Vitamin D is often known as the “sunshine vitamin” because it is synthesized in response to exposure to sunlight. There are two major natural sources of vitamin D, cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). Vitamin D3 is synthesized in the skin of many herbivores and omnivores upon exposure to UV light from sunlight.  Vitamin D2 is not found in green forages, but is formed when the dying leaves are exposed to sunlight. Thus, sun cured hay is a good dietary source of vitamin D2. Livestock utilize vitamin D3 much more efficiently than vitamin D2.

What Does Vitamin D Do?
Vitamin D is a precursor to a steroid hormone, calcitrol, which primarily functions to control the calcium and phosphorus balance in the body.  Calcitrol helps determine the amount of calcium and/or phosphorus deposited or depleted from the bones as well as the plasma levels of both minerals. Vitamin D has also been reported to be involved in magnesium absorption.

Vitamin D deficiency results in rickets. Less severe symptoms include decreased appetite, reduced growth rate, stiff gait, labored breathing, weakness and tetany. Since vitamin D is so crucial in the utilization of calcium and phosphorus, deficiency often mimics deficiency of these two minerals.

Vitamin D Requirements
Most livestock do not have a nutritional requirement for vitamin D as long as they have access to adequate sunlight.  Vitamin D becomes an important issue in the absence of sufficient UV radiation from sunlight. Radiation that reaches the earth contains only a small part of the UV range that promotes formation of vitamin D.  This UV radiation is more potent in the tropics than towards the poles; more potent in summer vs. winter; more potent at noon vs. morning or evening and more potent at higher elevations.  The best time for vitamin D production is between 10 am and 2 pm.

However, natural vitamin D production can be limited by several factors.  These include confinement, seasonal  periods of low light (more pronounced in  Northern latitudes during winter months),  extended cloud cover,  and limited exposure of skin to sunlight (for example, sheep with full fleece or horses with blankets) and even coat and skin color (irradiation is less effective on dark-pigmented skin).

Lack of UV exposure can be compensated for with adequate amounts of sun-cured hay. However; like vitamins A and E, unless vitamin D is stabilized it is destroyed by oxidation. This oxidative destruction is accelerated by heat, moisture and trace minerals.

Why Supplement Vitamin D?
Given the critical role of vitamin D in calcium and phosphorus metabolism, it is not worth the risk of relying solely on naturally derived sources of vitamin D.  Dietary supplementation of vitamin D acts to prevent devastating production losses.  Just like you pay for auto insurance every month even though you will go through the majority of your life not getting into an accident, supplementing vitamin D, even though it may not be needed a majority of the time, will prevent catastrophic losses during the periods when vitamin D is lacking.

Dietary supplementation will be most critical in confined livestock (think dairy cattle or stalled horses and show animals) or those covered with barriers to skin exposure to UV light (think full fleece on sheep or horses covered in blankets). Special consideration should be given to added vitamin D levels in feeds and supplements for these groups. The second group most susceptible to deficiency would be grazing livestock in Northern latitudes during winter months.  Livestock have limited capacity to store vitamin D and the short daylight hours will limit exposure to sufficient UV light.

Up until recently, the cost of adding vitamin D was minimal. Vitamin D was often used as a means to enhance the look of a product tag due to its relative low cost. However, given that the vitamin markets have taken a sharp upturn, you may notice that the vitamin D levels of some of your favorite supplements may have fallen.  In most cases this should not be a concern (unless you have livestock in the susceptible situations listed above). As stated earlier, many had been “over formulating” for vitamin D for years due to its low cost.

If you have any questions regarding vitamin D levels in your current supplement program, contact the Ridley Block Operations nutritional staff at 1-800-869-7219. You can also learn more about the wide variety of supplement options available at www.crystalyx.com .

Jackie Nix is an animal nutritionist with Ridley Block Operations. You can contact her at jackie.nix@ridleyinc.com.