The amino acids in horse feed sources, with a focus on lysine (which is often the limiting one), methionine and taurine

Biological significance

• Essential amino acid for horses (cannot be synthesized endogenously).

• Typically the first limiting amino acid in most roughage-based rations, especially in straw and grass with a low protein percentage.

• Main features:

  • Protein building, especially in muscle tissue and growth (foals, young horses, performance horses).

  • Supports enzyme and hormone production, including via carnitine synthesis (lysine + methionine → carnitine).

  • Has a role in bone growth, especially in young animals (via collagen production).

  • Supports the immune system through antibody synthesis.

Metabolic role

• Lysine is not metabolized in the intestine like many other amino acids, but is absorbed directly in the small intestine.

• Excess lysine cannot be converted into other essential amino acids → it is therefore crucial that rations cover the minimum requirement.

Reference values

• NRC (2007):

  • Adult horses (500 kg, maintenance): approx. 0.026 g lysine per kg body weight/day (≈ 13 g/day).

  • Foals and young horses have higher requirements (e.g. foals 4–6 months: approx. 4.3 g lysine per kg of gain + maintenance).

References

• NRC (2007). Nutrient Requirements of Horses , 6th Edition. National Academies Press.

• Ott, EA et al. (1979). "The lysine requirement of growing horses." Journal of Animal Science , 48(4): 867–874.

2. Methionine

Biological significance

• Essential amino acid, often the second limiting in horse feed after lysine.

• Contains sulfur → important for:

  • Formation of cysteine ​​and taurine synthesis (in most animals, but see the taurine section).

  • Keratin synthesis, i.e. hoof, skin and fur formation.

  • Methyl group donor (via S-adenosylmethionine, SAM) → epigenetics, DNA methylation, transmethylation reactions.

  • Antioxidant function: Methionine → cysteine ​​→ glutathione.

Metabolic role

• Methionine can be converted to cysteine ​​via the transsulfuration pathway.

• Some methionine is used for carnitine production along with lysine.

• Limited amounts in typical roughage → supplementation may be relevant in high-performing horses or with poor hoof quality.

Reference values

• There are no official NRC figures for methionine requirements in adult horses, but:

  • Methionine typically constitutes ~2–3% of the total essential amino acid requirement.

  • Pragmatic recommendation: ~ 30–40% of lysine intake (i.e. 4–5 g/day for an adult horse maintenance).

References

• NRC (2007), chapter on amino acids.

• Breidenbach, A. & Meyer, H. (1997). "Amino acid composition of equine muscle protein and the amino acid requirements of horses." Livestock Production Science , 51: 101–107.

• Staniar, WB et al. (2010). "Methionine and cysteine ​​requirements in equine diets." Equine Vet J Suppl , 38: 34–39.

3. Taurine

Biological significance

• Non-classical essential in horses — i.e. they can synthesize taurine from methionine and cysteine.

• Features:

  • Osmoregulation and cell volume regulation.

  • Antioxidant and membrane stabilizing functions.

  • Importance for bile acid conjugation (taurocholic acid) → fat digestion.

  • Stabilizes cell membranes in muscle and nerve tissue; has potential effects on muscle performance and heart function.

  • Immunomodulation and intestinal barrier function (shown in other species, assumed to be similar in horses).

Taurine in horses

• Horses can form taurine from cysteine ​​via the cysteine ​​sulfinate pathway → they are not dependent on dietary taurine (unlike cats).

• There are no NRC requirements.

• In practice, taurine in horse feed is low, as plant feed generally contains very little free taurine.

• Supplements are studied experimentally in relation to muscle function and recovery, but not standardized.

References

• Hintz, HF & Schryver, HF (1972). "Taurine biosynthesis in the horse." J. Nutr. , 102(6): 833–837.

• Swanson, TD et al. (2012). "Taurine status and metabolism in horses." J Anim Physiol Anim Nutr (Berl) , 96(3): 481–489.

• Ghosh, S. et al. (2018). "Taurine in nutrition and metabolism: A review." Nutrients , 10(5): 599.

4. Cysteine ​​in horses

• Cysteine ​​contains a thiol group (-SH), which forms disulfide bridges (-S–S–) in keratin and collagen → essential for the strength of hoof horns, hair/fur and connective tissue.

• Precursor to glutathione (GSH) along with glutamate and glycine. GSH is a central intracellular antioxidant and part of phase II detoxification.

• Cysteine ​​can be formed from methionine via the transsulfuration pathway. If methionine is low → cysteine ​​formation decreases. Conversely, cysteine ​​can save on the methionine requirement if supplied directly.

• In horses, cysteine ​​is converted to taurine via the cysteine ​​sulfinate pathway.

References

• Hintz & Schryver (1972). Taurine biosynthesis in the horse. J. Nutr.

• Meyer & Coenen (2002). Pferdefütterung.

• Staniar, WB et al. (2010). Equine Vet J Suppl , 38: 34–39.

Table 1. Summary table

1. Amino acid content of typical horse feeds

(All values ​​are average typical figures — there may be variation depending on variety, harvest time, drying, protein content, etc.)

2. Important observations

Lysine

• Grass, wrap and hay typically cover maintenance needs if the protein content is moderate to high — but not always the need for growth or muscle building.

• Alfalfa and sainfoin have higher lysine concentrations and can significantly increase the total ration.

• Soybean meal and pea protein concentrate are the most effective sources for correcting amino acid profiles, especially in straw-based rations.

Methionine

• Generally lower proportion in all vegetable feed materials, including alfalfa.

• Soy and pea protein have slightly higher methionine, but it is still often the 2nd limiting amino acid in practice.

• In case of high hoof load or poor hoof quality, supplementation (DL-methionine or MHA) may be necessary.

Taurine

• Very low in plants — most in feed comes from microbial synthesis or endogenous formation.

• Brewer's yeast may contain small amounts (0.5–1 g/kg DM), but the biological significance is unclear in horses.

• Horses can produce taurine themselves from cysteine ​​→ no standards.

Cysteine

• The cysteine ​​content often follows the crude protein content relatively stably, but has a slightly lower percentage than methionine in most vegetable feed materials.

• The total content of methionine + cysteine ​​typically constitutes 2.8–3.5% of crude protein, which is lower than for e.g. pork and poultry.

• Soybean meal and pea protein concentrate have the highest absolute content, but the percentage of RP is at the same level as alfalfa.

• Cysteine ​​can save methionine: If the feed contains sufficient cysteine, the need for methionine supplementation can be reduced slightly (typically 40–60% of the SAA requirement can be covered by cysteine).

• Keratin-rich structure (hooves, fur) requires both methionine and cysteine ​​→ rations for horses with hoof problems should be optimized for total sulfur-containing amino acids, not just methionine.

• Alfalfa and sainfoin contribute significantly to cysteine ​​in roughage-based rations and can relieve the need for synthetic supplementation.

3. Practical application

• For low-protein straw or wrap-based rations, 1–1.5 kg of alfalfa + 0.5–1 kg of soy meal or pea protein can cover both energy and essential amino acids for adult horses.

• For higher requirements (e.g. young horses, performance horses, pregnancy), the target is often 4–5% lysine of crude protein, and methionine around 30–40% of lysine.

• Brewer's yeast should be evaluated critically due to digestibility and the amount of non-starch polysaccharides in the cell wall — it can cause increased intestinal fermentation and gas.

Main references

• NRC (2007). Nutrient Requirements of Horses. National Academies Press.

• Meyer, H. & Coenen, M. (2002). Pferdefütterung. 4. Edition.

• DLG Futtermitteltabellen (2020). Deutsche Landwirtschafts-Gesellschaft.

• Breidenbach, A. & Meyer, H. (1997). Livestock Production Science, 51: 101–107.

• Hintz, HF & Schryver, HF (1972). J. Nutr. , 102(6): 833–837.

• Swanson, TD et al. (2012). J Anim Physiol Anim Nutr, 96(3): 481–489.

• EU Feed Materials Register & Feedipedia (INRA/FAO/AFZ).

4. Updated amino acid table (short form)

5. Is it necessary to add the amino acid Taurine to horse feed?

Taurine is not essential, as it can be formed from essential methionine, further to the other important sulfur-containing amino acid, cysteine, or directly from cysteine. Taurine may be conditionally necessary if the feed is low in the other amino acids, but as can be seen from table 4, all normal feed materials have a fairly even and balanced content, especially when looking at methionine and cysteine ​​together.

However, the conversion of methionine to taurine is a stepwise process that requires several enzyme steps, and therefore the nutritional co-factors needed for these enzymes to do their job effectively are important. These are the B vitamins B2 (riboflavin), B6 ​​(pyridoxine), B12 (cobalamin), folate, as well as the minerals magnesium, zinc, copper, manganese, iron , and also vitamins C and E.

If you feed a vitamin-mineral supplement that is deficient in some of these co-factors, the conversion processes may suffer. Regulator Complete's position is that with sufficient protein coverage in relation to the horse's needs and our optimally composed vitamin-mineral supplement, which among other things covers all nutritional co-factors, it is not necessary to add taurine.