The Basics of Protein in Horse Nutrition

A major constituent of the horse’s body is protein. Eighty percent of the horse’s fat-free, moisture-free body composition is protein. Protein is the predominant component of all tissues in the body as well as enzymes, hormones, and antibodies, and it is a critical part of the horse’s diet throughout its life.

Proteins are complex compounds composed of an infinite number of combinations of 22 different amino acids. Although all of them are needed for synthesis of body protein, some can be produced in body tissues and do not need to be supplied in the feed. These are referred to as nonessential amino acids, while those that must be provided in the diet are called essential amino acids. There are 10 essential amino acids. The essential amino acid most likely to be deficient in the diets of growing horses is lysine. A great deal of research has been done on the requirement for lysine by growing horses. Studies have shown that horses fed diets deficient in lysine will grow more slowly than horses fed a diet high in lysine, even if the crude protein percentages of the diets are identical. Researchers have suggested that the second limiting amino acid for growing horses may be threonine.

There are several sources of supplemental protein that are commonly used in horse feeds. These include milk proteins, alfalfa meal, and a number of by-product meals made from the production of oils such as soybean meal, linseed meal, cottonseed meal, safflower meal, canola meal, and sunflower meal. What is often overlooked, however, is the amount of protein and lysine that is supplied by the grain portion of a horse ration. Typically, cereal grains contribute about 40 to 50% of the total protein of a feed for growing horses. The amount of lysine supplied from these cereal grains, however, is only about 30 to 40% of the requirement, since cereal grains are fairly low in lysine. Therefore, the supplemental source of protein used in horse feeds should be high quality. Alfalfa, milk proteins, and soybean meal are all good sources of quality protein for growing horses. Protein supplements that are deficient in lysine include linseed meal, cottonseed meal, canola meal, and peanut meal.

Too little protein in the diet is cause for concern and may lead to sluggish growth, poor appetite, a dull hair coat, and a general unthrifty appearance. Overfeeding protein is also undesirable, especially when horses are housed indoors. A portion of the nitrogen in protein is excreted in the urine. This nitrogen, in the form of ammonia, can accumulate in poorly maintained or inadequately ventilated stalls. Ammonia fumes irritate the horse’s respiratory passages, contributing to breathing problems, particularly in performance horses.