A Close Look at Protein Quality

By Juliet M. Getty, Ph.D.

Did you know that there are literally hundreds of different proteins in your horse’s body?  Here is a short list of where proteins are found:

  • Vital organs (heart, lungs, liver, kidneys, etc.)
  • Skeletal muscles
  • Blood proteins
  • Skin, hair, and hooves
  • Joints and bones
  • Eyes
  • Connective tissue  
  • Enzymes  
  • Immune function
  • Red blood cells

To build these, and other body proteins, your horse needs the amino acids (building blocks), supplied by the protein in his diet. And that brings up the concept of “protein quality.”

The proteins quality is of concern when planning diets. This is because there are 10 amino acids that are considered “essential” – meaning your horse cannot produce them and therefore, they must be in the diet. For a protein source to be worthwhile, it must provide these 10, in their proper proportion.

Poor-quality protein

If you feed only one type of grass, day in and day out, without any other source of protein, the overall protein quality of the diet will be poor. Naturally, the vital organs such as the heart, liver, kidneys, and lungs must receive amino acids to support life. Other organs such as the pancreas, spleen, digestive tract, bone marrow, adrenal and pituitary glands (to name a few) are also on the priority list. But the skin, hooves, joints, eyes, and hair will only receive the amino acids they need once the priority tissues have been fed. Therefore, these areas are a window to the inside of the horse because if these tissues are in good health, there’s a good chance the horse is doing well on the inside.

Determining protein quality

How do you know if your horse’s diet has enough essential amino acids in the right proportion? To get this information, the first place you’re likely to look is the percentage of crude protein (CP) written on the feed label. Well, I’m sorry to tell you this, but CP tells you virtually nothing about the protein’s quality.

A better approach is to look at the list of ingredients on the label. This will tell which protein sources are included. Look for a variety of ingredients such as soybean meal, alfalfa meal, beet pulp, flaxseed meal (and other seed meals), and grain by-products such as rice bran and wheat middlings.

Can you feed too much protein?

Healthy horses can tolerate protein levels that slightly exceed the requirements. Any excess amino acids, beyond the horse’s need for body proteins, will be burned for energy or made into fat (for later energy usage). When this happens, nitrogen must be excreted in the urine as urea. For the most part this is not a concern; however in aging horses or those that have kidney problems, excess protein puts a strain on the kidneys.

Calcium can be lost. The kidney loses its ability to retain calcium when protein intake is high, resulting in urinary calcium losses. This can create an imbalance with other minerals such as phosphorus and magnesium. Growing horses and those that are exercised intensively not only need to maintain normal blood calcium levels for muscle function, but also for healthy bones.

It is best to combine grasses and legumes

Provide 1.5 to 3.5% of your horse’s weight as mixed forages (grasses plus legumes such as alfalfa and clover). Contrary to popular opinion, protein does not cause laminitis. Alfalfa is higher in protein that grass hays, but in moderate quantity it boosts the overall protein quality (by balancing the amino acids found in grasses), is low in sugar, and supplies additional minerals. However, I do not recommend feeding alfalfa as the only forage source; it should be mixed (no more than 50%) with grasses to keep protein levels in check and prevent intestinal enterolith formation.


In summary, protein is a necessary dietary component that is often ignored. Avoid the mistake of relying on crude protein to judge your feed or hay. Instead, evaluate the actual ingredients in order to make certain that you’re giving your horse a variety of protein sources; this will ensure a balanced amino acid profile.


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