<p>The requirement for protein depends upon the physiological state of the animal, and characteristics of the protein itself. In addition, the quality of the protein characterised, for example, by amino acid composition and digestibility, will interfere with the amounts of protein that you have to give the horse. The quality is of particular importance for growing horses. Lysine is usually the first limiting amino acid (the first amino acid to come into short supply in the diet).</p>