Vitamin D regulates calcium and phosphorus absorption and directs these minerals into bones and teeth. Horses synthesize vitamin D3 through their skin when exposed to ultraviolet light, and sun-cured hay also contains meaningful amounts of vitamin D2. Horses with regular outdoor turnout in sunlight rarely have vitamin D issues. Indoor-kept horses or those in regions with limited winter daylight may become deficient, though this is less common than in small indoor pets because even partial outdoor access provides significant synthesis.
A 500kg horse needs about 3,300 IU of vitamin D per day — comfortably provided by a few hours of outdoor turnout in sunlight combined with sun-cured hay. Horses with minimal outdoor access may benefit from a supplement, which most commercial feeds include. Do not supplement heavily without checking existing intake, as the margin between adequate and excessive is narrower than for many other vitamins.
0.0% of daily nutrient intake
Vitamin D makes up 0.0% of your horse's total daily nutritional requirements by weight. That's a tiny amount — but it matters.
Weak or soft bones, poor bone development in growing horses, reduced calcium absorption despite adequate dietary calcium, and increased fracture risk. Significant deficiency is uncommon in horses with any regular outdoor turnout.
Vitamin D toxicity causes excessive calcium absorption and deposition in soft tissues, including blood vessels, kidneys, and the heart. Symptoms include increased thirst, weight loss, and calcification of tissues. Toxicity is primarily a risk from over-supplementation or accidental ingestion of rodenticide containing cholecalciferol.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 3300 | 6600 | IU | For a 500kg horse. Met through combination of sun-cured hay and outdoor turnout. Horses with minimal sun exposure may need supplementation. |
Source: NRC 2007
Vitamin D regulates calcium absorption from the gut and directs it into bones where it belongs. Without adequate vitamin D, calcium passes through the gut without being properly absorbed, even when dietary levels are sufficient.
What this means: Ensure your horse gets regular outdoor turnout for natural vitamin D synthesis through the skin. Indoor horses on hay-only diets may need a vitamin D supplement, especially in winter or at high latitudes, to ensure their calcium intake actually reaches their bones.
Vitamin D is fat-soluble and requires dietary fat for absorption and transport. The fat in normal equine forage supports vitamin D uptake from both dietary sources and sun-cured hay.
What this means: The natural fat in your horse's forage supports vitamin D absorption without any need for additional fat supplementation for this purpose.
