Fridays with Finn

An Equine Nutrition Blog

Protein Requirements in Horses

By: Madeline Boast, MSc. Equine Nutrition

Defining Protein

Protein is one of 6 nutrient classes that horses have a requirement for. It is a major component of body tissues. Along with being a core component of tissues, proteins are also heavily involved in enzymes, hormones, and antibodies.

Proteins are long chains of amino acids, there are 21 amino acids that join in a multitude of combinations to form proteins. In equine nutrition, we divide the amino acids into essential and non-essential. The essential amino acids are those that the horse cannot produce and therefore must be supplied in the diet. They include: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Unfortunately, the individual requirements for each amino acid have not been elucidated for horses except for lysine. More research is required to be able to understand the exact dietary requirements for individual amino acids.

Now that we have established a basic understanding of protein in the equine diet, lets discuss which horses have a higher requirement for protein in their diet.


When Do Horses Need More Protein?

A horse’s requirement for protein depends on their physiological state as well as workload. Typically, horses do not have high protein requirements. In fact, most of the time their protein requirement can be met with forage as long as the protein levels in the forage are adequate. That being said, occasionally, protein in the forage will not be adequate and an additional source must be added to the nutrition program – again this just highlights the importance of a hay analysis.

Growing horses, pregnant mares, lactating mares, and horses under a moderate to heavy exercise regime will probably need additional protein. Mares in late gestation and those in the first three months of lactation have the highest requirements. Best practice for these horses would be to first have them on a protein rich forage such as legume-based hay, then add additional protein as needed.

Although this sounds simple, there is one more layer to the story. Not only do you need to ensure your horse has adequate protein in their diet, but it must also be high quality.


Protein Quality

Protein quality has to do with the amino acid composition of that ingredient as well as the digestibility. High quality protein provides essential amino acids in the correct proportions, whereas lower quality protein may be inadequate at providing essential amino acids or have poor digestibility. Therefore, when curating a diet for a horse that has increased protein requirements ensuring you are adding a good quality protein can make a significant difference in their nutritional well-being.

When comparing feedstuffs based on protein quality a general rule is that legume hay has a better protein profile than grass hay or pasture. Additionally, many cereal grains (barley, corn, oats) are considered to have low quality protein. Typically, soybean meal is the go-to high quality protein source for horses as it provides a high percentage of lysine.


Take Home

When evaluating your horse’s diet for protein a hay analysis should ALWAYS be done. This way a nutrition program can be built around the forage. After the hay analysis, if a nutritionist determines your horse needs an additional source of protein added they will either recommend a single ingredient such as soybean meal or add a balanced complete feed. When shopping for a complete feed it pays off to do your research not only on protein content but also on protein quality!!

If you have any questions regarding protein requirements or would like to discuss the protein quality of your horse’s diet, please email Madeline at 



Graham-Thiers, P. M., Kronfeld, D. S., & Kline, K. A. (1999). Dietary protein influences acid‐base responses to repeated sprints. Equine Veterinary Journal, 31(S30), 463-467.

Graham-Thiers, P. M., Kronfeld, D. S., Kline, K. A., & Sklan, D. J. (2001). Dietary protein restriction and fat supplementation diminish the acidogenic effect of exercise during repeated sprints in horses. The Journal of nutrition, 131(7), 1959-1964.

National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press.

Tanner, S. L., Wagner, A. L., Digianantonio, R. N., Harris, P. A., Sylvester, J. T., & Urschel, K. L. (2014). Dietary crude protein intake influences rates of whole-body protein synthesis in weanling horses. The Veterinary Journal, 202(2), 236-243.


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