How Much Sun Do Horses Need? - Equine Vitamin D Requirements

I have a 15-year-old TB that raced until 9 years old. He is boarded in upstate New York where there is not much sun. He receives a complete diet (designed by a nutritionist at Cornell) with 2 cups Empower. He goes out to a small paddock 4 hours per day. Is it enough to produce Vitamin D and generate a healthy circadian rhythm? Thanks! – GR via AAEP Ask the Vet

Dear GR,
First, let me praise you for your hard work in providing a complete and balanced diet for your horse! Next, let me encourage you to turn your horse out as much as possible. Unless there is a medical reason against it or your boarding situation prevents it, the more your horse can exercise at will in fresh air with other horses the better! I say this because in researching answers to your two questions, I was unable to provide you with evidence proving he needs more than the four hours of turnout daily he gets now.

The short answer to your first question is: yes, the amount of sunlight your horse is exposed to through fours hours of turnout per day is probably enough to meet his minimum Vitamin D requirement. While the sixth edition of Nutrient Requirements of Horses only said “There are no reports of Vitamin D deficiency to date in horses maintained in practical settings with some exposure to sunlight,” several other sources said “a few hours of sunlight a day” and even “as little as one hour of sunlight a day” would probably be sufficient. If you choose to supplement to ensure healthy calcium metabolism (the primary role of Vitamin D), 300IU/100lb BW is the recommended amount, or about 3000IU for a 1000lb horse.

Now to your second question! Circadian rhythm is the roughly 24-hour cycle in physiological processes of living beings, both plants and animals. Some of these physiological processes include: sleep/wake, body temperature, thirst, appetite, brain wave activity, hormone production and cell regeneration. Circadian rhythm is generated from within the animal itself, but it can be altered by external cues such as daylight, temperature, feeding/fasting periods, and other stimuli. As long as your horse receives some light each day, he should have a healthy Circadian rhythym.