Question

Have there been any studies to determine a horse’s need for increased vitamin D in the winter months in areas where sunlight is significantly reduced? I am trying to determine why my mare becomes moody during the time of year when we have reduced daylight hours only. This is the third winter I have ridden her, and it is definitely a pattern for her. For humans, vitamin D does have an effect on moods. Any thoughts? What is a safe amount of a daily dose of vitamin D for a horse?

Answer

According to Nutrient Requirements of Horses, published by the National Research Council in 2007, “there have been no reports of vitamin D deficiency in horses maintained in practical setting with some exposure to sunlight.” That exposure can be a little as 15 minutes per day. As far as I am aware, there has not been any research addressing a concern for vitamin D levels in performance horses in the times of limited sunlight.

Vitamin D is not only supplied by exposure to sunlight but is commonly added to commercial feeds in adequate quantities for those horses with little sunlight exposure. The limitation of vitamin D in feeds is dependent on the feeding rate of the feed. As long as the horse is getting at least the minimum recommended amount of that particular feed per day, there should be more than adequate vitamin D in the diet. There is also some vitamin D found in sun-cured hay, particularly alfalfa. According to Nutrient Requirements of Horses, the recommended amount of vitamin D for an 1,100-lb adult horse is 3,300 IU per day. The upper safe level of vitamin D supplementation is 22,000 IU per day.

Perhaps your mare’s shift in mood in the winter months may have more to do with her hormones than vitamin D. In general, mares become anestrus (not cycling) when the daylight hours shorten and their ovaries become inactive. She may be responding to low hormone levels or may be one of the few that remain cycling year-round. You may want to discuss this with your veterinarian.