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Defining Digestible Protein for Horses

horse in pen

Nutrition articles frequently refer to protein quality and essential amino acids. What’s often not discussed is the digestibility of protein.

When we use the term “crude protein,” we are essentially talking about a calculation based on measured nitrogen. Protein is about 16 percent nitrogen by weight, so if we measure the amount of nitrogen and multiply it by 6.25, this gives us a measurement of crude protein. It does not tell us anything about the quality of the protein. If you tested pure nitrogen this way, it would be 625 percent protein!

Digestible protein is the amount of the protein that is actually digested by the animal. In an over-simplified example, if you feed a horse 100 grams of protein and measured 50 grams in the feces, the protein would be 50 percent digestible.

The content of essential amino acids in the protein is extremely important to simple-stomached animals (horses included). We commonly talk about 10 essential amino acids (EAA) that must be in the diet as the horse cannot make them on their own (synthesize) them. These are:

  • Phenylalanine
  • Valine
  • Threonine
  • Tryptophan
  • Isoleucine
  • Methionine
  • Histidine
  • Arginine
  • Leucine
  • Lysine

A common memory aid in many nutrition text books is to use the first letters of these 10 as PVT TIM HALL (those of you who had a non-ruminant nutrition course still remember this acronym!).

The body can generally synthesize the other 12 amino acids, so they do not need to be in the diet, although there must be a supply of appropriate substrate to produce them.

In most species, lysine is the first limiting amino acid, with methionine and threonine close behind. We commonly talk about amino acids as the building blocks of protein. Once a horse runs out of an essential amino acid, it cannot build any more protein and the rest of the amino acids are used inefficiently for energy.

If you have a horse on a diet that is calculated to have adequate “crude protein” but essential amino acids are not present, the horse simply cannot use the protein to build and maintain muscle, hair, hoof and skin and you will see changes in the appearance of the horse, such as loss of topline condition, rough hair and scaly hoof surface.