Understanding Protein in Horses' Diets

The amino acid profile of a feed is more important for a young, growing horse than for a mature one; adult horses are far less sensitive to differences in protein quality than younger ones.

Photo: Anne M. Eberhardt/The Horse

Of all the components of your horse’s diet, protein is probably the most misunderstood. Long assumed to function as an energy source for the body, protein mainly functions to provide amino acids (the building blocks of bones, muscles, and soft tissues) for growth and repair.

What are amino acids good for? Virtually all of the horse’s vital processes, it seems. Amino acids are involved in the synthesis and the release of hormones, the synthesis of neurotransmitters and enzymes, and the regulation of sleep, appetite, and blood pressure, to name just a few functions. But primarily, amino acids are needed for the formation and repair of muscle tissue and other soft tissues throughout the body. On a fat-free, moisture-free basis, they account for approximately 80% of a horse’s total structure.

Growing horses, which are ‘building’ new tissues as they mature, and horses used for breeding, have higher protein requirements than do mature horses being used for pleasure or performance. Whether working or idle, most mature horses need surprisingly small amounts of protein.

Inside a Protein Molecule

Proteins are “chains” made up of various combinations of the twenty-two different amino acids that exist in nature. Amino acids are relatively simple organic compounds, consisting of a basic amino group and an acidic carboxyl group. Carbohydrates and fats also contain carbon atoms with hydrogen and oxygen atoms attached, but amino acids alone contain nitrogen and sometimes sulfur. The position and number of the amino acids in a single protein make up its “amino acid profile.”

When a horse ingests protein, enzymes and acids break up the chain of amino acids in the digestive tract, and the individual amino acids are absorbed through the wall of the small intestine and into the bloodstream via the liver. From there, they travel to the sites where they are most needed for growth or repair of tissues.

Although amino acids are absorbed from the small intestine in a format relatively unchanged from their original chemical composition, the horse’s body does have the ability to change some amino acids into different formats as the need exists, a process that occurs in the liver. However, the body does not have the ability to create all the amino acids it needs. Some amino acids can only be synthesized by micro-organisms or green plants. These are called the “essential” amino acids, and the horse must obtain them from his environment. (“Non-essential” amino acids are those the horse can synthesize himself.)

A good quality protein source is a food that provides a sufficient amount of these essential amino acids, particularly the amino acids lysine and methionine. Lysine is often called the “first limiting” amino acid—meaning that if insufficient quantities of lysine are present, then the horse’s body will have difficulty using any of the other amino acids available. Methionine is second in importance.

The amino acid profile of a feed is more important for a young, growing horse than for a mature one; adult horses are far less sensitive to differences in protein quality. Nor does it matter to the horse whether a particular amino acid comes from a natural source or whether it is chemically synthesized. Lysine and methionine are often deficient in horse feeds, and as they can be synthesized inexpensively, it’s quite routine for feed companies to add these ingredients to improve the overall amino acid profile biochemically. (Not all amino acids can be easily synthesized, however.)

Can protein serve as an energy source? Well, yes, but metabolically it’s an expensive process, producing three to six times more heat than the breakdown of carbohydrates or fats and yielding considerably less energy. The heating factor might be beneficial in a cold environment, but it also might contribute to excessive sweating and possible heat exhaustion during hard work, especially in a warmer climate. And because protein is one of the more expensive ingredients in a feed, it’s impractical to feed higher levels of protein in search of a performance advantage. You’ll do far better by upping the levels of carbohydrates and fats.

In a future article, we'll decipher how to assess the protein levels in your horse's diet.

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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