Protein in the Horse Diet

Structure

Proteins are made of amino acid building blocks arranged in specific ways, which creates the unique structures of each protein. During digestion, the proteins are broken down into their amino acid components in the small intestine and absorbed into the bloodstream. Amino acids are utilized in almost every body process, such as cellular development or enzyme signaling, and in the creation of many tissues, such as skeletal muscle.

Amino acids are classified as either essential or non-essential. Non-essential amino acids can be made in the horse's body, while essential amino acids cannot and must be provided in the diet. There are 10 essential amino acids: phenylalanine, valine, threonine, lysine, isoleucine, leucine, methionine, histidine, arginine, and tryptophan. Lysine is the most well-known because it is the first limiting amino acid, followed by methionine and threonine. This means that lysine is the first amino acid used up while making proteins, and further protein synthesis cannot continue until more lysine is supplied.

Think about building proteins like building a Lego house. You have 20 red Legos and 30 blue, building in an alternating pattern. Then you reach down to add the 21st red Lego, but there are no red Legos left. You must wait to keep building this pattern until you get more red Legos. The red Legos are your first limiting material; if these were amino acids, lysine is the red Legos.

Dietary Supplementation

Not all feedstuffs are created equally when it comes to protein amount. For example, legume hay tends to be higher in protein than most grass hay. Common sources of protein in equine diets are legume forages, cool season grasses (mid-maturity), alfalfa meal, soybean meal, flaxseed, fish meal, and linseed meal (among others).  However, despite differences in the amount of protein in different feeds, a true protein deficiency is not common in horse diets, and many can even be over-supplemented in protein.

Regarding caloric density, proteins are much lower in calories than carbohydrates and fats. As mentioned earlier, protein is important for various body processes and structure and is not considered a key source of energy (unlike fats and carbohydrates, which are important energy sources). Each horse's protein demands differ depending on weight, growth status, working status, and reproductive status (Table 1). Protein values are estimated and often referred to as "crude protein" because it is estimated from the amount of nitrogen (component of amino acids) in the feedstuff and multiplied by 6.25.

Most horses require 8–12% protein in their diet, but that demand is higher for lactating, growing, or heavily exercising horses. Since forage is the bulk and foundation of most horse diets, a simple hay test will be able to estimate the amount of protein in your hay.

The National Research Council (NRC, 2007) states that the protein requirement for the average 1,100-pound (500-kg) horse is 1.38 pounds (630g) per day. However, most feed labels report the amount of protein as a percentage. Therefore, you must calculate how many grams of protein your horse is consuming. This can be done by first multiplying the amount fed by the protein percentage listed, then multiple by 453.6 to get grams. For example, a 10% protein grass hay fed at 15 pounds per day supplies 1.5 lbs (680g) of protein:

15 lbs hay × 0.10(% protein) = 1.5 lbs protein × 453.6(g) = 680.3 g protein

This exceeds the requirement of the average 1,100 lb. horse. Work with an equine nutritionist to determine the protein needs of your horse and the best sources.

*Adapted from NRC, 2007

Common Misconceptions

Some of the common misconceptions are about the amount of protein supplied. Overfeeding protein will not give your horse a nice topline or create defined muscles; they also need sufficient exercise to build muscle. Keep in mind they cannot properly build muscle if the protein source is low in lysine and other limiting amino acids.

Protein is not as calorie-dense as carbohydrates and fats and should not be used as an energy source. High protein feeds are not necessarily high in calories; think about low-calorie ration balancers that are 30% protein. However, these ration balancers are fed at low feeding rates (1–2 lbs daily) and supply around 136g of protein per pound. They still need to be supplemented with a quality forage to meet daily protein needs for the average 1,100-pound horse.

Lastly, protein does not make a horse "hot." Many times, diets that are high in protein are also high in energy, and it is likely the excess of energy that increases activity from the horse.

Conclusion

Protein is a very important nutrient in the horse diet. Proteins are made from amino acids and are involved in multiple bodily processes. Amino acids are classified as essential and nonessential, with lysine, methionine, and threonine being the most important essential amino acids. Proteins are found in multiple feedstuffs, including hay, pasture, and commercial concentrates. Work with an equine nutritionist to determine the protein needs of your horse and the best sources of protein.