Covering Horses & Vitamin DIn Australia, covering to prevent insect bites.

(the Sunshine Vitamin)   Jenny Paterson B.Sc

Having passed the winter solstice we are on our way back to summer. However the days are still short and the mornings pretty cold and most people go to work in the dark and get home in the dark. Understandably it is not easy for them to take their horse’s covers off during the day.
Everyone knows that you need sunshine on your skin in order for the body to manufacture Vitamin D. The same goes for horses, except because the majority of their skin is covered with hair, they need at least 5-8 hours of sunshine to make their vitamin D rather than the 60-90 minutes we humans need.



Danny covered in the freezing rain.

Vitamin D the facts...
It goes without saying that horses that are covered all the time can develop a deficiency of Vitamin D.
 The two most useful forms of Vitamin D: areD2 and D3. While they come from different sources, the D2 form is made in grasses and plants by exposure to sunlight and the D3 form is made in the animal also from exposure to sunlight, they both carry out the same physiological functions.

Researchers have established that, besides calcium metabolism, the principle biological effect of vitamin D is gene control by promoting correct cell division and helping the regulation of cell death. This has major implications for horse and human health.

Vitamin D is actually a hormone. Hormones are ‘messengers’. When Vitamin D is sent to the small intestine it  increases calcium absorption  from the food, if there isn’t enough calcium in the food, Vitamin D is sent to message the skeleton to release calcium from bones into the bloodstream (in conjunction with parathyroid hormone and calcitonin).  When there is insufficient circulating calcium, Vitamin D will message the kidney to conserve calcium by reducing excretion via the urine. 

Correct blood calcium and phosphorous levels are vital for your horses’ nerves and muscles to function optimally and to maintain skeletal integrity.

The Problem...

It is the modern lifestyle of our domestic horses that can result in their developing a Vitamin D deficiency.

  • Horses that are stabled or covered 24/7 are at highest risk.
  •   Regular washing with shampoo inhibits the manufacture of vitamin D because the precursor oils in the horse’s skin are washed off.

Unlike many nutrients, Vitamin D can be stored in various organs for future use. Vitamin D accumulated in the summer months is used up over winter but needs to be regularly replenished by sunlight exposure and diet.

Health impacts of Vitamin D deficiency in horses include:

  • Decreased immune system function and associated increased susceptibility to infections and viruses
  • Reduced appetite, depression (Vitamin D is also known as the “Mood Hormone”)
  • Slow/faulty skeletal growth in young horses
  • Impaired muscle contraction
  • Insulin resistance
  • Tying up  
  • Poor muscle recovery time for performance horses

Vitamin D deficiency can lead to the secondary consequences of calcium deficiency:

  • nebulous lameness, swollen joints
  • bone and teeth demineralization
  • hyperactive reflexes
  • ‘peri-oral’ (around the mouth) numbness & tingling (the horse wants to rub his nose on his leg all the time progressing to boxing their nose with front feet, twitching of the muzzle etc)
  • paresthesias (chronic itching, skin crawling)

To prevent Vitamin D deficiency…

  • Give your horse ‘naked’ time at EVERY OPPORTUNITY, needs to be 5-8 consecutive hours. Well nourished horses don’t ‘fade’!
  • Check your multi minerals contain excellent levels of Vitamin D especially if your horse is covered more than you would like.