Horse Supplement Vitamin D 


Vitamin D is available in two forms; D2 which is found in plants and D3 which is produced by animals or can be delivered orally in synthetic form. Horses, like humans, produce vitamin D as a result of sunlight exposure. Once inside the horse’s body, D2 and D3 have the same function.


What is Vitamin D

Vitamin D is actually a hormone because it is transported in blood and has actions on various cells in the body. It goes through several steps once it enters the body, which includes going through processing in the liver and the kidneys. Its job is to ensure correct levels of calcium in the blood. This is important for proper functioning of bones, joints and muscles. It increases absorption of ingested calcium and if needed it can get the bones to give up calcium and instruct the kidneys to reduce calcium loss through urine.


Vitamin D3

Vitamin D3 is produced as a result of skin exposure to sunlight. Sunlight reacts with a chemical within a horse’s body oil and once inside the body’s system, it goes through several more processes before it is in its active form of vitamin D3. It can also be manufactured synthetically and delivered to horses through supplementation.


Vitamin D deficiency

Vitamin D deficiency is quite common. Horses can produce little vitamin D3 as a result of modern day equine practices. They are commonly housed indoors and often spend time outside rugged – this diminishes their ability to produce D3. As well as this, fly sprays and coat conditioners can affect production and washing with soaps and shampoos removes body oil required to produce D3.


Human studies have also found that sunlight is less intense in winter, so it has been assumed that this will also be applied to horses. Human studies have found that during winter, sunlight levels above the latitude of 51deg N (much of Canada, Great Britain, Scandinavia) are not adequate to produce D3.


Signs of Vitamin D deficiency

  • Reduced appetite
  • Unwell looking
  • Slowed growth
  • Bone demineralisation (risk of fractures as a result)
  • Poor muscle contraction


Equine sources of vitamin D

Fresh fodder like grass will provide horses with some vitamin D. Unfortunately, we have previously thought that sun cured pastures are good vitamin D sources. However, some modern day researchers believe they may not be as dense with vitamin D as we thought.




Optimal Vitamin D input

Current research indicates that horses need to receive at least 6.6 IU of vitamin D per kg of body weight. (3300 iu per 500kg horse per day.) It is believed that 5-8 hours of sunlight exposure (in ideal conditions) will produce this amount.


Vitamin D toxicity

This is an uncommon problem but is possible.

Unfortunately, it has similar signs and symptoms to deficiency

  • Reduced feed intake
  • Poor growth
  • Unwell appearance


Too much supplementation can result in too high of an intake so do not exceed the recommended dose. Fortunately, sunlight exposure will not lead to excessive vitamin D production.


Lack of equine D3 research

Unfortunately, more research is required into horses and vitamin D. Most of the research of vitamin D is based on humans or has been performed on rats and extrapolated for use in horses. Things that we do know is that regardless of latitude and weather, horses have low levels of vitamin D in their plasma anyway. There are also suggestions that animals with thick coats (like horses) are unable to produce vitamin D3 anyway. Limited research indicates that horses still produce D3 however it does not affect the levels in plasma.

It is also difficult to know how much horses rely on vitamin D3 compared to D2.



Key points to maintain adequate vitamin D levels:

Although more research is needed into this area, it is generally recommended that vitamin D is important for horses. So recommendations include;


  • Plenty of outdoor time for your horse
  • Pasture grazing (dry feed has very little vitamin D)
  • Avoid excessive use of soaps and shampoos
  • Avoid rugs/blankets when not needed
  • Avoid excessive use of coat conditioners and fly sprays