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The Case of the Weakest Link: Protein & Amino Acids

Updated: Apr 2

Protein is an important macronutrient, but did you know that a horse’s protein requirement is actually a dietary requirement for essential amino acids? Proteins are made up of amino acid chains, in different combinations and structures. Some amino acids cannot be produced in adequate amounts in the body, and thus are essential in the adult horse’s diet.

Essential Amino Acids

  • Lysine

  • Methionine

  • Threonine

  • Arginine

  • Histidine

  • Isoleucine

  • Leucine

  • Phenylalanine

  • Tryptophan

  • Valine

Not only is protein made up of chains, protein synthesis works like a chain as well, in that it is only as strong as its weakest link. Protein synthesis relies on all necessary amino acids to be present in adequate amounts - if one amino acid is limited (present in less than adequate amounts; “limiting amino acid”), or if one is over-supplemented and prevents other amino acids from being absorbed, protein synthesis will also be limited (NRC, 2007). Supplementing just 1 amino acid without a detailed analysis of the horse’s diet can only further disturb amino acid balance, and as a result, disturbed protein synthesis.

Based on a typical equine diet (forage and cereal grains), Lysine is considered to be the first limiting amino acid, Threonine the second, and Methionine the third (due to trends in pigs). However, equine Threonine and Methionine requirements remain unknown. Studies in growing horses (who thus have higher requirements) found that Lysine and Threonine supplementation increases daily gain and girth gain (NRC, 2007; Mok and Urschel, 2020).

Protein Deficiency (NRC, 2007).

  • Weight loss

  • Muscle loss (and poor performance)

  • Reduced feed intake

  • Poor hair growth, milk production & hoof growth

Protein Quality

The quality of a protein will depend on its amino acid profile, and its digestibility, rather than crude protein content. As such, high quality protein sources will provide sufficient quantities of essential amino acids, in ratios which match requirements, to allow for optimal utilization & protein synthesis (NRC, 2007).

While research has yet to identify a complete snapshot of equine amino acid requirements, Lysine requirement has been established, and is often used against muscle tissue amino acid ratios to extrapolate dietary amino acid requirements (NRC, 2007). Muscle tissue is the main product of amino acid utilization in the body, it is possible that it may reflect amino acid relationships in the horse (NRC, 2007). Unfortunately, without further amino acid requirement research, protein may be overfed (Mok and Urschel, 2020; Woodward et al, 2011).

Protein Excess: The effects of excess protein in horses has not received much research focus, despite protein being commonly overfed to horses (Woodward et al, 2011;NRC, 2007).

  • Lower blood pH at rest & during exercise

  • Increased calcium and phosphorus losses

  • Will bypass foregut digestion and reach hindgut, where it will be fermented.

  • Increased water losses as excess protein is excreted in urine, as urea.

  • Increased metabolic heat production, may impact thermoregulation

Protein Sources

The quality and type of forage will impact amino acid composition, as Lysine tends to be low in grasses, while Methionine tends to be low in legumes. Timothy hay has been reported to meet Lysine requirements for light exercise, while alfalfa hay can exceed the Lysine requirements for moderate exercise (Woodward et al, 2011), but this will also depend on the digestibility of the forage, and the amount of leaf matter present.

When feeding a low-quality protein forage, supplementation with a high-quality source is recommended so as to ensure amino acid requirements can be met (NRC, 2007). Soybean, alfalfa, canola and flax are commonly used to increase protein in the diet, as legumes and cereals have higher protein content than grasses (Novak, Shoveller and Warren, 2008).

Some feedstuffs, such as linseed meal (flax), cottonseed meal, copra (coconut) meal and peanut meal contain amino acid deficiencies (KER, 2012), so formulation to account for differences in composition is key to providing a complete amino acid profile to meet the horse’s requirements. When feeding to address protein deficits in the diet, supplement feedstuff selection is important to ensure the feedstuff you are supplementing will top up the remaining requirements (so that there are no weak links in the chain!).

Soybean meal is widely considered the best plant-based protein source for horses, and as such is commonly used in rations of growing horses (Hammer, 2010). Whey is another quality protein, common in human performance nutrition, that can be valuable for the horse, provided a lactose-free source is used.

Suspect a protein inadequacy in your horses diet, or unsure if your horse's requirements are met? We can help! Email with your questions, or to inquire about our nutrition consulting services.


KER, 2012. Choose Quality Protein for Horse Diets. Kentucky Equine Research. Accessed online from:

Hammer, C. 2010. Feedstuffs for Horses. North Dakota Extension Service. Fargo, ND. 2010.

Mok, CH., Urschel, KL. 2020. Amino acid requirements in horses. Asian-Australas J Anim Sci 33(5):679-695.

Novak, S., Shoveller, AK., Warren, LK. 2008. Nutrition and Feeding Management for Horse Owners. Alberta Agriculture and Rural Development. 1-123.

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

Woodward, AD., Nielsen, BD., Liesman, J., Lavin, T., Trottier, NL. 2011. Protein quality and utilization of timothy, oat-supplemented timothy, and alfalfa at differing harvest maturities in exercised Arabian horses. Journal of Animal Science 89(12): 4081-4092.


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