Vitamin D is an essential nutrient required for calcium and phosphorus metabolism. Metabolites of the vitamin function to increase blood calcium concentrations into parathyroid hormone, and specifically works to increase calcium (and phosphorus and magnesium) absorption from the gastrointestinal tract, increase calcium and phosphorus release from the bone and to conserve calcium in the kidneys.
The skin can produce one type of this specific vitamin, Vitamin D3, when it is exposed to ultraviolet light (sunlight). In the winter, sunlight intensity is decreased, daylight hours are shorter, hair coat is longer, and many horses tend to be blanketed. Therefore, Vitamin D synthesis within the horse’s body may be insufficient. Similarly, while the horse could also obtain Vitamin D2 by consuming sun-cured forages such as hay, these stores in the hay decrease over time and amounts may be limited. Because of this, most, if not all, commercial feeds in Canada are supplemented with it. Typically feeds will contain between 700-1,500 IU/kg of feed, which along with any small amount from the sunlight or dried forages, should meet the 3,300 IU/day requirement (for a 500kg mature horse).
It should be noted that over supplementation can be toxic, so one should avoid adding supplements onto commercial feeds that already contain it. Vitamin D toxicity has also resulted due to feed manufacturer error, and should be considered if horses suddenly go off of their feed.