Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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All the best,
Dr. Getty
Hi Jackie,
Yes! This is an excellent idea (of course, hay is missing other vitamins besides Vitamin D, which is why I recommend adding a comprehensive vitamin/mineral supplement to a hay-only diet that contains vitamins E, D, A (or beta carotene), and omega 3 fatty acids).
But, you're on target -- horses are able to produce vitamin D when exposed to sunlight (as long as they are not bathed too frequently or sprayed every day with chemicals). But during the winter, horses are often indoors more often, or they are blanketed, or they just don't get enough sunlight due to the angle of the sun during this time of year.
So, 5,000 IU of vitamin D would be very appropriate. If you need to give a supplement that has everything, I really like Glanzen Complete.
All the best,
Dr. Getty 
Author of Feed Your Horse Like A Horse
I was thinking of adding one of my 5,000 IU Vitamin D tablets to my horse's daily ration during the winter months. Good idea? Bad idea? Other?
Many thanks for any insight!
Jackie
Where are you from? Bayfield, CO