in forage-based equine diets is already high, additional supplementation should be approached with caution in laminitic or metabolically sensitive horses.

Vitamin A for my horse

Much fuss is made about vitamin A.  This vitamin is quite possibly the most over-supplemented vitamin in the horse feed world. 

Vitamin A, along with vitamins D, E and K are fat-soluble. Fat-soluble vitamins are stored in the fatty tissues of the horse’s body and the liver. 

These are easier to store than water-soluble vitamins, and they can be stored in the body as reserves for days, and sometimes months. As with humans, fat-soluble vitamins are absorbed through the horse’s intestinal tract with the help of fats, or lipids.

Vitamin A is present in the grass, hay or haylage-based diets in the form of carotenoids, primarily beta-carotene, which is converted to retinol in the intestine.

Adult horses feeding on even 2-year-old hay with a beta carotene level as low as 4 mg/kg (compared to 30 to 385 mg/kg normally present in forages have shown in research to have no symptoms of vitamin A deficiency.

What you need to know is that vitamin A is abundant in the grass and is very unlikely to be deficient in our UK horse hay, if it is under two years old.  When a foodstuff is naturally green, such as grass, hay, alfalfa, etc., this indicates good levels of vitamin A.

However, if you are feeding forage or feedstuffs which are not green, then levels are likely to be too low. Levels of vitamin A in beet pulp and copra are low, as is straw. So, if these are being used as a grass or hay replacer, then supplementation of vitamin A would be wise.

If you really need to be sure your horse has enough vitamin A available, just feed a carrot or two each day. A large carrot will contain around 8500 IU of Vitamin A (as beta carotene), and the requirement for a 500 kg horse is just 15,000 IU per day, so two carrots are a cheap way of supplementing vitamin A.

Do not feed carrots to horses with laminitis due to the high sugar content. It is better to use a horse feed balancer which contains good levels of vitamin A, if you are feeding straw or hay which is more than one year old to laminitis-prone horses or ponies.

The reason we add vitamin A to our Laminae Plus Balancer is that we recognise that many horses prone to laminitis will not be able to access vitamin A through eating grass, cannot have carrots and may be fed old hay or straw as a major portion of their diet.

Does a horse need vitamin D?

Vitamin D can be synthesised in the skin, and no dietary intake is needed if the horse has exposure to sunlight.  It is not known how much time is needed for optimum levels, but in humans, even 20 minutes of just hands would be sufficient. 

In addition, this vitamin is also abundant in the grass and cured hays. Hay less than a year old contains around 2000 IU/kg, hay over a year old but under two years will contain around 800 IU/kg.  Hay over two years old will contain around 400 IU/kg.

According to the National Research Council, a 500 kg horse requires approximately 3,300 IU of vitamin D per day, although endogenous synthesis through sunlight exposure is typically sufficient under normal management conditions [1].

An adult 500 kg horse eating 10 kg of less than one-year-old hay per day would be exposed to 20,000 IU.

Requirements for young horses are greater, but the amounts in grass, hay or haylage are still so great as to mean that supplementation is not something to worry over.  Only horses stabled out of direct sunlight and not on a forage-based diet or being fed feedstuffs or forage which is not green will need supplementation.

Again, we add vitamin D to the Laminae Plus horse feed balancer due to many horses and ponies prone to laminitis being stabled and being fed feedstuffs which are not green.

Toxicity of fat-soluble vitamins A and D in horses and ponies

At Forageplus, we believe that less is more with these two vitamins, so we do not routinely add vitamin A to any of our supplements apart from the horse feed balancer Laminae Plus, which is aimed at horses prone to laminitis.

These horses are more likely to be eating either hay, haylage or straw, which is not green, or be on a hay replacement diet of beet pulp and other feeds, which are not green.

Both these vitamins can be harmful if supplementation is too great. The upper safe limits are reported as 16,000 IU/kg of diet for vitamin A and 44 IU/kg of body weight for Vitamin D. 

Calculating exposure to these two vitamins in feedstuffs other than those reported in supplements fed is very important to maintain health and prevent poisoning.

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Does my horse need vitamin K?

Vitamin K is present in plants eaten by horses and is also synthesised by bacteria in the horse’s gut in the form of menaquinone. No diet-related deficiency of vitamin K has been reported in horses, as it is widely available in forage and synthesised by hindgut microbes [1].

The NRC (2007) discusses that the bacterial source might not be enough to meet the needs of horses in heavy work, however, studies in other large animals show large bowel absorption.  The form commonly supplemented to horses, however, is largely useless.

Supplementing your horse with vitamin E

For horses consuming conserved forage such as hay or haylage, vitamin E is the most important vitamin to consider supplementing. Unlike fresh spring and summer pasture, which is rich in natural vitamin E, levels decline rapidly during cutting, drying, and storage [1][3].

Vitamin E plays a critical role as a biological antioxidant, supporting muscle function, immune health, and cellular protection. Deficiency has been linked to neuromuscular disorders and reduced performance capacity.

Where your horse is on a hay or haylage diet, or only has limited access to fresh green growing grass (less than 6 hours), then basically you only need to worry about vitamin E, which is the number one antioxidant vitamin in the body.

In addition to vitamin E, mineral supplements can help ensure that horses receive essential trace minerals for optimal health and performance.

A potent anti-inflammatory, it protects individual cells every day, and supplemented levels should be increased where horses are performing at high levels or are sick.

More information about vitamin E and its levels can be found in our Forageplus Talk article on this vitamin.

At Forageplus, analysis of over 12,500 forage samples consistently shows that while macronutrients and minerals vary widely, vitamin E is the most consistently low vitamin in hay-based diets, particularly in winter feeding systems where fresh pasture is unavailable.

Vitamin Deficiencies in Horses

Vitamin deficiencies can occur in horses due to various reasons, including inadequate diet, poor quality feed, and increased demand for vitamins during periods of stress or growth. Possible vitamin deficiencies in horses include:

• Vitamin A deficiency: This can lead to night blindness, a dull coat, and a weakened immune system. However, adequate amounts are almost always supplied by the forage, and this is possibly the most over-supplemented vitamin for horses.
• Vitamin D deficiency: May cause bone deformities, osteomalacia (softening of the bones), and a compromised immune function. No dietary intake is needed if the horse has exposure to sunlight, with this vitamin also being abundant in grass and cured hays.
• Vitamin E deficiency: Often results in muscle weakness, a higher susceptibility to diseases, and impaired immune function. It is very important to supplement with vitamin E throughout the winter months.
• Vitamin C deficiency: True vitamin deficiencies are relatively rare in horses maintained on forage-based diets, but suboptimal levels may occur under certain conditions, such as intensive work, illness, or restricted access to fresh forage.

Signs and Symptoms

The signs and symptoms of vitamin deficiencies in horses can vary depending on the specific vitamin and the severity of the deficiency. Some common signs and symptoms include:

• Poor coat condition
• Impaired immune function
• Increased risk of disease
• Poor wound healing
• Muscle weakness
• Bone deformities
• Night blindness

Recognising these symptoms early and addressing them through dietary adjustments or supplements can help maintain your horse’s health.

Vitamin Interactions and Equine Health

Vitamins interact with each other and with other nutrients to maintain optimal health in horses. Understanding these interactions is essential to ensure that your horse is receiving a balanced diet.

• Vitamin D and calcium: Vitamin D is essential for calcium absorption, and a deficiency in vitamin D can lead to impaired bone health.
• Vitamin E and selenium: These two nutrients work together to protect cells from damage and maintain immune function.
• Vitamin C and iron: Vitamin C is crucial for iron absorption, and a deficiency in vitamin C can lead to impaired iron status; however, iron is almost always found at high levels in the forage, and vitamin C supplementation is only needed under certain circumstances.

These interactions are well recognised in equine nutrition and form the basis of NRC nutrient requirement models [1].

Understanding the Complexities

Vitamin interactions can be complex, and it’s always best to consult with a reputable company, veterinarian or equine nutritionist to ensure that your horse is receiving a balanced diet.

A diet that includes essential vitamins and minerals is crucial for maintaining optimal health in horses. Ensuring a balanced intake of these nutrients will support your horse’s overall well-being and performance.

Want to learn more about horse feeding? Check out our other academy articles here.