A fact sheet from A to K
Horses need vitamins to maintain many essential bodily functions and to stay healthy. Horses cannot produce all vitamins in their bodies, so they must obtain them through their diet. As in humans, vitamin requirements in horses vary by season and time of year. During the grazing season, vitamin needs differ from those in midwinter during indoor feeding. The further winter progresses, the fewer vitamins are obtained from hay and oats alone. At harvest time, up to 80% of the vitamins in forage can be destroyed, and levels continue to decline during storage by about 6–10% per month.
Fat-soluble vitamins dissolve in fat, and any excess is stored in body fat, the liver, and the kidneys. Water-soluble vitamins are not stored; they are excreted in the urine. There is no need to worry about excess intake or overdose of water-soluble vitamins— the body absorbs them as needed. The horse’s body can also recognize vitamin stores and adjust the absorption of precursors according to demand.
Water-soluble: B, C, and H vitamins
B: The B vitamins include B1, B2, B3, B5, B6, B9, and B12.
B vitamins are needed for the horse’s energy metabolism, protein synthesis, and the function of the nervous system and muscles. Sport horses and foals benefit the most from B-vitamin supplementation, as B vitamins support muscle energy production, nervous system function, and metabolism. B-vitamin supplementation is also useful for all horses during stress, illness, gastrointestinal disturbances, and antibiotic courses, when the horse’s own B-vitamin production in the gut may be disrupted. If normal microbial function is compromised, B-vitamin deficiencies must be compensated through feeding. Horses also benefit from B-vitamin supplementation during coat shedding, as hindgut production of B vitamins is often insufficient at that time. The horse’s own production of B12 requires sufficient chromium in the diet.
Folic acid, which belongs to the B-vitamin group, supports red blood cell production and protein metabolism, making it especially important for horses under heavy workload.
Sources: Good sources of B vitamins include green plants and herbs, grains, wheat bran, yeasts, seeds, leafy grass-based forages, legumes (e.g., alfalfa, fenugreek), and the horse’s own gut microbial activity. Brewer’s yeast is a versatile, natural source of B vitamins. In addition to vitamins, it contains beneficial amino acids, gut-friendly inactivated yeasts, and chromium.
Deficiency: B-vitamin deficiency often shows as reduced appetite and performance, decreased energy, a poor coat, and a generally “flat” or tired appearance. High-concentrate feeding can disrupt the horse’s own B-vitamin production in the gut.
C: Vitamin C is needed for iron utilization, absorption of other vitamins, and maintaining immune function. Vitamin C is a strong antioxidant and is also required for the body’s own collagen production. It is important during prolonged stress and illness, for weanlings, or whenever the horse’s immunity is compromised. Various bacteria and viruses can reduce the body’s ability to produce vitamin C. Vitamin C supplementation is recommended during long transports.
Sources: Vitamin C is synthesized from glucose in the horse’s liver. Pasture grass contains plenty of vitamin C. Good sources include rosehip, nettle, sea buckthorn, other berries, carrot, beetroot, yarrow, dandelion leaf, blackcurrant leaf, horsetail, angelica, and others.
Deficiency: Vitamin C deficiency may appear as reduced immunity and overall condition, with increased susceptibility to illnesses such as respiratory infections or pastern dermatitis (“mud fever”). Vitamin C deficiency halts collagen production, reducing the renewal and function of cartilage and tendons. Deficiency can also impair mucous membrane function, which may show for example as nosebleeds.
H (biotin): Biotin, also known as vitamin H and part of the B-vitamin group, is especially important for hoof health and coat condition. Biotin affects the flexibility and durability of new hoof horn as it grows, as well as the outer epithelial layer of the skin. Biotin also supports metabolism, including the utilization of fats, carbohydrates, and amino acids, and it influences reproductive functions.
Sources: Yeasts, oats, soy, whole grains, dandelion root, seeds, alfalfa, and dark green vegetables.
Deficiency: Biotin deficiency makes a horse look generally unthrifty: the coat becomes rough and tangled, the skin is poor, and mucous membranes may be dry. Diets high in concentrates can disrupt gut function and B-vitamin production.
Fat-soluble: A, D, E, K
A: Vitamin A (retinol) is important for mucous membranes, skin, and vision. It supports immunity and reproduction. Horses should be supplemented with vitamin A throughout the indoor feeding season—and also in summer for horses that do not graze. Horses receive vitamin A both as preformed vitamin A and as its precursor (carotenoids), which are converted into vitamin A in the body. Like vitamins E and C, vitamin A acts as an antioxidant that protects cells from free radicals.
Sources: Pasture grass, forages, carrot, sea buckthorn, nettle, rosehip.
Deficiency: Vitamin A deficiency impairs low-light vision and makes the coat coarse and poor in quality. It also reduces immunity and fertility.
D (D2 and D3): Vitamin D (cholecalciferol) regulates calcium and phosphorus balance in bones. It is important for bone formation and skeletal strength and supports immune function. Vitamin D is produced in the skin through sunlight exposure. Therefore, vitamin D supplementation may also be needed in summer if the horse spends a lot of time indoors or is turned out with a blanket. In winter/indoor feeding season, vitamin D supplementation is needed for all horses, especially broodmares and foals.
Sources: UV exposure via the skin (D3), fish oil products (D3), industrial concentrates and supplements (D3), alfalfa (D2).
Deficiency: Vitamin D deficiency reduces the intestinal absorption of calcium and phosphorus and can cause problems with bone mineralization. In growing foals, inadequate vitamin D intake can cause growth disturbances and weaken the skeleton. It is especially important to ensure sufficient vitamin D for horses in heavy training, foals, young horses, and broodmares.
E: Vitamin E (tocopherol) is an important antioxidant for horses. Antioxidants protect cells from oxidation and related damage. Pregnant and lactating mares, foals, young growing horses, and sport horses have the highest vitamin E needs, and adequate intake is particularly important for them. Vitamin E should be increased throughout the indoor feeding season and year-round if the horse does not have access to pasture. If the diet is high in fat, vitamin E requirements also increase. Vitamin E supports normal muscle and nervous system function and immune defense, and it also contributes to normal skin and joint function.
Sources: Pasture grass, early-cut forage, whole grains, seeds, CdGE – natural vitamin E.
Deficiency: Vitamin E deficiency can cause neurological issues, muscle problems, infertility, and reduced immunity.
K: Vitamin K supports blood clotting and bone formation. It also supports cellular metabolism. Growing foals benefit most from vitamin K supplementation, as it supports skeletal development. Vitamin K requirements may increase during illness and antibiotic treatments.
Sources: Pasture grass, forages, alfalfa, green leafy plants (e.g., thyme, nettle, dandelion, alfalfa), carrot, plant oils, and the gut’s own microbial activity.
Deficiency: Vitamin K deficiency appears as prolonged clotting time, bleeding, and anemia resulting from bleeding. Deficiency is extremely rare.
Rule of thumb:
The lowest need for additional vitamins is in horses that graze at least 2–3 hours per day. The highest need for supplementation is in intensively trained competition horses, broodmares, and growing foals during the indoor feeding season. In autumn, as horses transition from pasture back to indoor feeding, both the vitamins available and the needs for them change. Fresh pasture grass contains abundant precursors of vitamins A, E, and K.
