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Recently there was some push back around soy as an ingredient and more over as an amino acid requirement in feed and or ration balancers. The good ol “show me the scientific evidence” came right out. So lets talk about soy as an ingredient and why it matters even for horses that don’t have a direct sensitivity or allergy for soy. Today we will talk through a few different aspects including soys anti-nutrient properties, glyphosate issues, and the all important “what about amino acids then?”  

Sources

Lysine as the first limiting amino acid in horses

National Research Council (NRC). (2007).
 Nutrient Requirements of Horses (6th rev. ed.).
Washington, DC: National Academies Press.
Establishes lysine as the first limiting amino acid in typical equine diets and emphasizes amino acid balance over crude protein.

Lysine, threonine, and methionine requirements and ratios

Mansilla, W. D., et al. (2020).
 Amino acid requirements in horses: Current knowledge and future perspectives.
Animals, 10(4), 682.
https://doi.org/10.3390/ani10040682
Reviews essential amino acid requirements in horses; discusses lysine as first limiting, with threonine and methionine following depending on diet.

Threonine as a secondary limiting amino acid

Harris, P. A., & Pagan, J. D. (1999).
 Protein and amino acid nutrition in the performance horse.
Proceedings of the Equine Nutrition Conference, Kentucky Equine Research.Discusses lysine as first limiting and identifies threonine as potentially limiting when lysine is corrected.

Functional importance of lysine and threonine (equine studies)

Zhang, L., et al. (2023).
 Effects of lysine and threonine supplementation on milk yield, amino acid metabolism, and fecal microbiota of lactating mares.
Journal of Animal Science and Biotechnology, 14, 84.
https://doi.org/10.1186/s40104-023-00884-5
Demonstrates physiological and metabolic effects of lysine and threonine supplementation in horses.

Balanced amino acid profiles vs crude protein

Graham-Thiers, P. M., & Kronfeld, D. S. (2005).
 Amino acid supplementation improves muscle mass in exercising horses fed diets adequate in crude protein.
Journal of Nutrition, 135(9), 2144–2148.
https://doi.org/10.1093/jn/135.9.2144
Shows that horses fed “adequate” crude protein still benefit from targeted amino acid supplementation.

Ideal protein concept (amino acid balance over quantity)

Wu, G. (2014).
 Dietary requirements of synthesizable amino acids by animals: A paradigm shift in protein nutrition.
Journal of Animal Science and Biotechnology, 5, 34.
https://doi.org/10.1186/2049-1891-5-34
Introduces and supports the concept that amino acid balance determines protein utilization more than total protein intake.

Why crude protein is a poor metric

Crude protein vs usable protein

Millward, D. J., et al. (2008).
 Protein quality assessment: Impact of digestibility and amino acid composition.
British Journal of Nutrition, 99(5), 995–1011.
https://doi.org/10.1017/S0007114507868495
Explains why nitrogen-based protein estimates do not reflect biological protein value.

Amino acid digestibility and bioavailability

Gilani, G. S., et al. (2012).
 Protein quality evaluation: Impact of digestibility and amino acid availability.
Journal of AOAC International, 95(5), 1398–1411.
https://doi.org/10.5740/jaoacint.11-028
Demonstrates that digestibility and amino acid profile determine usable protein, not crude protein.

Conditionally essential amino acids (rate-limiting functions)

Wu, G., et al. (2013).
 Amino acids: metabolism,

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Audrey is not an MD or DVM and has never implied or claimed to be either. Audrey holds a Doctoral Degree of Traditional Naturopathy and a Masters Degree in Science. She created an evidenced-based anti-inflammatory nutrition program for equine and has successfully helped over 10k horses. This information is not meant to diagnose, prescribe for, treat, or cure, and is not a replacement for your veterinarian.  These are my personal interpretations based on my education, skill and experience. 

Equine Energy Medicine refers to non-invasive, supportive modalities focused on energetic balance, nutrition, and overall well-being, and does not replace veterinary care.