Yes, just like humans, horses can only make Vitamin D from the sun. Or they can ingest it from fresh, live grass (not hay). And if you live above 34 degrees North Latitude (draw a line from the middle of Los Angeles east to pass through Columbia in South Carolina – anything above that, needs Vitamin D supplementation), you and your horse cannot make enough Vitamin D from the sun anyway.
There are many other factors that can affect a horse’s Vitamin D production, and these include bathing your horse (her natural oils are needed to convert sun to Vitamin D), stalling your horse, blanketing or fly sheets covering your horse’s skin, or your horse standing under a roof for extended periods. It takes 5-8 hours of sun exposure per day for your horse to produce the required amount of Vitamin D for basic health.
According to Dr. Juliet Getty, “Horses do best when they receive at least 6.6 IU of vitamin D per kg of body weight. For an 1100 lb (500 kg) horse, this translates into 3300 IU/day. Sunlight exposure — 5 to 8 hours/day under optimal conditions – will produce this amount of vitamin D.”
Since the upper limit of Vitamin D (shown to cause toxicity) is 22,000 IU/day, I give my horses 4,000 – 6,000 IU per day, since we live in the very cloudy, rainy Pacific Northwest. The easiest way is to purchase a Vitamin D3 in liquid form and just drizzle it over your feed or supplement pellets.
Remember that Vitamin D is actually not a vitamin, but a hormone. So aside from the well-known effects of Vitamin D deficiency on bone strength and density, you also need to be aware of it’s effect on mood (depression), energy levels and immune system functioning.
One last important point to consider is that Vitamin D requires magnesium to be converted into its active form. So I also give my horses extra magnesium sprinkled over their feed. If I’m bulk-buying magnesium from the feed store (usually magnesium oxide) then I give 1 tablespoon per horse. Again, Dr. Juliet Getty has an excellent guide to magnesium supplementation for horses.
It’s very easy to find the upper limit for magnesium supplementation – just watch your horse’s poo. When their poop becomes really mushy or runny, that’s too much magnesium – simple!
While we’re on the topic, what other daily supplements do I give my horses? In addition to magnesium and Vitamin D3 I also give 1.5 tablespoons of dried seaweed, Hoffman’s Minerals, Natren Equiflora probiotics, 1.5 tablespoons of Redmond or Himalayan Salt. I sprinkle all these over 1 cup of a vitamin supplement feed. Currently, during the wet winter I’m also giving respiratory support herbs and I found the most fantastic dried herbs from a company in the UK called Wendals Herbs – I order them from HorseHealthUSA.com