Amino acids in horse feeds

Amino acids are necessary for vital processes in the body, for example muscle, fat and glucose metabolism is dependent on amino acids. But what are amino acids? What’s behind the terms essential and limiting amino acids? And how do you feed amino acids to your horse?

What are amino acids?

Next to fats and carbohydrates, they are the most important class of the so-called natural products and the building blocks for all cells. Amino acids are organic compounds containing an amino group, a carboxylic acid group and a side chain (R group). The special properties and functions are assigned to all amino acids via their side chains. There are over 400 known amino acids, whereby only 21 are needed by the organism to build proteins. Amino acids are rarely found alone – most form chains. Chains of less than 100 amino acids in length are called peptides. However, most chains have more than 100 amino acids. These are called proteins. Read more about protein in our article Protein in horse feeds | agrobs.de.

Amino acids fall into the following categories: essential, semi-essential and non-essential. Non-essential amino acids (such as alanine, asparagine, aspartic acid, glutamic acid, proline and serine) can be produced by the body and need not be supplied through food.

Semi-essential amino acids (such as arginine, cystine, glutamine, glycine and tyrosine) can be produced by the body but not in sufficient quantities.

Essential amino acids cannot be produced by the body and so the body needs to get them through food. Essential amino acids for mammals include isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Amino acid composition is an important factor in protein quality. Protein that is high in quality has a high amount of essential amino acids.

What do different amino acids do?

In the cell nucleus, every single amino acid has clear instructions as to where it is needed and the characteristic function it performs in the body. Some of these functions are described below.

Lysine: Lysine is an essential amino acid that helps to build collagen and antibodies. Lysine also supports growth.
Threonine: Threonine is an essential amino acid and regulates protein balance. Threonine is also a component of collagen and is present in skin, ligaments, tendons, bone and cartilage. Threonine is also important for the formation of antibodies.
Methionine: Methionine is an essential sulphurbearing amino acid that contains creatine. Creatine is the most important protein component in horn, whereas methionine is often found in complementary feeds to help hooves. Feeds containing methionine are known to enhance hoof growth in cows; however, this has not been proven to be the case with horses.
Isoleucine: Isoleucine is an essential amino acid found in high concentrations in muscle tissue. Isoleucine is responsible for the formation of haemoglobin; it also stabilises and regulates blood sugar levels.
Alanine: Muscles need large quantities of alanine, especially during exertion. Alanine can be converted into glucose in the liver and be available to the muscles as fuel.
Leucine: Leucine is an essential amino acid that is important for building muscle mass and for growth. This amino acid also promotes healing in bone, skin and muscle tissue.
Phenylalanine: Phenylalanine is an essential amino acid that assists in hormone metabolism, the building of other amino acids and the formation of important neurotransmitters.
Tryptophan: Several messenger substances and hormones are derived from amino acids; serotonin is derived from tryptophan, for example. Serotonin is a messenger substance that is associated with sedation and stress/anxiety relief.
Valine: Valine is an essential amino acid that supports the absorption of all amino acids in the muscles.
Histidine: Histidine is necessary for cell growth and cell repair. It is found in larger concentrations in skeletal muscle fibres.

What are limiting amino acids?

Limiting amino acids are always essential amino acids that are often not adequately found in the feed. Limiting amino acids can inhibit the entire metabolic process, which can be clearly seen through the use of the law of the minimum.

Liebig´s law of the minimum describes the dependency of individual nutrients to one another. A deficiency of just one amino acid can cause a complete undersupply to the body, independent of the quantities of other amino acids. This law can be illustrated by the use of the barrel (aka “Liebig’s barrel”). The barrel has vertical staves of different lengths, but can only be filled to the level dictated by the shortest stave.

Imagine that each stave represents an amino acid. If just one of these staves is shorter, filling the barrel with liquid will result in the liquid pouring out at this stave, regardless of how long the other staves are. You can make the other staves as long as you want, but that won’t change anything as long as this shortest stave is still present. The shortest stave represents the “minimum factor”, i.e., the first-limiting amino acid. In horses, threonine, lysine and methionine are often limiting amino acids, whereby threonine is the most scarce in forage.

Amino acid metabolism

Again, essential amino acids must be taken in through food. Important here is protein quantity, but also the composition of the amino acids. After feed is ingested, protein digestion occurs. First, the proteins are broken down into their components in the small intestine and the amino acids are absorbed by the intestinal mucosa. The body can now create the proteins it needs from the released amino acids. This process is known as protein synthesis. More than 100,000 different proteins can be created, which perform many different functions. For example, there are transport proteins, structural proteins and motor proteins.

There are two pathways for amino acids to enter the metabolism. Some amino acids enter via glucose metabolism; these are called glucogenic (or glucoplastic) amino acids.

Others enter via the fat metabolism (ketogenic amino acids).

There are also some amino acids that are used in both of these metabolic processes.

The amino acids are broken down into their components during the digestive process. This results in the production of ammonia as a cytotoxin. The ammonia is bound by glutamic acid and transported to the liver, where it is further metabolised. Urea is produced, which can then be eliminated in the urine.

Of course, all these metabolic processes are only possible when sufficient amino acids are present. If that is not the case, endogenous protein synthesis is reduced accordingly.

How and when should I feed my horse amino acids?

Most horses get adequate amino acids through their feed; in certain cases, feed may provide insufficient quantities.

Lactating broodmares are particularly susceptible to nutritional deficiencies with amino acids, because they are eating not just for themselves, but for the foal as well. They need about twice as much as regular mares. Read more about protein supply for mares in our article Proper feeding for broodmares | agrobs.de.
Foals need high-quality protein for growth. At first, they get it exclusively through the mare’s milk. As we know, lysine and threonine are often first-limiting amino acids. In studies, foals that got these limiting amino acids at the critical time after weaning showed no growth interruptions in comparison to other foals.

It is thought that some sport horses do not get enough amino acids. Their physical exertion leads to the breakdown of muscle proteins, with the body using some nutrients and minerals more quickly and/or in larger quantities than it does at rest. Here, for example, the increase in muscle mass and loss of nitrogen through the horse’s increased perspiration plays a role. A deficiency in certain amino acids in the feed can lead to an amino acid deficiency in the muscles for a period of time post-work. If amino acids are not sufficiently present in feed, this can result in an amino acid deficiency in the muscles after exertion. In that case, the horse can be fed forage or concentrates with adequate quantities of pre-caecal digestible crude protein, mineral feed with added amino acids and/or special amino supplements. The time you feed your horse should be linked to when he’s in work, so that the amino acids are available to the muscles in the body immediately after exertion. There is even a study with racehorses showing that feeding specific amino acid combinations helps them to perform better than horses given normal feeds. Read more about equine muscle development in our article Building muscle through diet and training | agrobs.de.

We should also not overlook the requirements of overweight horses, whose feed intake is regulated through a weight reduction diet. When a horse eats less feed it will lose not only excess fat but also muscle mass, because the muscles are not getting the high-quality protein they need. This can be prevented through the use of a high-protein feed. However, a high supply of protein does not necessarily guarantee that the horse’s body can use this protein for metabolic processes. Protein suitable for this purpose must be pre-caecal digestible protein. The digestibility of protein may vary, so look for precaecal digestible protein when buying feed. Feeds that are high in pre-caecal digestible crude protein include linseeds, sunflower seeds and brewer's yeast. Less suitable feeds include lucerne and cereals.

The following table illustrates how amino acid supply is represented in different horses via base feed and when amino acids should be added to feed:

Type of horse	Weight	Energy requirement (maintenance)	Total energy requirement including for performance	Precaecal digestible crude protein requirement	Requirement: Lysine	Requirement: Methionine+Cystine	Requirement: Threonine	Hay quantity	Contained precaecal digestible crude protein and amino acids

Warmblood sport horse	600 kg	63 MJ	94.5 MJ for moderate work	364 g / day	15.2 g / day	10.4 g / day	18.8 g / day	12 kg	pcdCP	600 g/kg
									Lysine	26.4 g/kg
									Methionine+Cystine	15.6 g/kg
									Threonine	26.4 g/kg
Warmblood broodmare	500 kg	55 MJ	107 MJ (after 15 days of lactation)	897 g / day	56 g / day	29 g / day	41 g / day	13 kg	pcdCP	650 g/kg
									Lysine	28.6 g/kg
									Methionine+Cystine	16.9 g/kg
									Threonine	28.6 g/kg
Haflinger (weight reduction diet)	400 kg	40.2 MJ	40.2 MJ	268 g / day	11.2 g / day	7.7 g / day	13.9 g / day	6 kg	pcdCP	300 g/kg
									Lysine	13.2 g/kg
									Methionine+Cystine	7.8 g/kg
									Threonine	13.2 g/kg

Source: Coenen, M.; Vervuert , I. (2020): Pferdefütterung. Based on data from the German Society of Nutrition Physiology. Recommended energy and
nutrient values for horses (2014)

This table clearly shows that horses, particularly those that must perform at a certain level of work, get a sufficient supply of protein and amino acids through hay when healthy amounts of hay are provided.
The values are almost adequate for weight reduction diets as well. Only the level of threonine, often found in insufficient quantities in forage, is shown to be too low and a limiting factor in muscle metabolism.
In broodmares especially, the requirements for pre-caecal digestible amino acids are clear and cannot be met solely through forage.
Sport horses generally do not need to be supplemented with amino acids through extra feed. If the horse still has difficulty building muscle, give him an appropriate feed close to work so that the amino acids are immediately available when needed.

With weight reduction diets, a concentrated amino acid product is advised because an amino acid deficiency can impede muscle metabolism, but at the same time the horse should be eating minimal extra feed and getting minimal extra energy/calories. This may be in the form of a mineral feed with added amino acids or a pure amino acid supplement.

Broodmares in particular will need an additional supply of protein. This could be met through a concentrate feed (in combination with an amino acid supplement or a mineral feed with added amino acids).

Keep in mind that the protein content in hay has been falling in recent years. Following the values given in the literature (as well as in this table), one finds an average of about 50 g of pre-caecal digestible crude protein in a kilogramme of hay. Hay analyses in 2021, however, showed that the analysed hay contained 10 g less crude protein on average, so a hay analysis is recommended. This way, you’ll know exactly how much protein is in your hay.

Do you want to know whether your horse is getting an adequate amount of amino acids and the best way to supplement your horse’s feed with them? Then contact us for a free feed consultation. Professional feeding advice | agrobs.de

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