The Role of Macronutrients in Equine Nutrition

Before creating a ration, it is essential to know the nutrient requirements for the type of animal you are feeding and the role of each nutrient class in the diet. Horses and other equids are hindgut fermenters, meaning they have different anatomy and physiology from both monogastrics and ruminants, which is explained in depth in the fact sheet, “The Gastrointestinal Tract of Horses.” As with other classes of livestock, macronutrients make up the majority of the nutrients in equid diets.

Macronutrients include water, carbohydrates, proteins, and lipids which are often referred to as fats. The following sections outline the basics of carbohydrates, proteins, and fats and their functions. Water is outside the scope of this fact sheet and will be detailed in a forthcoming article.

Carbohydrates

Carbohydrates are the main source of energy and calories for herbivores. Energy itself is not a nutrient, but it is required for maintenance of the horse—movement, breathing, maintaining body condition, digestion, blood circulation, and many other bodily functions. Certain classes of horses, such as lactating or late gestation mares or horses performing hard work, need more energy. Carbohydrates can be used to help meet the energy needs of the animal.

Carbohydrates can be separated into two categories: structural and nonstructural. Structural carbohydrates are fibers found in plants. They give the plant its shape and support upright growth. Nonstructural carbohydrates are the contents of the plant that are within the confines of the structural carbohydrates, such as sugars and starches. Another way to classify carbohydrates is as fiber or nonfiber carbohydrates because some structural molecules in plants do not classify as fiber.

Knowing the fiber and nonfiber fractions (Figure 4) of the diet can be the starting point to understanding carbohydrates in horse diets. Fiber includes cellulose, hemicellulose, and lignin. Lignin is considered an indigestible structural component and is measured in laboratory feed analyses. Cellulose and hemicellulose are the usable fiber fractions that are broken down in the large intestine. Horses, as hindgut fermenters, have large populations of microbiota in their cecum and colon that break down fiber for microbial use. for their own use. The byproducts of microbial fiber digestion are volatile fatty acids—specifically acetic, propionic, and butyric acids—that can be used as energy by the horse.

Nonfiber carbohydrates are mainly sugars and starches which, compared to fiber, are more readily digestible for use as energy substrates (Figure 1). Sugars are classified by molecule size and structure. Glucose, a monosaccharide, is the sugar that is used most for energy. Starches are categorized into two different classes: amylose (straight-chain starch) and amylopectin (branched-chain starch). Amylose can be digested by enzymes in the small intestine, while amylopectin digestion requires enzymes produced by microbes. Regardless of the starch type, the final product is glucose which is used for energy.

Proteins

Protein plays many roles in the body. Muscle development and growth are likely the first activities that come to mind. Proteins also act as enzymes and as some hormones. Enzymes are used in digestion and regulate a variety of metabolic reactions. Protein-based hormones have a wide variety of functions, including regulating growth and reproductive processes. While excess amino acids can be converted to energy, this is inefficient and increases nitrogen excretion as urea via urine. This can lead to increased ammonia levels in situations where horses are housed indoors, such as in stalls.

Mammalian proteins are made from 21 amino acids. There are nine essential, or indispensable, amino acids that must be provided in feeds because they cannot be adequately synthesized from existing molecules in the body. Another class of amino acids is nonessential, or dispensable. Nonessential amino acids do not need to be supplied in the diet because sufficient amounts can be made in the body. The third class of amino acids is conditionally indispensable. Under regular maintenance conditions, these amino acids are made in the body in adequate amounts, but during times of stress or sickness, they also need to be supplied in the diet.

Protein is reported in feedstuffs and on feed tags as crude protein (CP). Crude protein is the calculated estimate of protein within a feed based on the nitrogen content of the feed. This calculation is used because protein is the only macronutrient containing nitrogen. Protein is about 16% nitrogen on average; therefore, 1 gram (0.04 ounces) of nitrogen in a sample translates to about 6.25 grams (0.22 ounces) of protein.

Carnivores and omnivores consume meat or other animal-derived products to meet most of their protein needs. Herbivores, like the horse, consume plants for protein. Feeds such as soybean meal (Figure 2) or corn gluten meal can increase the CP level in the diet, provide high quality protein, and are excellent sources of indispensable amino acids.

Lipids or Fats

Lipids are a category of feedstuffs that include fats, oils, and waxes. However, fats and oils are the most fed types of lipids. They provide about 2.25 times as much energy as carbohydrates. Fats have important roles such as facilitating absorption and transportation of fat-soluble vitamins, serving as precursors to hormones for internal signaling, and forming an insulating layer around the muscles and under the skin. Energy balance and fat storage can be evaluated using equine body condition scoring systems. Horses can make their own fats from excess carbohydrates or amino acids. Most fats made within the body are saturated (single-bonded carbon with single-bonded hydrogen atoms), versus unsaturated fat (double-bonded carbon with single-bonded hydrogen atoms) like corn oil. Several essential unsaturated fatty acids must be provided through the diet, including linoleic (omega-6) and α-linolenic (omega-3) fatty acids.

These essential fatty acids are contained in pasture and grains that horses ingest. Some grains, such as flaxseed (Figure 3) or linseed, are exceptionally high in oils and can increase the energy density of equine diets. Oils themselves, such as corn oil, can be added to diets for the same reason. Despite being herbivores, horses can tolerate up to 10% of their daily dry matter intake as fat, especially if the fat is added to the diet gradually over time. Ensuring that added dietary fat does not provide unused, excess energy to the horse is critical because obesity has many negative health implications.

Omega-6 and omega-3 fatty acids are special classes of fatty acids that contribute to a horse’s overall well-being. The word “omega” simply refers to the structure of the unsaturated fatty acid, or where the double bond is placed within the carbon chain. While there is not a recommended omega-6:omega-3 ratio, typical equine diets are less than 10:1. Sources of omega-3 include fresh pasture, hay, or fish oil, which provides greater concentrations than forages but is more expensive. Higher levels of omega-6 fatty acids can be found in cereal grains. Omega-3 fatty acids have been known to mitigate inflammation and improve outcomes for horses suffering from inflammatory conditions , such as arthritis. Omega-6 fatty acids can also improve immune function in horses.

Other Feeding Recommendations

Dry matter (DM) refers to the fraction of feed remaining after water is removed. Dry matter nutrient composition is how diets are formulated to meet requirements and monitor feed intake. Horses eat approximately 2–3% of their body weight in dry matter per day, and half or more of their dry matter intake should come from fresh or conserved forage (Figure 5).

Another rule of thumb is to limit soluble carbohydrates, especially for horses with metabolic disorders such as laminitis, equine metabolic syndrome, or insulin dysregulation. Starch is very easily degraded, and an overload can cause colic or laminitis episodes, especially when suddenly added to the diet or when fed to certain types of horses. Therefore, the recommended upper limit on starch feeding is 2 grams per kilogram (0.032 ounces per pound) of body weight at each meal. For a 1,100-pound (500 kilogram) horse the recommended limit would be 2.2 pounds (1 kilogram) of soluble carbohydrates per meal.

Take Home Message

Horse owners and managers need a fundamental understanding of the major nutrients provided in the feedstuffs offered and consumed by horses under their care. This fact sheet provides practical information on the role of carbohydrates, proteins, and fats in equine diets, and offers examples of feedstuffs along with their average nutritive values. Information on vitamins and minerals can be found in the fact sheet “The Role of Micronutrients in Equine Nutrition.” Knowing the macronutrient classes and how they fit into a whole diet is essential for ensuring good nutritional health of the horse .

References

Geor, Raymond J. 2007. Equine Carbohydrate Nutrition: Implications for Feeding Management and Disease Avoidance. Timonium, Maryland: Proceedings of the 5th Mid-Atlantic Nutrition Conference:154–161.

National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press.
doi.org/10.17226/11653

Weir, J., H. Li, L. K. Warren, E. Macon, and C. Wickens. 2017. “Characterizing Ammonia Emissions from Horses Fed Different Crude Protein Concentrations.” Journal of Animal Science, Volume 95, Issue 8: 3598–3608.
doi.org/10.2527/jas.2017.1648

Worth, Melyni. 2010. The Horse Nutrition Handbook. Massachusetts: Storey Publishing, LLC.