In young, rapidly growing horses, horses at work and lactating mares the hay fed should be supplemented with concentrated energy sources to meet their energy requirements. Protein Horses use protein to synthesize various body tissues, such as muscle. Proteins are composed of amino acids and will vary in amino acid composition. Currently, the exact amino acid requirements of horses are not known. But feeding an adequate source of protein should ensure that horses get the composition of amino acids they need.
Protein requirements vary for different classes of horses.
Young, growing horses have a higher requirement for protein because they are growing body tissues like muscle and bone. Mature horses have a much lower requirement for protein than do young horses since mature horses need protein for maintenance of body tissue rather than growing new tissue. Note that horses with increased exercise do not need more protein than do horses not in training. They lose a small amount of nitrogen in the sweat, but the additional grain fed to meet the performance horse's energy needs will more than adequately provide for the increased nitrogen requirement without increasing the percent protein in the diet. When protein is fed beyond what the horse requires, the body uses it as an energy source and excretes the unused nitrogen in the urine.