Equine Diet Supplements: Rational Use in Clinical Practice
ACVIM 2008
Meri Stratton-Phelps, DVM, MPVM, DACVIM, DACVN
West Sacramento, CA, USA


Supplements are frequently added to the ration of a horse. Selection of a particular supplement is made based on a perceived benefit to the health or performance of the horse. Horse owners are influenced by supplement marketing, and by the recommendations of trainers, feed store employees, and advice from friends. The veterinarian may or may not be directly involved in a client's decision to use a particular supplement.

Before a supplement is chosen for use, the clinician should be aware of current equine diet supplement industry regulations and standards for product marketing and preparation. Whenever possible, current research that supports the safety and efficacy of product ingredients should be reviewed before the supplement is used. This presentation reviews the current regulatory standards in the equine supplement industry. Current knowledge about the efficacy of organic mineral supplementation and about natural botanical supplements is discussed. A systematic plan to develop a supplementation protocol is also presented.

Regulation of Equine Diet Supplements

The equine diet supplement industry is regulated by two different organizations; the Center for Veterinary Medicine (CVM) within the Food and Drug Administration (FDA), and the Association of American Feed Control Officials (AAFCO). In an effort to maintain the safety of foods for human consumption, the FDA strictly defines foods and food additives that can be included in animal feeds based on the Federal Food Drug and Cosmetic Act (FFDCA). Foods provide nutritive value, taste and aroma, and food additives include substances that will become a component of a food, and ingredients that alter the characteristic of a food in the manufacturing, processing, packaging and storing process.1 Feed ingredients that are generally recognized as safe (GRAS) are permitted in livestock feeds. AAFCO provides a definition of approved animal feed additives in their yearly publication. This publication also lists nutrients that are considered GRAS, and lists natural plant substances that can be used as seasonings and flavorings.2 Although AAFCO lists many herbs as seasoning agents, the amount of the herb needed for flavoring is usually much lower than the amount included in a supplement for a therapeutic effect.

Within the equine feed and supplement industry, certain ingredients are thought to have properties beyond basic nutrition, and are added to a feed or supplement to alter the health of the animal, to improve growth or enhance athletic performance, or to alter the function of a specific organ system. In the field of human nutrition, the 1994 Dietary Supplement and Health Education Act (DSHEA) permits general health claims for a human supplement. Because DSHEA does not apply to the animal feed industry, any health claims made for animal supplements are not permitted. Specific claims that a product is "intended for use in the diagnosis, cure, mitigation, treatment or prevention of disease" or that the product is "intended to affect the structure or any function of the body" result in a classification of the product as a drug.1 Any product that meets the definition of a drug must go through the New Animal Drug Application through the FDA, like any pharmaceutical agent. Although this definition of a drug is clear, many products marketed as equine diet supplements actually fall within the classification of a based on marketing claims, and intended product use (i.e., joint supplements, immune enhancing products). Currently, CVM's enforcement of the regulatory standards is extremely limited. The veterinarian remains the primary source for information on the safety and efficacy of equine diet supplements.

Organic Minerals

Organic minerals are formed when the mineral bonds to an organic ligand of an amino acid, a hydrolyzed protein, propionic acid, or a polysaccharide solution. The strength of the mineral to ligand bond, and the size of the ligand affect the bioavailability of organic mineral complexes, with greater stability noted with larger ligands and covalent bonds.1 Organic mineral complexes are defined as feed ingredients by AAFCO, and are permitted feed additives.3 Organic minerals may have a higher bioavailability compared to minerals that are complexed with compounds like carbonate, chloride, sulfate, and oxide.

In horses, studies comparing organic mineral to inorganic mineral supplementation have focused almost exclusively on organic trace minerals like copper, zinc and manganese. To date, only a few equine studies have shown a beneficial effect of organic mineral supplementation. Broodmares supplemented with either inorganic trace minerals or with a combination of inorganic trace minerals and trace mineral proteinates did not show any changes in serum trace mineral concentrations. Foals born to these mares showed increased serum zinc and copper concentrations in the mares supplemented with the proteinate chelated trace minerals.4 The same authors evaluated the effect of feeding copper, manganese and zinc proteinate on growth, body weight, and hoof development in yearling horses, compared to yearlings fed inorganic supplementation of the same minerals. Only hip height gain and hoof growth was greater in yearlings fed the organic minerals.5 Siciliano et al., found that hoof wall hardness and hoof wall copper concentration decreased more in weanling horses supplemented with inorganic copper, zinc, and manganese compared to weanling horses treated with amino acid chelates of the same trace minerals.6 Hoof wall growth rate, hardness, tensile strength, and trace mineral content was not affected when mares were supplemented with trace minerals that contained 50% amino acid chelates of copper, zinc and manganese compared to 100% inorganic copper, zinc and manganese.7 Using a similar study design, Siciliano et al., discovered that mares supplemented with 50% amino acid chelates of zinc, copper and manganese did not have an increase in the liver concentration of these trace minerals, and the cellular immune response was not changed in treated horses compared to horses fed inorganic trace minerals.8 Adult horses fed 45% of a mineral supplement that contained an organic form of zinc and copper had a lower copper digestibility and copper balance and a lower zinc balance compared to horses fed only an inorganic source of these trace minerals.9 In the area of organic macro mineral supplementation, calcium-amino acid proteinate had similar bioavailability compared to calcium carbonate when fed to Quarter Horse yearlings.10

Selenium yeast has recently been added to the list of tentative (versus official) feed ingredients recognized by AAFCO, and is approved by the FDA as an additive to equine complete feeds, not to exceed 0.3 parts per million added selenium. Selenium yeast appears to have a greater bioavailability in some large animal species (ruminants, swine), and may have an improved digestibility in horses, although additional research on selenium yeast still needs to be completed in horses.11-12

Based on the inconsistent beneficial results of organic mineral supplementation, at this time a significant health benefit cannot be guaranteed when horses are fed organic minerals. Additional studies in the future may support the use of organic minerals and selenium yeast over inorganic minerals, but at this time there is insufficient evidence to require organic mineral supplementation in equine feeds and diet supplements.

Botanical Diet Supplements

Botanical supplementation for the horse has recently gained popularity as eastern medical therapies have blended with more traditional western pharmaceutical-based medicine. Herbalists support the use of more natural substances to manage and prevent disease. Herbal product therapeutic claims for treatment and prevention of disease are not permitted, based on current FDA regulations. The current AAFCO regulatory guideline permits the use of a wide variety of herbs in animal feeds for the purpose of flavoring. To achieve a therapeutic health effect, a higher concentration of an herb is likely required, and the herb no longer meets the AAFCO definition of a feed additive. It is the responsibility of both the veterinarian and client to ensure that any botanical products are safe and efficacious before they are used in the ration of a horse.

Owners can purchase individual herbs, or can select herbal blends that are designed and marketed for use with different equine disease conditions. Botanical ingredients are often added to more traditional organ specific diet supplements to enhance the performance of the supplement. It is challenging to determine the efficacy and safety of herbal products because minimal research has been performed in any animal species. Often, the only information that can be used in making a decision to use a particular botanical ingredient for a horse is an anecdotal, historical report of efficacy. In some cases, a practitioner familiar with herbal therapeutics can be consulted to determine an appropriate treatment protocol for the horse. Even equine herbalists use anecdotal results and personal experience to establish herbal drug therapies.

After the physical examination and ration evaluation have been completed, the veterinarian can discuss botanical supplement use with the horse owner. During the discussion the client should be questioned about why they want to treat their horse with a particular herb or herbal supplement. The owner's decision may not be medically justified, and may result in harm to the horse. If the horse is actively competing in athletic events, the client must be made aware that many dietary herbs will cause a positive drug test. Some herbs that may not be used during competition in the United States include valerian, lavender, devil's claw, comfrey, chamomile and cayenne.13 This list is not complete, and the veterinarian should ensure that any herb used in a performance horse is evaluated to confirm that it is allowed during competition. Safe withdrawal times before competition have not been established for herbal products.

Contraindications to Herbal Supplement Use

Herbal supplements should not be administered to pregnant or lactating mares because no data on supplement safety has been established. This restricted use recommendation is extrapolated directly from herbal supplement recommendations in women who are pregnant or nursing. Certain herbs (burdock, black cohash, tumeric, yarrow, rosemary) stimulate uterine tissue, comfrey is teratogenic, and other herbs have abortifacient actions (sage, fenugreek).13 The author does not recommend use of herbal supplements in growing foals because of the lack of safety data for growing horses. If the horse has a medical problem like chronic colic, or hepatic disease, botanical products should be used with caution. If the horse is being treated with any pharmaceuticals, nutrient-drug interactions between herbal substances and the drug must be researched before herbal therapy is initiated. Immunosuppressive therapy may be antagonized by herbs that act as immunostimulants (Echinacea spp., astragalus).14 Herbs that act as diuretics (juniper, dandelion, uva ursi) may increase nonsteroidal anti-inflammatory drug toxicity.14

Herbal Ingredient References

Once an herbal product or supplement mixture is selected, safety and efficacy of the ingredients should be determined. Data often must be extrapolated from other animal species due to the lack of research studies in horses. Selecting an appropriate dose of an herb or herbal supplement is challenging because therapeutic studies have not been performed in horses. Information about the use of different herbs can be found in textbooks including Mosby's Handbook of Herbs & Natural Supplements (Skidmore-Roth), Veterinary Herbal Medicine (Wynn and Fougere), and in the PDR for Herbal Medicines, 4th Edition. On line resources include the International Bibliographic Information on Dietary Supplements (IBIDS) Database (http://grande.nal.usda.gov/ibids/index.php), the National Institutes of Health, Office of Dietary Supplements (http://dietary-supplements.info.nih.gov/), ConsumerLab (www.consumerlab.com), and Quackwatch (www.quackwatch.com).

Selection of Diet Supplements

A wide variety of diet supplements are marketed for horses. The spectrum of products ranges from basic vitamins and minerals to gastroprotectants, joint supplements, and mixtures of botanical herbs. Product selection by a client is often based on recommendations from trainers, feed store employees, and on marketing campaigns from the supplement industry. In many cases, the chosen products may not be ideal for the horse. Although clients often want to manage the supplement program of their horse with multiple products to solve numerous health or performance issues, the choice of products is often random, resulting in a suboptimal diet for the horse. The veterinarian is responsible for educating their client about the different types of supplements, and about the appropriate use of the different products. The veterinarian may not be included in the initial decision to start a supplement program, and may only learn about the therapy during a routine examination. Because some supplements are contraindicated in certain horses, and under certain conditions, the equine veterinarian should always be included when equine diet supplements are used. The following guidelines can be followed to develop a supplementation program.

Types of Diet Supplements

Equine diet supplements can be broadly categorized into three different groups. The first group includes vitamins, minerals, and vitamin and mineral mix products. The second group of supplements includes non-herbal products designed to enhance the health of specific organ systems (joint, gastrointestinal, hoof, etc.). The third group consists of herbal products that are formulated to manage a particular organ system. Some horses will benefit from one type of supplement, while others may show improved health when they are treated with a combination of supplement products.

Evaluate the Health of the Horse

The first step in the supplement selection process is to evaluate the medical history and clinical health of the horse. Botanical supplementation in horses with a history of colic, and horses with hepatic disease should be approached with great caution. Herbs that interact with pharmacologic agents may be contraindicated. Pregnant mares, lactating mares, and young growing foals should not be fed herbal supplements, or products that contain herbal supplements because the safety of the herbs has not been tested in horses, or other animals (including humans). Herbal supplements should also be used with caution, if at all, in horses that compete in athletic events to avoid positive drug testing and disqualification from the competition.

Evaluate the Ration

The second step in the supplement selection process is to review the horse's diet history and current ration. Different types of forages, consumed as pasture grass or hay, contain variable concentrations of essential nutrients. Forage analysis is the most accurate way to evaluate the ration. Equi-Analytical Laboratories (http://www.equi-analytical.com/) performs a comprehensive analysis of forage for as little as $30.00 and is useful for clients that purchase a large volume of hay. If forage analysis is not practical for a client, then a nutrient database can be used to estimate the nutrient profile of a forage. Horses that only consume pasture grass or hay may benefit from supplementation with a basic vitamin and mineral product to ensure that their nutrient requirements are met. Trace mineral blocks often do not provide a consistent concentration of nutrients due to variable intake by individual horses. Supplementation can be ensured when a daily dose of a vitamin and mineral product is added to the ration.

Supplemental commercial feeds that are added to the ration often contain vitamins, minerals and protein. Cereal grains, by-product feeds (bran, beet pulp), oil, and protein supplements also add nutrients to the ration. Horses fed a mixed ration of forage and other feeds may not require supplementation with a vitamin and mineral product. Singular nutrients like vitamin E or other antioxidants may benefit horses consuming a mixed ration. Over supplementation with fat soluble vitamins and trace minerals should be avoided. Selenium is especially toxic to horses, and should not be included in the ration in a concentration that exceeds 2 mg of selenium per kilogram of the diet on a dry matter basis. The 2007 Horse NRC serves as a reference for daily nutrient requirements.

Develop a Supplementation Program

The first priority is to ensure a balanced ration that meets the nutrient requirements for the life stage or athletic activity level is fed to the horse. A nutritionist can be consulted for assistance in balancing a ration for horses that have elevated nutrient requirements (horses in moderate to very heavy work, reproductively active mares), adult horses with medical problems, and foals with developmental orthopedic disease.

Once basic nutrient requirements have been met, additional supplementation with organ-specific products can be considered. All supplements should be evaluated to see if they will meet the goal of product use. Supplement ingredients should be reviewed for safety and efficacy before they are administered to a horse. Many supplements contain ingredients that are neither GRAS nor approved feed additives, but that appear to be safe based on studies in other species. Products that contain ingredients with questionable efficacy and safety, and that may cause an adverse interaction with pharmaceutical agents, should be avoided.

Most equine supplements do not have any scientific literature to demonstrate their efficacy or safety, and anecdotal evidence to support use of the product in a particular animal will need to be made based on the horse's response to the supplement. A trial period of 2-3 months is a reasonable time to evaluate the efficacy of a product. If a positive response has not been observed within this time, then the supplement should be discontinued. A different product can be used with a similar treatment protocol if the veterinarian and horse owner agree to test a new supplement. If at any time during treatment the horse develops any health complications, the supplement should be discontinued and re-evaluated once the medical problem has resolved.


1.  Nutrient Requirements of Horses, 6th ed., 2007, p.183.

2.  AAFCO (Association of American Feed Control Officials), 2007. Official Publication, p. 393.

3.  AAFCO, 2007. Official Publication, p. 307.

4.  Ott E, et al. J Eq Vet Sci 1994;14(2)93.

5.  Ott E, et al. J Eq Vet Sci 2001;21(6)287.

6.  Siciliano P, et al. Proc 18th Eq Nutr Physiol Soc Symp 2003, 96.

7.  Siciliano P, et al. Proc 17th Eq Nutr Physiol Soc Symp 2001, 143.

8.  Siciliano P, et al. Proc 17th Eq Nutr Physiol Soc Symp 2001, 419.

9.  Baker L, et al. Proc 19th Eq Nutr Physiol Soc Symp 2005, 162.

10. Highfill J, et al. Proc 19th Eq Nutr Physiol Soc Symp 2005, 37.

11. Pagan J, et al. Proc 16th Eq Nutr Physiol Soc Symp 1999, 135.

12. Richardson S, et al. J Anim Sci 2006;84:1742.

13. Wynn S, Fougere B. Veterinary Herbal Medicine, Mosby, 2007, 411.

14. Poppenga R. Vet Clin North Am: Eq Practice 2001;17(3)455.

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West Sacramento, CA

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