One of the nine essential amino acids required from dietary sources lysine is regarded as being the first limiting amino acid for growth and tissue repair. In other words, regardless of how much protein is present in the diet, if lysine is insufficient the animal cannot grow.
Lysine is the only amino acid for which the requirement of horses has been firmly established for all stages of growth, adulthood, work, pregnancy, and lactation. For this reason, it is used as a measure of protein quality and generally the lysine content of feeds is recorded on feed labelling. Good sources of lysine include beans like soy, legumes like Lucerne, lupin, and rice bran. The recommended lysine requirement for a 545kg horse ranges from 30g/Day for maintenance up to 92g/Day for a mare in peak lactation equivalent to 4.3% of the crude protein intake.