- And what does this mean for horses’ vitamin supplementation?
In Finland, the need for vitamin supplementation in horses varies by season. Horses obtain plenty of vitamins A and E from pasture grass. In spring and summer, vitamin D is formed in the horse’s skin through exposure to sunlight, but during the dark winter months—and in blanketed horses—this does not occur. The vitamin content of cut hay decreases during storage, and the longer winter progresses and the closer spring and the new grazing season approach, the fewer essential vitamins remain in the hay. For this reason, during the indoor feeding season horses require vitamin supplementation, of which vitamins A, D, and E are the most important.
Where do the vitamins in hay go?
Storing hay over the winter inevitably affects its vitamin content. Protein and mineral levels are not affected, but vitamins—especially vitamins E and A—are significantly lost during winter storage.
Fresh green grass contains large amounts of vitamins A and E, but a substantial proportion of these vitamins is already lost during haymaking. After haymaking, the carotenoid content of hay decreases on average by about 7% per month of storage. This is an important factor to consider especially when feeding year-old hay. Early-harvested hay contains more carotenoids (precursors of vitamin A) than later-harvested, stemmier hay. Green-colored hay contains more carotenoids than yellowish hay.
Weather conditions at harvest also affect hay’s nutrient content: rain and wetting reduce vitamin levels. Drying hay in sunny conditions reduces vitamins A and E but increases vitamin D content. In addition, vitamin E in hay is destroyed by moisture and oxygen. For this reason, all horses that do not receive fresh green forage daily should have vitamin E added to their diet. A good rule of thumb for vitamin A content in hay is: the greener the hay, the more carotenoids it contains; the yellower the hay, the fewer carotenoids it contains.
Vitamin supplementation during the indoor feeding season: remember at least A, D, and E
The greatest need for vitamin supplementation is in competition horses, broodmares, and growing foals. The lowest need is in horses that graze at least 2–3 hours per day. During winter, horses whose diet consists mainly of forage, grain, and basic mineral supplements always require additional vitamins. Horses in heavy work need more vitamin supplementation than horses at rest.
Vitamins are divided into fat-soluble and water-soluble vitamins based on their solubility. In winter feeding, the fat-soluble vitamins A, D, and E are particularly important until the horse returns to pasture or receives hay from the new harvest. During summer, horses can store some of these vitamins in their bodies. As winter progresses, these stored vitamins are depleted within a few months. At the same time, forage vitamin content declines and sunlight is insufficient for vitamin D synthesis, making it necessary to add these vitamins to the daily diet. The required amount depends on the vitamin content of any compound feeds being fed.
Water-soluble vitamins include the B vitamins and vitamin C. These do not accumulate significantly in the body and therefore must be obtained daily from the diet. Horses also produce these vitamins themselves through their intestinal microflora. Excess intake of water-soluble vitamins—even from synthetic sources—generally does not cause adverse effects, because they are not stored in large amounts in the body, unlike fat-soluble vitamins. Horses can generally synthesize vitamins C, K, and B from their feed. Grains, bran, and yeast are good sources of B vitamins. Vitamin C is synthesized in the liver and stored in many tissues.
Different vitamins behave differently in the body. The behavior of water-soluble vitamin C is particularly well understood. As intake exceeds requirements, intestinal absorption decreases. For example, about 100 mg of dietary vitamin C is absorbed almost completely, but of a tenfold dose only about half is absorbed, and of a hundredfold dose only 10–20%.
The most common mistakes in vitamin feeding
Mistakes in equine vitamin feeding often result from either excessive use of vitamin supplements or poor-quality feed. Many compound feeds and industrial mineral mixes already contain added vitamins, so it is important to avoid overlap from multiple feeds and supplements to prevent overdosing.
In plants—the horse’s natural diet—vitamins always occur as precursors. These precursors are converted into vitamins in the horse’s body only if there is a deficiency. Synthetic vitamins, by contrast, are absorbed fully in the digestive tract, unlike precursors, which are converted only as needed. There is no risk of overdosing from natural vitamins obtained directly from feed.
In Finland, horses most commonly suffer from deficiencies of fat-soluble vitamins, especially vitamins D and E. Adequate intake of these vitamins is particularly important for sport horses and growing foals, as insufficient vitamin D can cause growth disturbances and weaken the skeleton. Vitamin E plays a significant role in many basic bodily functions, such as immune defense and the functioning of the nervous system and muscles. The greatest need for vitamin E is in pregnant and lactating mares, foals and young horses, and sport horses. It should also be noted that feeding vegetable oils increases the horse’s requirement for vitamin E. In addition, competition horses encounter many stressful situations that can impair the utilization of water-soluble vitamins as well.
Separate vitamin supplements are especially necessary if the normal state of the horse’s gut microbiota is disturbed, for example due to stress, medication, or diarrhea. Gut microbial activity—and thus vitamin synthesis—can be supported by feeding high-quality forage and avoiding concentrate-heavy diets.
Weaned foals may also benefit from vitamin K supplementation during their first indoor feeding season, as vitamin K supports skeletal development. In sport horses, supplementation with B vitamins may be justified because they are involved in energy metabolism. B vitamins may also be given during coat shedding. Horses with hoof or coat quality issues may benefit from additional vitamin H (biotin).
With synthetic fat-soluble vitamins in particular, there is a toxicity threshold—excessive intake can cause poisoning. However, deficiencies are far more common. Even so, all vitamins are safest when provided in moderate amounts in line with recommendations.
Sources:
- The Horse: Nutrition Loss in Stored Hay
- The Northwest Horse Source: Hay Is Missing Key Nutrients
- Mid-Rivers Equine Centre: Vitamin Loss in Cured Hay
