The EQUUS Vitamin Guide

Here are the vital statistics on the must-have vitamins that fuel a variety of your horse's body functions. By Christine Barakat for EQUUS magazine.

For all the attention they receive, carbohydrates, protein and fats may seem to be the most important ingredients of a horse’s diet. But vitamins also play a vital role in a wide range of his body processes.

These organic compounds are readily available from fresh and dried forage and, to a lesser extent, grains. As a result, most pleasure horses can meet their daily vitamin needs by consuming good-quality grass or hay. On the other hand, broodmares, youngsters, elite equine athletes such as racehorses, and horses recovering from illness or experiencing stress often require more vitamins than even the best hay can provide.

For these horses, a commercial feed formulated for the appropriate stage of life or activity level will typically provide the needed additional vitamins, each in the appropriate amount. Another option is targeted supplementation, adding a specific vitamin formulation to the daily ration.

To minimize the risk of inducing a potentially harmful imbalance or excess, learn the nutrient values of your horse’s current forage and feed before choosing a supplement. Adding one to a fortified grain ration, for instance, may double the quantity of a particular vitamin and/or interfere with the availability of other nutrients the body requires. Feeding several supplements simultaneously can pose the same risks. When making any changes to your horse’s ration, consult your veterinarian or an equine nutritionist.

VITAMIN GUIDE Vitamin AWhat it does:

  • chief component of the light-sensitive pigment in the rods of the retina; important to vision, particularly at night
  • facilitates cell differentiation and regeneration, making it critical to growth and healing


  • produced in the body from carotene pigments acquired from green forage and stored in fat cells; excess vitamin A is kept in the liver for future use. Also contained in most commercial feed mixes.

Imbalance risks:

  • Deficiency – very rare in mature horses fed adequate forage. Signs: Foals deficient at birth may be weak, slow to grow and/or experience night blindness.
  • Excess – occurs only through oversupplementation; pasture and hay cannot supply the toxic level. Signs: dull coat, fragile bones, poor liver and kidney function; disfigured foals in mares fed an excess during pregnancy

Vitamin B1 (thiamin)What it does:

  • used to metabolize carbohydrates and extract energy from the diet


  • produced by gastrointestinal bacteria (microflora) but in an amount insufficient to meet a horse’s needs. Fresh and dried green leaves and cereal grains with husks supply the balance.

Imbalance risks:

  • Deficiency – unlikely in horses fed good forage; however, intestinal disease or parasites can interfere with the vitamin’s synthesis in the gut. Some toxic plants, such as bracken fern, can inhibit absorption. Signs: lethargy, loss of appetite, nervousness
  • Excess – nearly always due to injections given to boost energy in performance horses. Signs: excitability; in extreme cases, labored breathing, convulsions

Vitamin C (ascorbic acid)What it does:

  • facilitates the synthesis of collagen, the primary component of connective tissueSource:
  • manufactured in the liver from glucose. Ingested vitamin C is broken down by gastrointestinal bacteria before reaching the bloodstream.

Imbalance risks:

  • Deficiency – very unlikely since the body controls production
  • Excess – Toxicity through supplementation has not been documented. However, recent studies raise the concern that prolonged or unnecessary supplementation may cause a horse’s body to stop production. Then, if supplementation is stopped, he could become deficient.

Vitamin DWhat it does:

  • Both forms–D2 and D3–bind to calcium and magnesium to maintain electrolyte balance in bone formation.
  • regulates the excretion of phosphates in urine


  • synthesized in the body when the sun’s ultraviolet rays combine with a form of cholesterol in the skin. Also provided by good hay.

Imbalance risks:

  • Deficiency – seen only when induced experimentally. Signs: diminished bone density, swollen joints, difficulty in moving
  • Excess – occurs only through oversupplementation. Signs: calcification of heart muscle and other soft tissues

Vitamin EWhat it does:

  • works with the mineral selenium to counteract the potentially harmful effects of oxygen by-products (free radicals) produced during normal cellular metabolism


  • growing forages, particularly alfalfa, timothy and Kentucky bluegrass; grains contain only small amounts.

Imbalance risks:

  • Deficiency – Recent studies suggest that deficiency results from an inability to absorb vitamin E, rather than inadequate intake. Signs: In young horses, rapid degeneration of cardiac and skeletal muscles. In adult horses, certain muscle disorders, such as equine motor neuron disease, have been linked with relative deficiency.
  • Excess – has not been observed.

Vitamin H (biotin)What it does:

  • helps to synthesize fats, proteins and glucose within the body
  • studies show that it improves hoof and hair quality, although its mechanism for doing so is unproven.Source:
  • synthesized by gastrointestinal bacteria. Small amount also found in plants.

Imbalance risks:

  • Deficiency – has not been reported. Although analysis has shown that horses with shelly hooves do not seem to have lower than normal vitamin H levels, studies have found that daily supplementation of 10 to 20 milligrams can improve poor-quality hooves over several months.
  • Excess – Toxicity has not been reported.

Vitamin KWhat it does:

  • necessary for blood clotting and the utilization of several proteins throughout the body


  • K1 is found in fresh and dried green, leafy plants. K2 is synthesized by gastrointestinal bacteria. A synthetic form, K3 (menadione), is often included in vitamin supplements.

Imbalance risks:

  • Deficiency – occurs only when the gut cannot synthesize K or liver damage makes it impossible for the body to absorb it. Some drugs can interfere with vitamin K production. Signs: internal and external hemorrhaging
  • Excess – Overdose occurs only as a result of injecting K3 when a horse is not deficient. Signs: renal damage, laminitis, death within 12 hours

More About B These seven vitamins of the B complex are of minimal significance in your horse’s diet because the body makes its own. There are no reports of adverse physical effects caused by their deficiency or excess. Still, you’re likely to see them on feed and supplement labels. Here’s what they do and where they’re naturally found.

choline–component of cell membranes; necessary for the transmission of nerve impulses; helps to metabolize fat. Abundant in forages and grains.

cobalamin (B12)–essential for the production of red blood cells. Produced by gastrointestinal bacteria; stored for weeks or months in the liver to compensate for any disruption in production.

folicin (B9, folic acid)–involved in cellular metabolism and red blood cell formation. Abundant in grass; also created by gastrointestinal bacteria.

niacin (B3)–used by every cell in the body for metabolism and DNA repair. Found in forage; synthesized in the intestinal tract from the amino acid tryptophan.

pantothenic acid (B5)–used in general metabolism. Found in forage and grain; also produced by gastrointestinal bacteria.

pyridoxine (B6)–used by enzymes to metabolize protein, fats and carbohydrates; contributes to red blood cell formation. Found in forages and grains; also produced by gastrointestinal bacteria.

riboflavin (B2)–used to convert feed into energy. Found in hay and grain; synthesized in the small intestine by gastrointestinal bacteria.

This article originally appeared in the May 2007 issue of EQUUS magazine.




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