In the latest edition of the My Horse University news, letter, Holly Spooner, Ph.D. of Middle Tennessee State University gives important details about protein in the feeding of horses. According to Dr. Spooner, protein may be the most misunderstood nutrient class when it comes to feeding horses.
Balancing your horse's diet
Protein is used in the formation of tissue, such as muscle, as well as to make hormones, enzymes, and antibodies and while the horse's body is able to make some amino acids, others must come from the diet.
Protein is used in the formation of tissue, such as muscle, as well as to make hormones, enzymes, and antibodies. Protein is composed of chains of amino acids. There are twenty-two amino acids, and while the horse’s body is able to make some of these, other amino acids cannot be made and must come from the diet.
Those amino acids which must be consumed from the diet are referred to as “essential amino acids.” Essential amino acids for the horse include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
For protein synthesis to occur, the necessary amounts of all amino acids must be present. If a single amino acid is missing, protein synthesis may be halted.
Dr. Spooner uses the analogy of a beaded bracelet to make her point: "One of the easiest ways to understand this is to think of stringing a beaded bracelet. You are using colored beads (amino acids) in a repeating pattern. When you run out of a certain color, you must stop and cannot continue beading until that color is replenished."
"The first “colored bead” or amino acid to be in short supply to a particular animal or in a particular ration is known as the limiting amino acid. For horses, the first limiting amino acid is generally lysine.
While it is important to understand the role of amino acids, we often think of feeding horses whole proteins, a complex source of many amino acids. The amino acid make-up or profile of feedstuffs is highly variable. How well the amino acid profile fits the animal’s need is generally referred to as one element of protein quality. Protein sources for horses with desirable amino acid profiles include soybean meal and cottonseed meal.
The other element of protein quality is digestibility. Not all protein is created equal when it comes to the horse’s ability to digest and absorb it. Protein in the horse is digested primarily in the small intestine.
Fermentation in the large intestine may “free up” some additional dietary protein; and synthesis of proteins from microbes occurs there, but the contribution of the hindgut in absorption of amino acids is negligible, with ammonia being the main nitrogen product absorbed by the hind gut.
This is also the reason the addition of urea, a non-protein nitrogen source that is converted by rumen microbes to beneficial proteins in ruminants, is not useful in horse rations.