Amino acids are the building blocks of proteins. You can think of amino acids as different colored beads. To create a specific protein, the colored beads have to be strung in a specific order. There are some beads that are in shorter supply than others. Those are the limiting amino acids.
Studies in various species have indicated that the limiting amino acids are lysine, methionine, and threonine (Richardson & Hatfield, 1978). There are other essential amino acids that are necessary, but those get little attention.
What are the factors that determine which amino acids are “more” necessary than others?
Dry matter intake: How much protein did your horse eat?
Amino acid profile of the consumed protein: Different protein sources have different amino acid profiles. For instance, hay has a different amino acid profile than oats.
Microbial digestion: Microbes in the digestive tract can utilize the protein amino acids, resulting in those not being available to the horse.
Intestinal digestibility: The forage quality and the amount of non-digestible fractions of the feed (Dias et al., 2023).
Physical status of the horse: Horses with muscle loss due to illness, age, or lack of caloric intake have different protein needs than those that are readily exercising or in good flesh (Deutz et al., 2014). (As an aside, this is an excellent article on protein needs with aging!)
Most of the studies available are in humans or other species. But the basic ideas can be extrapolated to horses. A good bet on the limiting amino acids are lysine, methionine, and threonine. Top 3 Aminos provides these amino acids.
Deutz, N. E., Bauer, J. M., Barazzoni, R., Biolo, G., Boirie, Y., Bosy-Westphal, A., . . . Calder, P. C. (2014). Protein intake and exercise for optimal muscle function with aging: recommendations from the ESPEN Expert Group. Clin Nutr, 33(6), 929-936. https://doi.org/10.1016/j.clnu.2014.04.007
Dias, A. V. L., Messana, J. D., Granja-Salcedo, Y. T., Alfonso, Y. F. M., Silva, L. G., Camargo, K. D. V., . . . Berchielli, T. T. (2023). Efficiency of Amino Acid Utilization in Nellore Cattle Grazing Low-Quality Forage Supplemented with Different Sources of Nitrogen. Life (Basel), 13(8). https://doi.org/10.3390/life13081622
Richardson, C. R., & Hatfield, E. E. (1978). The limiting amino acids in growing cattle. J Anim Sci, 46(3), 740-745. https://doi.org/10.2527/jas1978.463740x