Amino acids are the building blocks of protein, which makes up all body structures, as well as enzymes, hormones and other essential body components. Protein consists of chains of amino acids, and it is the length of these chains and the types of amino acids they contain that makes one protein different from another.
The horse – like us – has a dietary requirement for amino acids, rather than protein. Individual amino acid requirements, however, are not known for the horse (apart from for lysine), so nutritionists have to formulate balanced diets using protein and lysine levels.
There are twenty amino acids that make up body proteins, and ten of them are believed to be essential to the horse. The other non-essential amino acids can be made up in the body from the essential amino acids. The essential amino acids for the horse are: lysine, threonine, methionine, leucine, isoleucine, valine, arginine, histidine, phenylalanine and tryptophan. Lysine is the amino acid required in the largest amount and it is the most often ‘limiting’ amino acid, or the one that is most likely to be lacking in a diet and therefore limits protein synthesis in the body.
Protein is digested in the stomach and small intestine of the horse, where enzymes break down the chains of amino acids into shorter chains called peptides and single amino acids, allowing them to be absorbed. Further breakdown of peptides to amino acids occurs within the gut wall. The ‘freed’ amino acids are then transported in the blood to the liver, where they are processed depending on the body’s requirements.
Adult horses at maintenance have an average requirement for protein that can be expressed as bodyweight x 1.26 crude protein per kg of bodyweight per day. A 500 kg horse would therefore require 630 g crude protein daily, equating to 6.3% total based on an intake of 10 kg dry matter per day (2% of their bodyweight; a typical intake). The lysine requirement for maintenance is calculated as the crude protein x 4.3 per cent, which would be 27 g daily. A good grass hay fed ad lib (free choice) would fulfil these requirements.
Horses in work require a little more protein, and breeding stock and growing youngstock also have higher protein requirements. Lactating mares have the highest protein requirements. You can find out more about actual protein and lysine requirements from books with nutrient requirement tables such as my own ‘The Truth About Feeding Your Horse’ (JA Allen, 2007) or ‘The Nutrient Requirements of Horses’ (National Research Council, 2007).
Most feedstuffs for horses contain protein, with the exception of vegetable oils. The amount of protein varies, as does the quality of protein, which reflects the amount of essential amino acids it contains. The richest vegetarian source of both protein and lysine for horses (in the UK) is soya bean meal, followed by linseed and sunflower seeds. All three are commonly used in compound horse feeds. Corn gluten meal and brewers grains are also useful sources of protein for horses, and in other countries other oilseed meals are commonly used, including cottonseed and peanut.
Synthetic amino acids can be used to improve protein quality and researchers have investigated adding lysine and threonine to the diets of growing and exercising horses. Such additions need to be done with care, however, because the overall balance of the essential amino acids – described as ‘ideal protein’ is believed to be important. Synthetic amino acids might also be added to the diet of a horse with a particular health problem, e.g. branched chain amino acids (leucine, isoleucine and valine) are useful for horses with liver challenges and may be useful for those with suspected delayed onset muscle soreness (DOMS). Glutamine is used as a source of energy for gut cells and might be useful during immunosuppression or other illness.