High-Quality Protein: Understanding Amino Acids in the Equine Diet

When creating a nutrition plan for horses, it is important to understand how much protein they need and which protein sources are most suitable for each individual.

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Feeding your horse high-quality protein in the right amounts will help support the muscles they are working while under saddle. | Getty Images
Proteins are large molecules composed of many smaller building blocks called amino acids. Protein is a vital nutrient for horses, and a horse’s protein requirement varies depending on age and workload. Currently, equine nutrition researchers do not know how much of each individual amino acid horses need, so they’ve based nutrient requirements on crude protein (CP) level and lysine, also known as the first-limiting amino acid.

Some of the amino acids horses need are essential, meaning they cannot be synthesized by the horse and must be supplied in the diet. When selecting a protein source and understanding protein in the horse’s diet, you must consider various factors—simply looking at the percentage of crude protein does not supply sufficient information.

High-Quality vs. Low-Quality Protein

Indeed, meeting a horse’s protein requirement is more complex than simply checking the CP content. Horse owners have various protein sources to choose from, some of which are considered higher quality than others. “There are two characteristics that define high-quality protein: 1) it needs to be easily digestible in the small intestine; and 2) the protein needs to have a good amino acid profile,” says Lexington, Kentucky-based Caroline Loos, PhD, an equine nutritionist and the head of research at Cavalor.

It is critical for horse owners to understand the difference between high- and low-quality protein sources due to the specific amino acids horses need to consume. “There are 21 main amino acids that make up a protein, and 10 are essential to provide in the diet,” says Loos. “However, the ratio and type of amino acids differs between protein sources. Therefore, the protein source/protein quality in the diet will determine which amino acids will be available to the horse.”

High-Quality Sources of Protein for Horses

Because protein quality is more important than quantity, says Loos, owners should examine the ingredient list to see which protein sources were used and how much of each. Some high-quality sources include soy, canola, flax, alfalfa, or a blend thereof, she adds. Other sources include canola meal, flaxseed meal, sunflower meal, and certain beans. Note whether higher-quality sources are listed at the bottom of a feed or supplement’s ingredient list, because this might indicate very little was used.

Consider forage when choosing high-quality protein sources for your horse, says Carey Williams, MS, PhD, professor in the Department of Animal Sciences at Rutgers University, in New Brunswick, New Jersey. Alfalfa and other legume sources of protein are favorable, followed by soybean meal if a horse needs more protein beyond what forage can provide, she adds.

Evaluating Protein Content in the Equine Diet

Prior to choosing the best protein source for your horse, evaluate his current diet—both forage (hay and pasture) and concentrates—so you know what he’s currently consuming. “Most horses only need about 8-10% crude protein in the diet,” says Williams. If you have your hay tested and it meets this requirement, you might not need an additional protein source for your horse, she says.

Concerns With Oversupplying Protein

Excess protein is a common problem in equine diets, especially when owners do not know the protein content of their forage. When protein is oversupplied, the horse’s body does not store it for later use like it does carbohydrates or fats. Therefore, it must be broken down and excreted from the body. If a horse is healthy, consuming excess protein might not have immediate consequences.

“As long as a horse has healthy kidneys, they will excrete the excess protein as urea and ammonia; however, I would be concerned with older horses, as prolonged excess protein in the diet can become hard on the kidneys,” says Williams. Quality protein sources are often expensive, so excess protein in the diet means you’re wasting money, she adds.

Oversupplying protein over a long period has negative implications for horse health, including increased heat production, urine output, and plasma lactate concentrations, says Loos. High ammonia levels in the stall can also irritate the mucosa in the eyes and respiratory tract.

Muscle Development and Maintenance

Horse owners often correlate their horses’ protein intake to muscle development. In her recent study Loos notes that about 0.25 grams CP/kilograms body weight (~ 140 grams CP for the average 550-kilogram/1,200-pound horse) of a high-quality source was sufficient to result in near maximal activation of the muscle synthetic pathways. “Feeding more than this amount provided no additional stimulation,” she says. “Practically, these studies illustrate that we don’t necessarily need to feed a lot of protein to provide maximal stimulus to the muscle—more important is the quality (of the protein) we feed.”

Although nutrition plays a key role in muscle development, it is important to remember there are other contributing factors. Correct exercise is also important for muscle development and maintenance, says Loos. Combining a quality training program with a diet that includes adequate protein will support optimal muscle development and maintenance, adds Williams.

Take-Home Message

When selecting protein sources for horses, it is crucial to consider high-quality sources that are easily digestible with good amino acid profiles. It’s also important to evaluate the protein content of feed sources, especially hay, to determine if your horse needs additional protein supplementation. Avoid oversupplying protein, as it can lead to several issues and increase your feed expense.


Written by:

Madeline Boast completed her master’s in Equine Nutrition at the University of Guelph and started an independent nutrition company known as Balanced Bay. She has worked with a variety of equids—from Miniature Ponies to competing Thoroughbreds. Boast designs customized balanced nutrition plans that prioritize equine well-being, both for optimal performance and solving complex nutritional issues and everything between. 

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