Lysine is an essential amino acid - it must be present in the diet, it cannot be synthesized in the horse's body.

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Deficiency of any of the essential amino acids will lead to poor growth and poor tolerance to stress. Lysine deficiency may cause poor quality hooves and coat, delayed wound healing and poor tolerance to the stress of exercise with little ability to build muscle, poor red blood count and decreased fertility.

Diets based on grass hays or grass hay and grain may be deficient in lysine. General supplementation guidelines for a 500kg horse, based on activity and diet are as follows:-

Activity: Maintenance Light work Medium work Heavy work V Heavy work
Grass hay 5.5g 5.5g 7.75g 11g 11g
Grass hay & oats 1.4g 1.4g 2.5g 9.8g 9.8g


Digestion of many nutrients decreases with age, and protein is no exception. To assist the older animal, up to 10g of lysine can be given per day.


A lactating mare's lysine requirements are extremely high, up to three times maintenance levels in the first month of milk production.