Protein: An Important Nutrient

Protein may be the most misunderstood nutrient class when it comes to feeding horses. Protein is used in the formation of tissue, such as muscle, as well as to make hormones, enzymes, and antibodies. Protein is composed of chains of amino acids. There are twenty-two amino acids, and while the horse’s body is able to make some of these, other amino acids cannot be made and must come from the diet. Those amino acids which must be consumed from the diet are referred to as “essential amino acids.” Essential amino acids for the horse include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. For protein synthesis to occur, the necessary amounts of all amino acids must be present. If a single amino acid is missing, protein synthesis may be halted. One of the easiest ways to understand this is to think of stringing a beaded bracelet. You are using colored beads (amino acids) in a repeating pattern. When you run out of a certain color, you must stop and cannot continue beading until that color is replenished. The first “colored bead” or amino acid to be in short supply to a particular animal or in a particular ration is known as the limiting amino acid. For horses, the first limiting amino acid is generally lysine.

Ball and stick structure of lysine

Lysine is the limiting amino acid for horses. Photo by flickr/widdowquinn.

While it is important to understand the role of amino acids, we often think of feeding horses whole proteins, a complex source of many amino acids. The amino acid make-up or profile of feedstuffs is highly variable. How well the amino acid profile fits the animal’s need is generally referred to as one element of protein quality. Protein sources for horses with desirable amino acid profiles include soybean meal and cottonseed meal.

Table comparing protein supplements

Comparing common protein supplements for horse feeds. From the NRC 2007.

The other element of protein quality is digestibility. Not all protein is created equal when it comes to the horse’s ability to digest and absorb it. Protein in the horse is digested primarily in the small intestine. Fermentation in the large intestine may “free up” some additional dietary protein; and synthesis of proteins from microbes occurs there, but the contribution of the hindgut in absorption of amino acids is negligible, with ammonia being the main nitrogen product absorbed by the hind gut. This is also the reason the addition of urea, a non-protein nitrogen source that is converted by rumen microbes to beneficial proteins in ruminants, is not useful in horse rations.

Horse digestive tract

Protein in the horse is digested primarily in the small intestine.

Total tract protein digestibility in forages may range from 73-83% for alfalfa, to 57-64% for coastal bermudagrass; however, prececal digestibility (that which is occurring in the small intestine) is only about 28.5% and 16.8% respectively. On the other hand, total tract protein digestibility in cereal grains (e.g. corn, oats, and sorghum) is generally greater than 85%, and prececal digestibly is generally greater than 50%. When formulating rations, then, we must not only consider the amount of protein consumed, but also the protein quality both in terms of amino acid profiles and digestibility, particularly prececal digestibility. In addition to natural feedstuffs, the addition of synthetic amino acids to commercial concentrates may improve both amino acid profile and digestibility, and is commonplace particularly in rations designed for young, growing animals.

Protein supplied to the horse beyond requirements cannot be stored by the animal for later use. Instead, it is degraded. While this degradation can provide some energy for the animal, urea is produced and must be excreted in the urine. This provides additional work for the kidney and increases water loss from the body. At the same time, research suggests that excess dietary protein may affect acid-base balance as well as calcium retention, both of which may be of concern in athletic and growing horses. This, combined with the fact that protein is one of the more expensive aspects of horse rations, suggests that horses should be fed to meet protein (or more appropriate, amino acid) needs without greatly exceeding them.

Protein Requirements

Protein required by the mature horse at maintenance is simply to maintain homeostasis. At all times, the body is breaking down and rebuilding tissue, as well as creating hormones, enzymes, and antibodies. At the same time, a small amount of protein is lost via feces, urine, and sweat. Still, most mature horses at maintenance can easily have their protein requirement met by good quality forage (either hay or pasture). Horses at maintenance that are deficient in protein will generally have weight loss, in the form of muscle loss, even as their energy needs are being met.

Horse grazing in a pasture

Mature horses at maintenance can meet their dietary protein requirements with good quality forage.

Protein needs in young, growing horses are greater than the mature horse as the younger animal is laying down tissue such as muscle. Again, protein quality is an important concern and rations must contain adequate amounts of lysine, methionine, threonine, and other essential amino acids. Protein deficiency will result in reduced growth. Popularly believed, but unsubstantiated by research, is that overfeeding of protein is a culprit in development of developmental orthopedic diseases (DODs). Research suggests feeding protein at levels to maintain a high plane of growth does not result in increased incidence of DODs, although calcium balance could be altered with protein in excess of requirements.

Two foals standing in a pasture

Young growing horses need protein for bone and muscle development.

Also contrary to popular opinion, exercise only slightly increases the protein requirement of the mature horse. Generally, additional feed supplied to meet the increased energy demands of the working horse is adequate to meet the slightly increased amino acid need. Again, overfeeding of protein to the athletic horse should be avoided as it may negatively impact acid-base balance and calcium retention. Exercising horses that are still growing should be carefully fed to maintain proper growth without over-supplementation.

Horse and rider chasing down a cow

Exercise only slightly increases dietary protein requirements.

Protein needs of the broodmare are similar to that of the maintenance horse until the last third of gestation. It is during this late gestation period that the fetus is doing the majority of its growth, and thus additional protein should be supplied to the mare. Mares not receiving adequate protein during this time will draw upon their own body reserves to support the developing foal; this should not be encouraged. By the same token, the lactating mare will have increased amino acid requirements over maintenance as a result of supplying protein to the foal in the form of milk. Protein deficiency in the lactating mare may result in reduced milk production.


2011 Reviewers

  • Debra J. Hagstrom, MS University of Illinois

  • Jenifer A. Nadeau University of Connecticut

  • B. D. Nielsen, PhD Michigan State University

  • Carey A. Williams, PhD Rutgers University

2011 Section Chief

  • Colleen Brady, PhD Purdue University