Most of these animals eat stored forages, which can be limited in nutrients if not properly supplemented. When comparing these two opposing feeding regimes—fresh pasture versus stored forages—some of the critical nutrients affected are vitamin E and beta carotene, the precursor of vitamin A. And a lack of sunlight may result in vitamin D deficiency.
When fresh pasture is simply not an option, it's important to provide proper supplementation of fat-soluble nutrients to preserve the horses' long-term health and immunity.
HORSES ON PASTURE
For centuries, horses roamed extensive areas of pasture, their natural food source. They traveled over and were adapted to grazing on vast land areas. Unlike other grazing species, such as sheep and cattle, horses are selective and efficient grazers that primarily enjoy grass as opposed to other vegetation. Horses have been observed to prefer young, new growth rather than less-digestible mature grasses that are also less palatable and lower in nutrients. Horses also tend to prefer mixed-grass pastures to legumes and herbs, and they will often move to grazing areas, particularly in the spring, with plenty of new, young growth, regardless of the grass height or thickness.
The National Research Council (NRC) notes that horses spend about 10 to 17 hours each day grazing and that they need to graze for approximately 17 hours daily to meet their nutritional needs. A horse's gender, age and breed all influence its time spent grazing. According to the NRC, forages are the main component of horse diets.1 Whether a horse is fed solely on pasture, given stored forages in confinement or fed a mix of these two, forage and grass intake should be at least 1 percent of body weight per day.
There are some concerns associated with overgrazing lush spring pasture, including the potential for laminitis, colic or obesity in horses prone to metabolic syndrome. Horses turned out onto pasture after winter confinement may overeat because of the enhanced palatability of lush green grass. As a result, the transition to pasture should be gradual.
Because all fat-soluble vitamins are metabolized differently, each nutrient's presence in a supplementation program must be considered individually.
The natural vitamin E source found in fresh grass pasture—RRR-alpha-tocopherol, also known as d-alpha-tocopherol—is preferentially transported to and retained in tissues at higher levels than the synthetic-source vitamin E typically found in complete feeds. In horses, as in all animals, vitamin E is not stored in tissues (unlike vitamin A, which is stored in the liver).
Excess dietary vitamin E is excreted in the urine as alpha-CEHC, a water-soluble metabolite. Tissue levels of vitamin E in standardbred horses are reportedly highest in adipose tissue, followed by the liver, skeletal muscle and serum.2 Tissue levels deplete within three to seven weeks after feeding. It's preferable to give a daily supplement of vitamin E to maintain a constant level in serum, the liver and skeletal muscle.2
For horses grazing pasture, vitamin E intake is essentially continuous, maintaining optimum levels (> 4 μg/ml plasma). Once these horses are removed from pasture, it's estimated that vitamin E depletion occurs within about one month.
Vitamin E deficiency
Signs of potential vitamin E deficiencies have been extensively described in foals and adult horses. Skeletal and cardiac muscle degeneration is the most common symptom. Tongue muscles may also be affected and may interfere with normal nursing.
Vitamin E benefits
Let's take a look at how vitamin E helps horses during all life stages and performance levels.
> Mares and foals. Breeding horses benefit from vitamin E supplementation. Since mares usually foal during the winter months in the northern hemisphere, the availability of green pasture containing a lot of vitamin E is usually limited. It's beneficial to feed mares in late pregnancy and early lactation higher levels of vitamin E to ensure that levels will be adequate in the colostrum and milk. This will also help ensure that essential antibodies are passively transferred to the foals, enhancing their immune systems.
> Exercising horses. Low levels of vitamin E have been implicated as a contributing factor in exercise-induced muscle damage in horses (i.e. "tying up"). It has also been shown that exercising horses receiving a daily supplementation of 2000 international units (IU) of natural vitamin E acetate (d-alpha-tocopheryl acetate) for six weeks had higher plasma alpha-tocopherol levels pre- and post-exercise compared with horses receiving equal IUs of synthetic vitamin E acetate (dl-alpha-tocopheryl acetate).
> Horses with neurologic disease. Oxidant damage is believed to play a role in the etiology and progression of many equine neurologic diseases (i.e. equine degenerative myeloencephalopathy and equine motor neuron disease, both linked to vitamin E deficiency).
Two studies were conducted at the University of California-Davis to measure the effects of vitamin E supplementation on serum and cerebral spinal fluid alpha-tocopherol levels to investigate the benefits of vitamin E to treat neurologic disease (see "UC-Davis shows vitamin E role in treating equine neurologic disease" in the December 2009 issue of dvm360).
The first study demonstrated that daily oral administration of 10,000 IU of vitamin E as micellized d-alpha-tocopherol was able to cross the blood-brain barrier in healthy horses. This allows vitamin E to reach the target site, the brain, and potentially benefit horses with neurologic disease.
The second study reported that supplementation with natural vitamin E showed greater bioavailability than an equal amount of synthetic vitamin E. Vitamin E as water-soluble, natural micellized d-alpha-tocopherol was transferred across the blood-brain barrier more effectively than an equal quantity of synthetic vitamin E. It is now common practice to supplement horses diagnosed with neurologic disease with 10,000 IU daily as micellized, natural-source vitamin E (RRR- or d-alpha-tocopherol).
Vitamin E supplementation
The daily-recommended levels of vitamin E for horses in confinement are as follows (animals on fresh pasture have no need for supplemental vitamin E):
> foals and yearlings: 500 to 1,000 IU
> pregnant or lactating mares: 2,000 to 4,000 IU
> stallions: 1,000 to 2,000 IU
> working horses: 2,000 to 4,000 IU
> horses with neurological diseases: up to 10,000 IU daily.3
Beta carotene and vitamin A
Beta carotene is the naturally occurring feed source of vitamin A as consumed from lush green pastures (or at much lower concentrations from stored forages). A 500-kg horse on pasture consuming 1.5 to 2 percent of its body weight as forage would consume approximately 3,000 mg of natural beta carotene daily. It has been reported that horses on pasture have plasma carotene concentrations eight to 13 times higher than horses kept in stables.4
Like other fat-soluble nutrients, beta carotene must be emulsified via the action of bile acids before absorption. It can be converted in the small intestinal mucosa to retinal by the action of the mucosal enzyme 15,15'-dihyroxygenase and subsequently reduced to retinol, or it can be absorbed intact. Though there are no data to necessarily support this, beta carotene conversion appears to be inefficient in horses. After absorption, it can be converted to retinyl ester or used as beta carotene. It may then be metabolized to vitamin A (retinol), stored in the liver or passed to the tissues.
Beta carotene benefits
For horses, beta carotene is mainly recognized as a precursor to vitamin A. But it can provide additional benefits, especially to those horses not consuming adequate beta carotene from lush green pastures. Beta carotene, like vitamin E, can serve as an antioxidant and enhance the immune system. It has been shown to enable immune cells to act more efficiently by increasing lymphocyte response to mitogens and to assist helper T cells and natural killer cells. Animals with high circulating beta carotene levels also show disease resistance.5 Whether the action is due to the beta carotene-moiety or its conversion to vitamin A is still debated (see "Is beta carotene an essential nutrient?" in the October 2004 issue of dvm360).
In horses and other species, beta carotene supplementation reportedly enhances reproduction and possibly has a direct role in regulating ovarian and uterine functions. Beta carotene is found in the equine corpus luteum, but its role there is poorly understood. Fertility, especially for females, can be improved through the consumption of adequate beta carotene.
Beta carotene deficiency
Reduced beta carotene intake occurs mainly when horses do not have access to feedstuffs containing high levels of beta carotene, such as lush green grass. Confinement feeding or feeding diets low in beta carotene has been shown to reduce circulating levels of beta carotene in animals. In cattle, beta carotene deficiency has resulted in higher incidence of silent estrus, decreased conception rates, increased embryonic death, early abortion and poor-quality colostrum (see "Is beta carotene an essential nutrient?" in the October 2004 issue of dvm360).
Sufficient intake of vitamin A is also necessary to prevent night blindness, although horses are somewhat resilient to this form of deficiency. More important, this vitamin is necessary for normal growth and hematopoiesis, and horses are deficient at less than 10 μg/dl (although liver concentration is a better indication of marginal vitamin A status). Like beta carotene, vitamin A is necessary for reproduction, though the nature of the involvement has been poorly studied in horses.
It's important to emphasize that horses grazing pasture consume beta carotene, not vitamin A. Therefore, dietary intake of beta carotene is not the same as supplementation with vitamin A. Excessive supplementation of vitamin A may be harmful because, unlike vitamin E, it is stored in the liver and can become toxic.
Beta carotene and vitamin A requirements
Horses may require both beta carotene and vitamin A, though these requirements are poorly defined.1 The vitamin A requirement has been noted as:
> maintenance: 30 IU/kg
> growth: 45 IU/kg
> pregnant and lactating mares: 60 IU/kg
> exercising horses: 45 IU/kg.1
Beta carotene is referred to as a necessary antioxidant in horses, though the required amount is not clearly defined (generally in horses, 1 mg beta carotene is equivalent to approximately 400 IU of vitamin A; in pregnant mares, 1 mg beta carotene is equivalent to 555 IU and in growing horses it's equivalent to 333 IU).
Though vitamin D plays a significant role in calcium-phosphorus metabolism and therefore bone formation, equine dietary requirements are less understood for this nutrient than they are for vitamin E, beta carotene and vitamin A. Because horses can produce vitamin D systemically, there are few, if any, reports of vitamin D needs for horses, especially those with access to UV sunlight exposure. There are also limited data on the needs of vitamin D supplementation for mature adult horses in confinement and for foals and yearlings.
According to the NRC, horses in normal farm conditions that are worked regularly and exposed to sunshine probably do not need vitamin D supplementation. If horses are confined or not exposed to plenty of sunshine, or if they're fed a diet designed to produce rapid growth and bone development (such as for racing at an early age), there may be some basis for supplementing with vitamin D. Research on vitamin D requirements is not available, but information on other species indicates that 300 IU of vitamin D per 100 pounds live weight daily should be adequate to meet the horses' needs.1
Excess vitamin D supplementation may lead to soft tissue calcification. The vitamin D metabolite found in the blood, 25-OHD3, is a measurement of vitamin D status, and the active form, 1,25-(OH)2D3, is also a key to vitamin D's role in immunity.
Vitamin D requirements
For growing horses, the NRC offers estimates by age group:
> 0 to 6 months: 22.2 IU/kg
> 7 to 12 months: 17.4 IU/kg
> 13 to 18 months: 15.9 IU/kg
> 19 to 24 months: 13.7 IU/kg.1
However, the NRC concludes, "the true minimum dietary vitamin D requirement for horses exposed to sunlight has not been defined. The metabolic requirement for vitamin D is assumed to be met by exposure to sunlight. The above estimates may be useful for horses with limited exposure to sunlight and during winter months when UV light is poorly absorbed (e.g. horses maintained predominantly indoors)."1
More research is needed to further determine the optimum requirements of vitamin D for confined horses.
As we've discussed, while it may be ideal for horses to have unlimited access to nutrient-rich grass, it's not always possible. But with the proper supplementation, even those fed stored forage can maintain proper nutrient levels.
Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.
1. National Research Council. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, D.C.: The National Academies Press, 2007.
2. Ronéus BO, Hakkarainen RV, Lindholm CA, et al. Vitamin E requirements of adult Standardbred horses evaluated by tissue depletion and repletion. Equine Vet J 1986;18(1):50-58.
3. Kane E. Stuart RL, Pusterla N, et al. Influence of source and quantity of supplemental vitamin E on equine serum and cerebrospinal fluid, alpha-tocopherol and its implication for neurologic diseases, in Proceedings. Am Assoc Equine Pract 2010;56:345.
4. Ahlswede L, Konermann H. Experiences with oral and parenteral administration of beta carotene in the horse. Prakt Tierarzt 1980;61:47-52.
5. Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: The National Academies Press, 2000.