Nutrition
Amino Acids
Basic Information 
Indications
Amino Acids
• There are approximately 20 amino acids categorized as either essential or nonessential.
• Essential amino acids: Considered to be of greater concern when formulating diets because they cannot be manufactured in the horse’s body at an adequate rate or amount to satisfy the horse’s needs.
• There are 10 essential amino acids: Arginine, histidine, methionine, isoleucine, leucine, lysine, phenylalanine, threonine, tryptophan, and valine.
• Lysine is the first limiting amino acid (most likely amino acid to be in short supply compared to demand).
• Threonine is speculated to be the second limiting amino acid.
• Recent interest in developing “ideal” amino acid profile for the horse.
Protein digestibility
• Protein digested in the foregut via enzymes yields amino acids for absorption into the bloodstream.
• Protein fermented in the hindgut yields ammonia.
• Therefore the goal is to have the majority of protein digested in the foregut in order for the horse to enhance the amino acid pool for use in the body.
• Concentrates tend to have higher foregut digestibility compared to forages and would therefore be a better source of amino acids. It is possible to provide a horse with a high protein diet from a forage like alfalfa yet not provide enough lysine or other amino acids due to the amino acid profile of the protein source and the digestibility of the protein source. Consideration should also be made for horses with a lowered ability to digest feeds such as aged horses.
• Digestibility of the protein source will affect the supply of amino acids in the body.
• Concentrates tend to provide higher quantities of amino acids and are generally more digestible especially in the foregut.
• At least 60% of the total protein in the diet should be provided by the concentrate portion of the diet for growing horses and lactating mares.
• Older horses may require additional dietary protein and amino acids due to decreased digestibility with age.
Lysine
• The first limiting amino acid for growing horses
• Increasing the lysine content of the diet has resulted in an improvement in average daily gain (ADG) for growing horses of all ages (foals, weanlings, yearlings).
• Diet content of lysine for the lactating mare becomes of utmost importance not only to ensure an adequate supply in milk for the foal but help the mare maintain her own body tissues without resorting to using reserves in muscle mass to support milk production.
• In exercising horses, supplemental lysine has resulted in improved muscle mass compared to those that did not receive supplementation.
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