If the ration contains enough protein, but too little lysine, the quality of the protein may be poor. A good protein quality is of particular importance for young growing horses and for lactating mares.
Deficiency of lysine in the diet impairs growth and development.
Good sources of lysine are commercial feeds where lysine has been supplemented. Soyabean meal is also high in lysine and can be fed as a supplement to the diet. Pasture is a good source of lysine but varies over the year with a high content in the spring and low content in the winter.