What Vitamins Do Horses Need?

What vitamins do horses need in their daily diet? Learn about what vitamins are and why they are vitally important at the right levels in both summer and winter horse feeding.

Understand what vitamins you need to focus on supplementing and what will be provided through the forage, hay, haylage or grass in the diet.

What are vitamins for horses?

Vitamins are organic compounds that are needed in very small quantities to sustain whole horse health and life.  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, if not all of the vitamins your horse needs should come from the grass and hay eaten every day. 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 plenty of fresh, green grass in the summer they will not be deficient in any vitamins. However, for horses with compromised hooves, there is research evidence to suggest that extra supplementation of B vitamins may be beneficial this is likely to be because stress may increase the body’s demands for the B vitamins. Horses can be stressed if they undergo a lot of physical activity or if they have poor health.

B vitamins for horses

The B vitamins are water-soluble, which means that where 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.  It is important then to supplement additional B vitamins to horses who 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 itself. Research has shown biotin at 10 plus mg per day to be useful for improvement in hoof quality. Alfalfa however is high in biotin especially when fed fresh.  Baled alfalfa has a level of around 0.24 mg/kg

Vitamin B12 is not present in the horse’s diet but the bacteria living in their intestines manufacture B12 which then the horse can absorb.  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 also water-soluble. Water-soluble vitamins do not stay in the body for long. The body cannot store them, and they are soon excreted in the urine. Because of this, water-soluble vitamins need to be replaced more often than fat-soluble ones.

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’.

Where 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 due to this vitamin increasing the uptake of iron.

Vitamin A for my horse

Vitamin A, along with vitamin D, E and K are fat-soluble vitamins. 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 in the body as reserves for days, and sometimes months. as with humans fat-soluble vitamins are absorbed through the horse intestinal tract with the help of fats, or lipids.

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

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 is 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 synthesized 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 of vitamin D but in humans, even 20 minutes of just hands would be sufficient. 

In addition, vitamin D 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 NRC (2007) figures a 500 kg horse will require 3300 IU of vitamin D per day.  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 of vitamin D.

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 vitamin A and D in horses

At Forageplus we believe that less is more with these two vitamins and 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 minerals in feedstuffs other than reported in supplements fed is very important to maintain health and prevent poisoning.

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. 

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 horse vitamin E

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.  

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.

Summary of horse vitamins needed

• Save money by not worrying too much about any vitamin other than vitamin E. Vitamin E is the only vitamin that everyone will need to supplement in the winter.
• The fat-soluble vitamins, A, D and K, as well as water-soluble vitamins C and B, are either abundant in a forage-based diet or are sufficiently synthesised in the horse’s body to not need to be supplemented unless a horse is laminitis stabled out of sunlight and not fed green forage.
• Vitamin C may be useful to increase in the diet of a hard-working performance horse.
• The B vitamins may be useful where hay or haylage is being soaked or a horse is stressed through travelling or performance work. There is also some evidence that additional B vitamins will support the health of skin and hooves.
• Vitamin A is needed where old hay is fed or the main forage is not green – like straw or very sun-cured hay.

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