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Soy for Horses: Facts and Fallacies

Filed under: Health & Training |     

Clydesdale horse gets loose from field and is found in nearby a field eating soybean crop. Photo by Carol Hamilton courtesy of AdobeStock.

From Kentucky Equine Research:

Soybeans and soybean byproducts are commonly used as ingredients in horse feeds. Whole soybeans are legume seeds primarily used as a source of protein and fat in horse feeds. Soy oil is a palatable and readily available source of fat in horse feeds and is sometimes top-dressed to increase fat and calories in a horse’s diet. Soybean meal, which is a byproduct of oil extraction, is the most common protein source in animal feeds, including horse products. Soy hulls are another byproduct of soy oil extraction and are a valuable source of highly fermentable fibers for animals with the capacity for microbial fermentation, including horses and ruminants.

Whole soybeans are an excellent source of high-quality protein and fat for horses. The protein provided by soybeans is particularly high in the limiting essential amino acid lysine and contains favorable concentrations of other essential amino acids, which makes it a particularly high-quality plant source of protein for monogastric animals. However, raw soybeans should never be fed to horses because they contain a protein that inhibits the action of trypsin, an enzyme that initiates the breakdown of proteins into smaller molecules. Soybeans should be roasted or otherwise cooked before feeding so the trypsin inhibitor is denatured and triggers no ill effects on the digestion process.

Soy oil is often included in the diets of horses, as fat is a dense source of calories, providing more than double the calories per pound than carbohydrates or protein. Research in horses has reported several benefits of fat supplementation in horse rations beyond an increase in caloric density, such as providing a fuel source for aerobic activity (and thus exhibiting a glycogen-sparing effect for hard-working horses), reducing the glucose/insulin response of a meal compared to feeding similar calories from nonstructural carbohydrates, and supplying essential fatty acids, including omega-3 and omega-6 fatty acids. Compared to other plant oils, soy oil provides a moderate ratio of omega-6 to omega-3 fatty acids; more favorable than corn oil, but possibly less than canola oil. Soy oil is typically found in commercial horse feeds.

Soybean meal is a major protein source used in horse feeds. Palatable to most horses, soybean meal contains a high concentration of protein (typically 43-53% as fed) and provides a high-quality source of essential amino acids, including lysine, tryptophane, threonine, and isoleucine, which are typically low or nil in cereal grains. Soybean meal is generally considered the highest quality source of plant protein in meeting amino acid requirements in horses.

Soy hulls, the skin covering of soybeans, are high in pectin and other digestible fibers, and low in nonstructural carbohydrates (NSC). Soy hulls are readily fermented by microbes in a horse’s hindgut, and have been shown to provide more digestible energy than forages, and only slightly less digestible energy than cereal grains such as oats and barley. Since soy hulls contain few NSC and are primarily digested in the hindgut, the use of soy hulls in horse diets poses little risk of colic or laminitis. Soy hulls are usually quite palatable for horses, although some decrease in palatability has been seen with higher inclusion rates of dietary soy hulls.

Q&A: Concerns Associated with Feeding Soy to Horses

Are soybeans genetically modified?

More than 93% of soybeans grown in the U.S. are genetically modified (GM), and more than 80% of soybean varieties worldwide are GM. The most common genetic modification of soybeans is the resistance to the herbicide glyphosate, which is commonly used to decrease weeds and increase crop yields. The typical practice for soybean producers in the U.S. is to apply herbicide early in the soy growth phase, so weeds will be killed and, as the soybean plants grow, the leaves will spread and block light to potential new weed growth, thus reducing the total amount of herbicide needed for optimal crop production. By the time the soybeans are mature and harvested, the herbicides used in early growth are no longer present.

Are GM soybeans and soy products safe for horses?

According to research studies, there is no difference in how genetically modified and nonmodified feedstuffs affect the health and safety of animals. The new genetic material introduced to soybean plants (initially developed in 1985) to make them resistant to glyphosate has not been shown to affect digestibility of soy ingredients, and was found to be nontoxic to mice even when fed at 1,000 times the normal amount. There is a lack of research in horses comparing GM to non-GM feed ingredients currently, but there is no evidence to suggest that feeding GM soy ingredients is in any way harmful to horses.

What are phytoestrogens?

Phytoestrogens are plant-based compounds that are chemically similar to the hormone estrogen and that can potentially bind to estrogen receptors in the body. Interestingly, various phytoestrogens are estrogen antagonists (block the action of estrogen) or estrogen agonists (enhance the action of estrogen). Many plants contain phytoestrogens, including soybeans, alfalfa, clovers, and flax.

In humans, phytoestrogens have some positive effects, such as helping reduce and prevent certain types of cancer, and can be good for supporting cardiac health. However, there are some detrimental effects, including adversely affecting fertility and causing hormonal issues.

The highest concentration of phytoestrogens in soy ingredients is found in soybean meal. Soy oil and soy hulls contain very little phytoestrogen, and should not be of any concern to horse owners. While soybean meal contains higher levels of phytoestrogens, the amount of dietary soybean meal is typically a very small percentage of a horse’s total diet and therefore will contribute a fairly insignificant amount of phytoestrogens. Broodmares provided alfalfa as their primary source of forage will consume significantly more phytoestrogens than that possibly provided by soybean meal as a source of protein in a feed or ration balancer.

There have been few studies on the effects of dietary phytoestrogens in horses, other than research indicating that phytoestrogens are absorbed in the digestive tract and can be measured in the blood. At this time, there is no indication that phytoestrogens in soy products have any effect on the fertility of broodmares or any other negative effects on horses.

Is soy allergy or sensitivity common?

Horse owners are sometimes concerned that their horses are allergic or have a sensitivity to dietary soy. While soy is a common allergen in humans and dogs, soybeans and soy ingredients typically do not elicit allergic responses in horses. Food allergies, which are uncommon in horses overall, are triggered by an immune response to a dietary protein. The typical symptoms of allergies include hives and itching in horses. Food sensitivity or intolerance is triggered by improper digestion, and symptoms are usually related to the gastrointestinal tract (such as excess gas, diarrhea, or colic).

Food allergies are often diagnosed by serum allergy testing (SAT), but research has shown that SAT commonly yields many false positives for food allergens, and is not reliable nor accurate for diagnosis of food allergies. Intradermal skin testing (IST) is the “gold standard” for diagnosis of skin hypersensitivities but has several limitations in horses that make such testing difficult. IST is difficult to perform in horses that are sensitive to needles, particularly in the field, and purified allergen extracts are difficult to obtain. Further, IST is not suitable for the diagnosis of food allergies.

To diagnose food allergies (or potential food sensitivities) most accurately in horses, an elimination diet is recommended. To perform an elimination diet, all suspected feeds and supplements must be removed from the ration, and if symptoms resolve within 4-8 weeks, then a food allergy or sensitivity may be present. To determine which ingredient is the allergen, grains and supplements should be slowly added back one at a time (over 3-6 weeks) until clinical signs reappear. If clinical signs do not reappear, then the original condition was not due to dietary ingredients.

Does soy increase inflammation when fed to horses?

In recent years, much attention has been devoted to the fatty acid composition of various fat sources commonly fed to horses. One of the primary concerns is related to the effects of the essential fatty acids linoleic acid (LA), an omega-6 fatty acid, and alpha-linolenic acid (ALA), an omega-3 fatty acid. Both omega-6 and omega-3 fatty acids have roles in the body’s immune response in mediating inflammation.

Omega fatty acids are a type of polyunsaturated fatty acid (PUFA) that have a chemical similarity of more than one double bond between carbon atoms, compared to “saturated fats” which contain no double bonds. Both LA and ALA are PUFAs. However, LA is metabolized in the body to produce arachidonic acid (AA), which is then further metabolized to prostaglandin E2 and other potent mediators of pain and inflammation. ALA is metabolized to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are important mediators of inflammation.

In general, omega-6 fatty acids are portrayed as “inflammatory,” while omega-3 fatty acids are considered “anti-inflammatory,” but the physiologic reality is not so simple. In the cascade of chemical reactions to reach biologically active compounds, multiple enzymes are shared in both pathways and the competition for these enzymes can affect which fatty acids impact body functions.

While the ratio of dietary omega-6 to omega-3 fatty acids is important, ideal ratios for all species have not been established. In humans, ratios of 2:1 to 10:1 have been suggested as adequate, depending on the medical conditions evaluated. Fatty acid ratios in horses have not been established, but deficiencies have not been reported at this time. While forages, the base of most horse diets, contain relatively low amounts of fat, the fatty acid content of that fat is significantly higher in omega-3s than omega-6s. Since most horses consume significant amounts of forage, the ratio of fatty acids is not problematic. For horses that require large amounts of feed, such as intensely exercising performance horses, the fats in many feed ingredients and oils, such as corn oil, are much higher in omega-6 fatty acids, so there is concern among horse owners that very high dietary omega-6s may contribute to medical conditions associated with chronic inflammation.

To maintain the balance of omega-6 and omega-3 fatty acids, much attention has been devoted to minimizing dietary omega-6s and supplementing omega-3s. Soy oil has been suggested as a feed ingredient that may contribute to inflammation, as it contains more omega-6s than omega-3s (7.5:1 ratio). However, horse owners must keep in mind that omega-6 fatty acids are also essential for optimal function of the immune system. Further, the total dietary amounts of PUFAs, including omega-3s and omega-6s is important, not just a ratio. The amount of soy oil typically found in feeds when compared to the total PUFA intake is usually appropriate to maintain an appropriate balance of fatty acids in horses. Soy oil is considered a moderate source of omega-3 fatty acids, particularly compared to other typical plant oils found in horse rations, such as corn oil (44:1 ratio of omega-6s to omega-3s).

If a horse is suffering from a medical condition associated with chronic inflammation, such as osteoarthritis or inflammation of the gastrointestinal tract, research suggests that supplementation with omega-3 fatty acids may be beneficial. In these situations, marine-derived oils are the only direct sources of EPA and DHA, and these would be the most potent, effective sources of long-chain omega-3 fatty acids available.

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