How Much Protein Does My Horse Need? Amino Acid Considerations for Horses

Horse owners are usually aware of the necessity of dietary protein to support and maintain health and performance, especially when considering topline and hindquarter muscling. They often do not, however, realize the intricacies of protein nutrition in equine diets.

First, horses do not actually have a nutritional requirement for protein per se but do require specific amino acids, the building blocks of protein, in their diets. There are 21 primary amino acids used to build proteins in the horse’s body, and 10 of those amino acids are considered essential, meaning that the molecules cannot be synthesized in the body and must be provided in the diet. Second, due to the unique architecture of the equine digestive tract, it is helpful to understand a bit about protein digestion and amino acid absorption to determine whether a horse’s essential amino acid requirements are likely being met.

Amino Acid Requirements

While the requirements for specific amino acids have been studied to a great extent in many species, only one amino acid has a documented nutritional requirement in horses: lysine. Researchers have conducted many studies to determine the lysine requirement in growing horses, and it is considered the first limiting amino acid in horses, i.e., the amino most likely to be deficient in an equine diet. In other words, when one essential amino acid is deficient, the body can only synthesize proteins to the level of availability of that amino acid, and lysine is the most likely first amino acid to be deficient.

Research studies suggest that the second limiting amino acid for growing horses is likely threonine, and, based on other species, methionine may be the third limiting amino acid in horses. However, the nutritional requirements of essential amino acids beyond lysine for horses are not documented at this time.

To ensure that essential amino acid needs are met, particularly for growing, breeding, and working horses, it is important to provide not only adequate quantities of dietary protein, but also appropriate quality protein. Protein quality is defined by how well the protein source meets the animal’s requirements of essential amino acids (and physiological needs). For horses, the highest quality protein sources are soybean meal and whey protein. Canola meal, linseed meal, and legumes are also considered quality protein sources in equine diets, although the amino acid profiles may not provide adequate lysine and potentially other essential amino acids to meet the needs of growing horses.

Amino Acid Absorption and Availability

An additional concept to keep in mind when considering whether essential amino acid needs are adequately supplied in a horse’s ration is whether the amino acids are absorbed from the digestive tract into the bloodstream. The horse’s digestive tract anatomy dictates that the majority of nutrient absorption, including amino acid, occurs in the small intestine. However, fiber digestion primarily occurs in the large intestine, known as the horse’s hindgut, through microbial fermentation.

Studies on digestion and absorption of protein in horses indicate that protein digested in the small intestine is primarily absorbed as amino acids, while the protein digested/fermented in the hindgut is primarily absorbed as ammonia. So, for protein to be of value to a horse in providing essential amino acids, it must be digested and absorbed in the horse’s small intestine. Since much of the protein in forages is contained within the cells and fibrous cell walls, the higher the fiber content, particularly in hays, the more protein passes undigested through the upper gut, so much of the protein is not available for digestion until it has already passed through the small intestine.

In one study, researchers compared small intestine and hindgut dietary protein digestion of various hays. Although the total nitrogen digestibility, a measurement of protein digestibility, throughout the digestive tract ranged from approximately 60 to 74%, digestion in the small intestine was only 2-28.5%. This means that depending on the hay source, very little of the total protein is digested in the small intestine, so the absorption of essential amino acids may be much less than expected. The fiber content of hay is greatly dependent on the maturity of the plant at the time it is harvested, as more mature plants have higher fiber content, particularly in the cell walls, and therefore less small intestine digestibility.

In another study, the relative small intestine digestibility of the highest quality hay provided was only 38%, indicating that even high-quality hay may not contribute as many essential amino acids as expected.

Studies comparing protein sources in horses have shown that oilseeds and meals (soybeans, soybean meal, canola meal) and fresh pasture plants, particularly legumes, provide more small intestine digestion of protein than hay. Even alfalfa hay, although considered a high-quality protein source, may have limited small intestinal digestion of protein, so may not be an appropriate source of essential amino acids, especially for horses with higher requirements (growing horses, broodmares, and working horses).

To ensure that amino acid requirements are met for horses whose diets are primarily forages, particularly those horses with higher needs, a ration balancer that provides concentrated protein from sources easily digested in the small intestine is recommended. Horses that are fed recommended amounts of well-formulated, balanced feeds that contain protein sources such as soybean meal, whey protein, and sometimes added individual amino acids typically consume adequate amino acids to meet their requirements.