What is Protein?

Horses require six nutrient classes to meet their nutritional requirements, one of which is protein. Protein is a major component of body tissue and plays a key role in enzymes, hormones, and antibodies. Proteins are composed of amino acids; the 21 amino acids join in a large variety of various combinations to form proteins.

In equine nutrition, there are essential and non-essential amino acids. The essential amino acids must be supplied in the diet as the horse cannot produce them. The current published nutrient requirements for horses (NRC, 2007) states a requirement for lysine, but research has not fully elucidated the horse’s requirements for the other essential amino acids. Despite this, it is well reported that the first three limiting amino acids in an equine diet are lysine, threonine, and methionine.

Protein Quality

When discussing protein, it is important to understand the concept of protein quality. Protein quality has to do with the amino acid composition of the feedstuff as well as how readily it is digested. A high-quality protein is one that provides essential amino acids in the correct proportions; lower quality protein may not adequately provide essential amino acids or may have poor digestibility.

Increased Protein Requirements

The amount of protein a horse requires depends on their age, physiological state, and workload. There is protein in forage, and when a quality hay is being fed, it often meets the horse’s protein requirements. However, this highlights the importance of a hay analysis, as you need to know the exact protein content in the forage.

Young horses that are growing, horses under a heavy exercise and pregnant or lactating mares will likely require supplemental protein. Mares in late gestation and in the first few months of lactation have the highest requirements. Feeding recommendations for these horses is to first provide them with a protein-rich forage, then add supplemental protein as needed such as a complete feed.

Consequences of inaccurate protein supplementation

Research has illustrated that protein deficiency is less common than excess protein in equine diets. Precise ration balancing in regard to protein is important as there are negative consequences to both over- and under-supplying protein. Protein deficiency in mature horses may present as weight loss, muscle loss, inadequate feed intake, and poor hoof and hair growth. For pregnant mares, they may experience fetal loss and poor milk production if they are deficient in protein. Young horses that are still growing will have a reduced growth rate if protein is not adequately supplied.

Although there are many negative consequences of protein deficiency, excess protein can also be a serious problem. When a horse is supplied with too much protein, the urea content in the urine will increase, and when there is additional urea, the horse will experience an increase in water loss. This will in turn, increase the water requirement of that horse which could be problematic for horses under intense exercise or those who may already struggle with hydration. Other problems with increased urea excretion are that it can negatively impact air quality in the barn, therefore leading to respiratory problems. Issues with the acid-base balance when exercising is another consequence that has been shown in research when the horse is consuming too much protein. Research has also reported earlier fatigue during exercise when protein is over-supplied.

Knowing What to Feed Your Horse

Now that you have some background information on protein in equine diets, how do you know what feed option to choose for your horse? Step #1 is to get your hay tested; this will give you a crude protein percentage for the forage, which is likely the primary component of your horse’s diet.

As an example, let’s say the horse weighs 1,100 lbs and consumes about 20 lbs of forage per day. Your hay analysis results come back and shows that the forage is 9% crude protein. So, how do you know if this is meeting your horse’s requirements? Below is a calculation for the grams of protein in 20 lbs of forage with 9% crude protein:

20 lbs x 0.09 = 1.8 lbs of protein, or 816.5 g

This means that this forage will be meeting the crude protein requirements for an 1,100-lb horse if they are at maintenance, in light exercise, or in moderate exercise. Excess crude protein would only need to be supplied if the horse was in heavy or very heavy exercise. Below is a table showing the NRC, 2007 crude protein requirements for a mature 1,100-lb horse in various workloads:

Crude protein chart


Most frequently, horse owners will discuss protein as percentages in the feed; however, the NRC, 2007 guidelines are not given in percentages, they are given in grams. Therefore, knowing how to do the basic calculation above can save you money and spare your horse from the negative consequences of inadequately supplied protein.

Most grass hay will average 8-12% protein, and legume-based hay will generally be higher in protein than grass hay, so why are you worrying about the protein percentage in your complete feed? It is likely that your horse doesn’t even need the additional protein.

To conclude, protein is an essential component of the equine diet. However, it is often over-supplied, which can negatively impact the horse’s well-being. Therefore, prior to supplementing protein with commercial products, have your hay tested to evaluate if you need to be adding an additional protein source. Working with an equine nutritionist is a great way to ensure a precise, balanced diet.