Feb, 2019
Protein is a essential nutrient in your horse's diet. This article will look at:
  • What is protein?
  • Protein quality
  • Protein sources
  • Protein requirements
  • What happens when a horse has too much protein
  • What happens when a horse has too little protein
  • Protein-to-calorie ratio
  • Protein for:
    • Broodmares
    • Youngstock
    • Performance horses

A vital nutrient

Protein serves a multitude of roles in the body. It is used to build and repair body tissues, and is a major component of the body forming muscles, skin, tendons, blood vessels, organs, bone, hair and hoof. It is not only important in building cells and repairing tissues, but also aids in regulating many of the bodies’ systems and functions.

Proteins form antibodies to combat invading bacteria and viruses; they make up enzymes and some hormones; they build nucleoproteins such as DNA; they carry oxygen throughout the body and they participate in muscle activity. After water, the major constituent of the body is protein. In fact, eighty percent of the horse’s fat free, moisture free body composition is protein.

Wrongly accused?

So if protein is such an important nutrient of the body, why does it get wrongly accused of so many problems in the horse? Feeds have been traditionally categorised by protein content rather than starch content. Consequently, it is understandable that when a high protein (and high starch) feed is being fed, protein is believed to be the cause when problems such as laminitis, epiphysitis, excitability and colic arise.

With traditional cereal based compound feeds such as mixes and cubes, it usually follows that the higher the protein level the higher the starch level, as the feed provides more energy for a harder level of work or a more demanding life stage.

Research has shown that high starch diets and the imbalance or deficiency of other nutrients, such as minerals, are contributing factors to muscle related problems, digestive problems and bone and soft tissue developmental problems in horses – not protein.

What is protein?

Protein is a long chain molecule made up of amino acids joined by peptide bonds. The types of amino acids incorporated into a protein chain – as well as the length of the protein chain -differentiate one protein from another.


In total, there are 22 different amino acids needed for protein synthesis in the body. Ten of these are considered ‘essential’ and must be provided in the diet. These are arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Non-essential amino acids do not have to be supplied by the diet, as they can be synthesized by microorganisms in the horse’s caecum and anterior portion of the large colon and by metabolic processes in the body.

All the necessary amino acids required to make a particular protein must be present at the same time. One that is present in less than adequate quantities is referred to as a limiting amino acid – because it will limit protein synthesis.

The challenge in feeding horses is to provide adequate quantities of protein that will allow for sufficient concentrations of circulating amino acids in the blood that the body can draw on to synthesise tissues, enzymes and hormones as well as repair tissues.

Sources of feed protein which contain an assortment of amino acids which approximate the needs of the animal are considered of high quality (high biological value), while those which do not are considered low quality.


Protein quality is a function of the amino acid profile and the digestibility of the protein source. The higher the digestibility (especially the foregut digestibility) of the protein source, the higher the absorption of amino acids to contribute to the amino acid pool for tissue synthesis and repair. Unfortunately, the digestibility of the protein in many ingredients commonly used in horse feeds has not been adequately determined.


The quality of the dietary protein should be considered when selecting a protein source for the horse’s diet. Not all dietary protein sources supply the proper balance of amino acids to the horse.

Soyabean meal has the highest biological value and has a 48% protein content on an as-fed basis. The quality (biological value) of a protein supplement is based on a comparison of the amino acids that make up soybean protein to the amino acids required by the horse to make up his proteins.

Not only do they need to have the same amino acids, but they should be present in the same relative percentages. Soybean meal is especially high in lysine, which is commonly low in most grains. Soybeans should not be fed to horses in their raw form. In the raw form, they contain an inhibitor of protein digestion in the horse. Raw soybeans are especially detrimental in foal rations.

Similarly to soya, alfalfa is also a member of the legume family and possesses the ability to ‘fix’ nitrogen from the atmosphere and incorporate it into the plant. Alfalfa is becoming a more common source of protein in a horse’s diet in the form of alfalfa chaff, alfalfa hay and alfalfa meal.

Other protein sources such as sunflower and canola meals can be used in horse feeds. Brewer’s grains, distiller’s grains and gluten are commonly used in horse feeds as protein sources but would be considered low in essential amino acids.

High protein feedstuffs % g/kg
Soya Beans 44-48% 440-480 g/kg
Linseed 32% 320 g/kg
Peas 23% 230 g/kg
Alfalfa 15-23% 150-230 g/kg


Low protein feedstuffs % g/kg
Maize 8% 80 g/kg
Wheat/Barley/Oats 10-12% 440-480 g/kg
Bran 13-15% 130-150 g/k

All cereals are particularly low in essential amino acids.

Cereal Based Agriculture


When a decision is to be made on which feedstuff to use, consideration for the amount of protein in the feed, the amount of feed that will need to be fed to provide adequate protein and the quality of the protein sources used in that feed should be considered.

Take the two following diets as an example. Which diet would be more beneficial to a horse in full work in terms of meeting the horse’s protein requirements and promoting good digestive health, a diet consisting of 10 pounds of a 20% ration or a diet consisting of 20 pounds of a 10% ration?

Both diets provide the horse with equal quantities of protein, but the diet consisting of 20 pounds of the lower protein feed is more likely to create digestive related problems such as tying up, excitability, ulcers, colic, etc.

Another example would involve two diets of similar crude protein content. One diet was deficient in the amino acid lysine. Youngstock on the lysine deficient diet grow more slowly than horses fed a diet high in lysine – even though the crude protein percentages (14%) of the diets were identical.


Protein is required by all ages of horses, but the amount and quality required depends on the horse’s age and physiological status. Young, growing horses and broodmares need the most and best protein, while performance horses require less protein. Some young horses can tolerate more protein and can use it to grow muscle, but many breeds of easy-keeping horses do poorly on excess amounts of protein when young.

For horses in work, protein needs do not increase very much. An increase in the total amount of feed would increase the total amount of protein given and be sufficient to meet extra protein needs caused by work. An evaluation of protein supplied from the forage should be accounted for when considering protein supplementation.

Forage % g/kg
Pasture – Spring 30% + 300 g/kg DM
Pasture – Summer 10-14% 100-140 g/kg DM
Pasture – Autumn 14-20% 140-200 g/kg DM
Pasture – Winter 10% 100 g/kg DM
Hay – Seed 4-8% 40-80 g/kg DM
Hay – Meadow 6-12% 60-120 g/kg DM
Haylage 9%-15% 90-150 g/kg DM


When the body utilises excess protein in the diet and converts it to energy, it snips off the nitrogen end of the protein strand and breaks up the remaining amino acids for participation in several energy-producing metabolic pathways. The excess nitrogen goes through several chemical reactions to form urea, a waste product.

As a result, blood urea nitrogen content increases and the extra urea is excreted in the urine. Water intake increases, which results in greater urine volume and a noticeable ammonia smell, especially in poorly ventilated stables. If a strong ammonia smell exists in a barn, it could indicate that too much protein is being fed.

Wetter stalls and ammonia odour add to management problems by increasing bedding and labour needs, as well as costs. Ammonia close to the stall floor can be a problem with young foals, as they have an immature respiratory system, making them more susceptible to respiratory diseases. High ammonia levels have been associated with respiratory problems in foals, as well as other animals.

Excess protein, besides being expensive, could be detrimental to the overall health of the mature horse. Too much protein throws the intestinal tract digestive process out of balance.

Poor digestion and an altered pH are often the result. High urea and ammonia in the blood can affect the nervous system, causing irritable behaviour and restlessness, and can disturb energy production during exercise.

If the horse already has a kidney or liver weakness and is unable to handle the excess urea or ammonia, there will be a build up of lactic acid with the resulting loss of performance or even ‘tying-up’ or ‘bursting’ – or simply fading at the end of a race.

High blood urea and ammonia levels on a blood test can indicate that the horse is getting too much protein in the diet. However, if horses are getting insufficient dietary energy and are breaking down body tissues for energy, this will also elevate blood urea and ammonia levels.

In the pasture, areas of scorched grass and accelerated grass growth where a horse has urinated and defecated are indicators that the horse is receiving a high dietary protein intake.


Since protein is needed for many body functions and growth, a deficiency of protein can result in serious problems. A common sign of protein deficiency is lower feed intake. However, a number of other factors can also cause a drop in feed intake.

Other commonly observed signs of protein deficiency are a rough coat, reduced hoof growth, and abnormal skeletal development in young, growing foals. In lactating broodmares, milk production will be reduced, which results in slower foal growth.

Reduced growth would also be evident in weanlings and yearlings fed a protein deficient ration. However, these conditions have also been observed when the ration’s protein content was apparently adequate, but the protein-to-calorie ratio was low.


The relationship of protein to energy (calories) is important, especially when feeding young horses. Some of the problems associated with creep feeding young foals, as well as feeding weanlings and yearlings, may stem from an improper protein-to-calorie ratio.

When feeds with a lower protein-to-calorie ratio were fed, weanlings ate less feed, grew slower both in daily gain and wither height, and had less body condition. However, reduced growth can also happen when too much protein exists in a ration compared to its amount of energy.

This situation often occurs when horse owners add too much supplemental protein to a commercial feed already formulated for young growing horses. The same can be said of farmers applying fertilizer (nitrogen) too early in the Spring.

Without the energy provided from the sun, the grass cannot utilise the applied nitrogen. Dietary protein provides the building blocks but it is the calories that fuel the construction and enable the body to utilise the protein.



Research indicates that the first 40 days of gestation may be a critical time for the pregnant mare. During this time period, early embryonic death has been shown to be high when mares were on poor-quality feeding programs. However, generally for the first 8 months of gestation, the protein requirements for a broodmare are similar to the maintenance requirements for an adult horse.

Protein requirements increase greatly in the 9th, 10th, and 11th months of pregnancy, with protein needs during these months increasing more than that of energy. Protein quality is very important in early lactating mares, especially for those which are being re-bred.

Non-lactating broodmares are often placed on a higher plane of nutrition 30 to 45 days before being bred. Such a program includes adding grain to the ration to increase energy and protein intake. It seems advisable to continue such a feeding program until broodmares are pronounced in foal at 40 days of pregnancy.

It has been shown that broodmares can be conditioned to store body fat for energy use during late pregnancy and in early lactation. In contrast, horses cannot store any appreciable amount of protein in their bodies.

As noted previously, protein fed above the body’s requirement is converted into energy and/or stored as body fat and/or excreted. Consequently, feeding an adequate amount of protein in late pregnancy and early lactation is very important, since mares do not have body stores of protein to draw upon.



Young, growing horses have the highest protein requirement. High-quality protein promotes not only proper weight gain, but also skeletal and muscular growth in weanlings. Protein quality is critical as a specific amount of the dietary amino acid lysine is required.

New research has shown that a lower protein ration (9%) supplemented with adequate lysine (0.6%) and threonine (0.4%) resulted in equal or greater growth in young, growing horses from birth to yearling age compared to feeding a higher protein ration (14%).

This shows the importance of protein quality in diets for young horses. It is possible that future research will show other amino acids to be limiting in the rations of young, growing horses. Feeding rations that incorporate high-quality protein sources, such as milk by-products, soybean meal, and alfalfa meal, should ensure an adequate intake of amino acids.


Red Mills

Horse owners and trainers often feed more protein to performance horses compared to amounts fed to mature, maintenance horses. This practice is not necessary. While there is an increased protein need for performance, considering the nitrogen loss in sweat and exhaling, this increase is not great.

The major nutritional concern with performance horses is an increased energy need. Consequently, performance horses are usually fed more concentrate. Since more concentrate is fed, the performance horse actually has a greater intake of protein, which readily satisfies its need for more protein.

It is not always necessary to increase the protein percent of a concentrate feed to 14%-16% for mature, performance horses. A 10%-12% protein feed fed at the recommended feeding rates can be more than adequate when fed with good quality hay, as the amount of concentrate fed is increased to meet the energy need.

The extra concentrate fed provides the amount of additional protein needed due to increased performance. Most commercial racing feeds are formulated with 14% protein content. This can allow for the dilution effect should the feed be mixed with oats or beet pulp.


Many race horses receive below the recommended daily intake of forage. Feeding a higher protein feed will ensure protein requirements are met. Also younger race horses still growing will benefit from a higher protein percentage racing diet.

In order to excrete large amounts of excess nitrogen and urea, the horse must drink more water and produce more urine, which in turn has a higher ammonia content. This places an added demand on body water and electrolyte reserves and the increased ammonia fumes can irritate the upper respiratory tissues and reduce the efficiency with which the horse copes with other respiratory allergens such as fungal spores.

A certain amount of the excess nitrogen will also be excreted in the sweat. Horses on a high-protein diet will often have thick, patchy, lathery sweat which is less effective in cooling than a thin, clear, watery sweat.

Another detriment of excess protein in the diet is that it produces more body heat in breaking the proteins into energy as compared to using carbohydrates or fats for energy. This is not ideal in the performance horse.


All horses require protein – it is absolutely necessary for the body to survive – but the amount and quality of protein needed vary considerably among the different life stages of horses and the use of the horse. A surplus is equally dangerous as a deficiency, so a delicate balance is required between feeding enough to ensure best results and overfeeding enough to cause disorders and inhibit performance.

There are several important factors concerning protein which should be evaluated when selecting a feed for the horse: the digestibility of the protein, the amino acid content of the protein and the protein to energy ratio (PER) of the ration. These factors are especially important when considering the requirements of the growing horse.

While protein is vital for the proper daily functioning of the body, this does not make it “superior” to any of the other nutrients, carbohydrates, fats, minerals, or vitamins. Other than water, no one nutrient is more or less important than the other. A balanced diet is key in feeding horses regardless of age, work load and breeding status.

Protein is absolutely necessary for body building and tissue repair and in the formation of muscles, bones, blood cells, enzymes, hormones, tendons, hooves, skin, the internal organs such as heart and liver, and indeed almost all body tissues in all classes of horses. Dietary protein is both a greatly overemphasized and misunderstood nutrient and possibly should be considered more friend than foe.



  • Protein in Horse Diets: The Balancing Act. Frederick Harper. University of Tennessee
  • Protein Requirements and Digestibility: A Review. Joe D. Pagan. Kentucky Equine Research Inc
  • The Effects of Feeding Excess Dietary Protein. Ellen Collinson


About the Expert 

  • Louise Jones 

United Kingdom 

  • Qualification 

BSc, MSc, R.Nutr 

  • Designation 

UK Head of Equine 

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For further information on protein, contact the RED MILLS nutrition team.

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