By Madeline Boast, MSc. Equine Nutrition

Protein is important for general well-being, health, and performance of horses. It is a major component of body tissues, second only to water. Aside from the inclusion in tissues, protein is also essential in the formation of enzymes, hormones, and antibodies. The vast use of protein in the body highlights its importance for health. 

The building blocks of protein molecules are amino acids. Proteins are differentiated by both the composition of amino acids and the length of the molecule chain. When protein in the equine diet is discussed, most of the time the term “crude protein” is used. For example, when evaluating a feed tag or hay analysis, a percentage of crude protein will be listed.

A hay analysis is the only reliable way to determine the nutritional value of forage. Photo: Shutterstock/Kateryna Puchka

Using this information to ensure that your horse’s protein requirements are met can be confusing as the nutritional requirement is not for protein, but for essential amino acids. Even when two products have the same crude protein percentage, their essential amino acid content can differ making them of unequal value to the horse. Understanding protein quality and essential amino acids helps owners make informed decisions about optimal protein supplementation and prevent deficiencies in their horse’s diet.

Protein Requirements 

There are 21 amino acids used to make proteins in the horse. They can be classified as essential, conditionally-essential, or non-essential. Essential amino acids cannot be synthesized by the body and must be provided in the diet. Conditionally-essential amino acids can be synthesized, but the horse may be unable to produce enough to meet their body’s demands due to a variety of reasons such as stress, performance, or disease. Non-essential amino acids are not required to be supplied in the diet as they can be synthesized adequately in the body.

Unfortunately, research has not determined exact requirement recommendations for all the 10 essential amino acids; in fact, there is only an established requirement for lysine. Nutritionists currently rely on a recommended intake of crude protein and lysine to determine if protein requirements are being met.

Limiting Amino Acids — Lysine is the most discussed amino acid for horses as it is generally the first to limit protein synthesis. For protein to be made, all the necessary amino acids must be present at the same time. If an amino acid is not present in adequate quantities, it is referred to as the limiting amino acid because it is limiting protein synthesis in the body.

In growing horses, lysine has been found to be the first limiting amino acid. Therefore, when discussing protein requirements, both the lysine and crude protein amounts in the diet are relied on. The other two essential amino acids that you may come across on a feed label are methionine and threonine.

Related: How to Accurately Feed Your Horse a Forage-Based Diet

Lysine, methionine, and threonine comprise what is thought to be the first three limiting amino acids in horses. However, most of the time only lysine will be included on a guaranteed analysis.

A simple way to picture what this means is with the analogy of a wooden barrel. If the staves (the narrow, curved strips of wood that form the sides of the barrel) are differing heights, then the barrel can only hold as much water as the lowest stave. Once above that level, the barrel will not hold more water, even if the other staves are a greater height (Figure 1). To relate this back to protein, when an amino acid is limiting (generally lysine, methionine, or threonine), the horse can only synthesize protein to the rate of the lowest essential amino acid.

Crude Protein — As mentioned, crude protein is listed on hay analyses and feed labels. The term “crude” is used because it is not truly a measure of protein but a measure of nitrogen. The protein’s quality and digestibility are not considered. It would be ideal to use digestible protein as a measurement of protein in feed; however, there is insufficient research to be able to use this measure in horses.

Calculating Requirements — The National Research Council (NRC) 2007 Nutrient Requirements of Horses provides crude protein and lysine requirements in grams based on the horse’s age, weight, and workload. Generally, lysine should account for 4.3 percent of the crude protein content in the diet.

Related: Understanding Equine Dietary Supplements

The hay analysis and guaranteed analysis on the feed tag will provide the crude protein value as a percentage; hence, the grams of crude protein supplied must be calculated based on the feeding rate. The following calculations demonstrate how to determine your horse’s crude protein intake.

Crude protein is listed as a percentage on feed tags and hay analyses, but meeting your horse’s actual protein needs depends on essential amino acids. To assess adequacy, calculate the grams of crude protein based on the feeding rate, since only crude protein, not specific amino acids, is listed. Photo: Clix Photography

A mature horse that is 500 kg and in light exercise requires 699 grams of crude protein and 30.1 grams of lysine daily. (These requirement values are taken from 2007 Nutrient Requirements of Horses.)

Diet #1 information

• 10 kg of grass hay fed daily (6 percent crude protein), 150 g vitamin mineral premix (not providing supplemental protein).
• Forage = 10,000 g x 0.06 = 600 g crude protein 
• This diet is supplying 600 g of crude protein daily and is not meeting this horse’s protein requirement. Using a ration balancer that is fortified with protein would be recommended to balance the diet, instead of a vitamin mineral premix.

Diet #2 information 

• 10 kg of grass hay fed daily (8 percent crude protein), 500 g of a ration balancer (20 percent crude protein)
• Concentrate = 500 g x 0.20 = 100 g crude protein 
• Forage = 10,000 g x 0.08 = 800 g crude protein 
• This diet is supplying 900 g of crude protein daily and is meeting the crude protein requirements of the horse. 

Feed calculation examples like these highlight how critical it is to test your hay and use that information to guide feeding decisions. As hay often comprises the majority of the horse’s diet, it is critical to know the nutritional value of the hay to determine if nutrient requirements are being met.

Protein Deficiency — With protein being a part of all tissues in the body and being involved in important bodily functions such as enzymes, hormones, the immune system, and even the transport of nutrients within the body, it is easy to imagine the negative impact deficiency has on horse health.

The body is unable to store amino acids for later use, hence when an amino acid is not present but is required for that protein, the protein synthesis stops. Poor hair growth and reduced hoof growth are common symptoms of protein deficiency, along with muscle and weight loss. In pregnant mares a protein deficiency can result in fetal loss, and in lactating mares can cause reduced milk production. Adequate protein supplementation is also critical for growing horses. When young horses are fed a diet that is deficient in protein their growth rates are negatively impacted. 

When deficiency occurs, the body will prioritize protein synthesis for essential functions. This is why protein deficiency is commonly noticed due to poor hair coat and hoof health.

Protein Excess — When it comes to optimized nutrition, balance is key. Excess protein in the diet can cause issues as well. Normally, excess protein is not concerning for healthy horses, but in some cases there can be a negative impact depending on the level of over-supplementation.

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When amino acids are provided in excess, the body does not store them for later use and therefore expels them. To do this, they are metabolized into urea and then excreted from the body in urine. This excess urea results in smelly stalls, negatively impacting the air quality of the barn, and can lead to excess water loss in severe cases. These are a couple of reasons why it is recommended to be cautious of significantly oversupplying protein.

Since we do not have exact requirements for all of the essential amino acids, balancing crude protein intake and essential amino acids content of the diet to ensure there are no deficiencies — while avoiding unnecessary over-supplementation — often depends on the quality of the protein in the diet.

Protein Quality 

Research has shown that horses are sensitive to protein quality, and this should be a key factor when deciding which protein source to use for your horse. Protein quality is a function of the digestibility of the protein source and the profile of amino acids.

When large amounts of poor-quality protein sources are fed to a horse, some of the amino acids will be provided in excess, and the key limiting amino acids may not be present in adequate quantities. The unused amino acids will be metabolized into urea which is then excreted in the urine. Protein synthesis will also be stopped at the point where the essential amino acids are not present. A high-quality protein source is digestible and contains high levels of essential amino acids.

Protein Digestibility — The horse’s gastrointestinal tract can be divided into two basic areas: the foregut and the hindgut. Dietary protein is primarily digested in the foregut of the horse (mainly the stomach and small intestine) by enzymatic digestion.

It is beneficial for protein to be digested in the foregut of the horse as this allows for greater absorption of amino acids to support body function and tissue repair. Protein that is digested by microbial fermentation in the hindgut is regarded as lower quality.

Horses that have an elevated requirement for protein, either from workload, or physiological state (such as a lactating mare), may benefit from supplemental protein sources that are readily digestible in the foregut.

Amino Acid Content — The amount and ratio of essential amino acids is the other factor that determines protein quality. In equine nutrition, when the lysine content of a protein source is greater, the quality is generally regarded as higher. For example, soybean meal is one of the most commonly used protein sources in equine feeds and regarded as high quality due to its high lysine content and the fact that the amino acid profile closely resembles the essential amino acids required by the horse. 

Protein is vital for a horse’s health, performance, and well-being. It supports tissue structure and is essential in the formation of enzymes, hormones, and antibodies. Illustration Copyright Horse Community Journals Inc./Photo: iStock/Stock Planets

The quality of protein will vary among sources as they have differing digestibility and amino acid compositions. All horses require protein, but when optimizing an equine diet, going beyond the crude protein content of the diet and evaluating the quality of the protein fed is recommended.

Common Sources 

There are many different protein sources used in equine diets. Soybean meal, whey protein, and alfalfa are examples of high-quality protein sources. Many commercial feeds will incorporate several of these sources and may also be fortified with essential amino acids.

Related: Nutrition for Weanlings and Young Horses

Soybean meal is what remains after the oil has been extracted from the soybeans, and the flakes cooked and ground into a meal (it is important to note that horses cannot have uncooked soybeans). This is regarded as a high-quality protein source for horses as it has a high lysine content and an overall crude protein content of about 45 percent.

Alfalfa is easily accessible to horse owners and is available in numerous formats, such as hay, forage cubes/pellets, and incorporated into commercial products. Feeding alfalfa also provides significant supplemental calories making it a great option for harder keepers.

Although horses are herbivores, they can safely consume whey protein, which is a byproduct from the dairy industry. Animal protein sources are considered to be higher quality protein sources as they contain more essential amino acids; however, most are, of course, not feasible to feed herbivores.

To determine the protein source in a product you must go beyond the guaranteed analysis and look at the ingredient list. 

Reviewing the ingredient list will help you make educated decisions on what product is preferable from a protein quality standpoint. Ingredients like soybean meal, whey protein, dehydrated alfalfa meal, along with directly added amino acids like L-lysine or DL-methionine are high-quality ingredients. Products that rely heavily on cereal grains such as ground wheat, corn, or oats might not be as robust in essential amino acid content.

To identify the protein source in a product, look past the guaranteed analysis and examine the ingredient list. This helps you assess protein quality and decide if a ration balancer is needed.

When comparing products with similar crude protein contents and ingredients it is beneficial to see if the guaranteed analysis provides an amount of lysine or even the first three limiting amino acids. With this information, you can opt for the product that uses higher quality ingredients and guarantees a certain amount of lysine.

Aside from the forages and concentrates that we feed our horses, amino acid supplements are also available. These products can be beneficial to increase the content of essential amino acids in the diet without adding unnecessary calories or increasing concentrate intake. Oftentimes, these products will be a combination of lysine, methionine, and threonine.

Understanding that not all protein sources offer the same nutritional value, when selecting a feed product for your horse it’s important to remember that the crude protein percentage alone doesn’t provide enough insight into the quality of the protein it contains. A review of the crude protein percentage, ingredient list, and amount of lysine in the product is recommended to determine if your horse’s protein requirements are being met. 

Take Home Message 

Adequate amino acid content of the diet is critical to protein synthesis. With protein playing such a significant role in body tissues and processes, it is of the utmost importance to ensure it is adequately supplied to support equine health. Reviewing the crude protein content of your feed and hay is a recommended first step, and when choosing between feed options, comparing the ingredient list to evaluate protein quality is beneficial.

If you have further questions about the protein content of your horse’s diet, contact a qualified equine nutritionist. 

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Related: How to Adjust Your Horse's Diet as They Age

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Main Photo: iStock/JackF