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What is Protein?

Chestnut horse standing side on showing good muscle build and shiny coat

Well muscled horse

Protein is a vital requirement for all horses, the demand is greatest for the growing horse and the late trimester mare, and the least for the spelling horse, with performance horses in-between. Protein is utilised in the tissues of the horse, predominantly the muscle, blood, organs and skeleton, but  also needed for hormones, enzymes, antibodies and other body functions.

Protein is made up of amino acids – 22 of them – of which many can be made within the tissues of the horse, but the rest need to be provided in the feed and are usually all readily supplied by a basic diet, except possibly lysine, which can be lacking in many  foodstuffs. Lysine is the first limiting amino acid in the horse and threonine the second. A limiting amino acid is described as one, that if not present, will prevent protein from being made even if other amino acids are present in adequate quantities. The amount of protein in a diet has to be assessed as underfeeding (especially the growing horse) has negative consequences for the development of the structure of the horse; however overfeeding has negative consequences too for all horses, some of these effects will be highly detrimental to the young developing horse. Generally in New Zealand protein is most commonly over fed, deficiencies are rare in managed horse care.

How Much Protein Does a Horse Need?

Bay horse front on, showing over muscled neck, shoulder, back and hind quarters.

Over muscled horse

Measuring the amount of protein is sometimes confusing with feed labels stating a crude protein (CP) figure as a percentage of the feed, the digestible protein (DP) is the actual relative amount that the horse takes into the system and that is quality protein. The usability of the DP is affected by the quantity of the limiting amino acids. The actual quality of the protein is determined by its amino acid composition. There are many attempts to work out an equation to provide the ratio between DP and CP. These measurements require faecal testing and a scientific input, also the results vary according to the composition of the actual feed and forage ratio, studies show that quality protein (DP) is greater in grain diets than forage. DP is always lower than the CP and so when assessing total protein check whether figures are CP or DP, and allow for a higher % need of protein if the horse is predominately on a forage/pasture diet.

Protein is measured in tables in grams, but the details on the feed labels are often in %. Remember this is the CP% of that particular feed and, since it is invariably mixed with other feeds in the diet, it is not the overall % of protein in the horse’s diet. If a feed states a protein level of 14% it does not mean that the horse is getting an overall 14% protein content. The true answer  is about the quantity and the mix of the feed intake.

The NRC is an internationally recognised institution that provides data on feed requirements for horses. One of their tables shows the CP in grams required by various types of horses.


Weight (kg)

CP (gm)Lysine (gm)
Maintenance 50065623
 Breeding Stallion 500 820 29
 Pregnant Mare -9 months 500 801 28
 -10 months50081529
-11 months 500866 30
 Lactating Mares – Foaling to 3 months 500 1427 50
 – 3 months to weaning 5001049 37
 Working Horses – Light work 500820 29
 – Moderate Work 500 984 34
 – Intense Work 5001312 46

To consider how much protein to feed, first of all look at overall feed total intake as shown below:

Amount of total intake as a % of Body Weight

Mature Horse maintenance
(eg avg horse 500 x 0.016 = 8 Kg)
 Pregnant Mares1.6
 Lactating Mares2.3
Lactating 3 months to weaning2.0
Working Horse Light Work1.8
Moderate Work1.9
Hard Work 2.3
Weanling 6 months 2.5
Yearling 2.3
18 months 2.0
2 year old 1.9

(ref Equi-analytical laboratories)

Therefore a lightly worked 500kg horse should consume at least 8kg of feed forage (at least 65%) and concentrate feed (35%). A lactating brood mare should intake 11.5kg in the first few months and 50% forage and 50% concentrate. So just for a simple example let’s take a lig