Protein and the Horse’s Diet

Dietary protein supplying amino acids is a necessary component of the horse’s diet.  Proteins are required for a multitude of key bodily functions. These include major components of muscle, enzymes, hormonal roles, the immune system, and transport of nutrients across membranes and in blood. Protein assists in tissue repair and growth, hence the amount of protein required in growing, pregnant, lactating or heavy exercising horses is increased. Proteins are made from various combinations of amino acids, and consist of carbon, hydrogen, oxygen, nitrogen and some Sulphur atoms. In horse feeds, protein is expressed as crude protein (CP) because it is an indirect or ‘crude’ estimation of that feed’s protein content. Digestible protein (DP) would be more ideal to describe a particular feed, but there is insufficient research data for DP values for horses. Typically, DP varies from ~40 to 80% of CP.

Amino acids are either dispensable (nonessential) where they can be produced by the horse in its body tissue or indispensable (essential) where the horse can’t synthesize them in quantities to meet the horse’s requirement. Essential amino acids (EAA) must be provided in the diet. Requirements for all of the EAA have not been established for the horse. High quality protein feedstuffs are those which contain high amounts and proportions of the EAA. The best indicators listed on commercial feed tags are the EAA lysine, methionine and threonine. Only lysine is routinely listed.

The horse consumes a variety of ingredients from roughage to grains that each have varying levels of protein quantity and quality. Table 1 lists the protein, lysine, methionine and threonine content in common feedstuffs for horses. High quality protein includes legumes, young grass pastures, soybean meal, canola meal, and linseed meal. Soybean meal is rated the highest quality due to the amino acid profile being superior to most other plant products. Moderate to low quality protein include cereal grains. Grain by-products often contain moderate or low-quality protein, even though they may be relatively high in crude protein.

Table 1. Crude protein, lysine, threonine and methionine content of common feeds fed to horses1

	Crude Protein, %	Lysine mg/g CP	Threonine mg/g CP	Methionine mg/g CP
Corn	9.1	29	36	21
Whole Oats	13.6	42	35	17
Flax seed Meal	32.6	37	38	18
Soybean, Meal, 48	53.8	63	40	14
Grass Hay, mid-maturity	13.3	35	36	13
Legume Hay, mid-maturity	20.8	51	44	16
Mixed Grass & Legume Hay, mid-maturity	18.4	38	38	14
Grass pasture	26.5	35	36	14
Legume pasture	26.5	52	44	16

¹Percentages are given on a 100% dry matter basis.

In order to provide adequate dietary protein, it is important to understand dietary sources of protein available to the horse and the protein and amino acid requirements for horses. The Nutrient Requirements of the Horse (2007) lists protein requirements as a % of dry matter intake (DM) or on a weight basis as grams of crude protein. The concentration in the feedstuff (% CP) multiplied by the weight of that feedstuff fed will give the absolute weight of CP supplied.

Example, a 1,200-pound pregnant mare (late gestation) may need the total ration to supply 2.0 pounds of protein per day to meet her needs (Table 2). The percentage crude protein is the portion of the total weight of the feed that is crude protein. To determine the amount of crude protein supplied in a given amount of feed, multiply pounds of feed by percent crude protein. For example 15 lbs. of an alfalfa-grass mix that is 18% CP supplies 2.7 lbs. of CP (15 lbs. X 18% CP = 2.7 lbs. of CP).

Most horse’s requirements are met by feeding pasture and/or hay. Commercial feeds and/or protein supplements are added when the requirements are higher than what the roughage provides. As shown in Table 2, the mature, idle horse has a low dietary crude protein requirement of 8.5%. Adult working horses require a level of 12% C.P. To meet increased energy requirements for performance horses feed intake is increased resulting in a greater protein intake. This usually satisfies the increased protein needs of the horse. The protein requirements for non-lactating mares during breeding and early gestation are identical to the requirements of a horse at maintenance. Minimum levels of 14-16% are better for growth, reproduction and lactation.

During growth both the amount and quality of protein are very important. Growing horses should receive a diet that is high in good quality protein. In fact, the lysine level is more critical than the protein content for a young, growing horse. The diet for growing horses should provide at least 0.65% lysine and 0.5% threonine on a dry matter basis. As the growing horse matures, the lysine requirement decreases to 0.45 percent for a yearling. Soybean meal, milk protein and alfalfa are feed ingredients high in lysine. Grains and grasses are typically low in lysine.

Table 2. Crude Protein and Lysine Requirements for various classes of horses.

Class of Horse	Min. % Crude Protein in Ration	Crude Protein (lb./day)	Lysine (lb./day)
Maintenance	8.5	1.4	.060
Pregnant mare – Late pregnancy	11.5	2.0	.084
Early lactation	14.0	3.4	.187
Late lactation	11.0	2.9	.157
Working horses – light work	10.0	1.5	.066
Working horses – medium work	11.0	1.7	.073
Working horses – hard work	12.0	1.9	.082
Weanling	14.5	1.5	.064
Yearling	13.0	1.8	.080
Two-year old (24 months)	11.0	1.7	.080

 

A protein deficiency is rare in equine diets. If a dietary protein deficiency occurs it is usually in horses consuming mature grasses or poorly digestible or heat damaged proteins. Signs would include general non-thriftiness, decreased feed intake, weight loss and poor hoof and hair coat quality. Inadequate muscle mass over the horse’s back and rump. visible withers and hip bones might also indicate that better quality protein is required. In growing horses, a deficit of an EAA can lead to lower average daily gain rates. In lactating mares an amino acid deficit can lead to reduced milk production, increased weight loss, and reduced rates of foal growth. Athletic horses also require enough essential amino acids to maintain their increased muscle mass or replace nitrogen losses in sweat.  

Excessive protein intake is more often observed when high amounts of protein-rich feedstuffs such as alfalfa or protein supplements are included in the diet. If excess protein is consumed, the nitrogen component must be removed via urea in urine. Excess production of urea causes the horse to drink more water and urinating more to rid the body of the extra urea. This causes more ammonia which can be noticed in poorly ventilated stables. In performance horses where water and electrolyte retention is essential, high protein diets should be avoided. The excess drinking and urinating can lead to elevated water and electrolyte losses from the body.

1. Urshel, K.L. and Lawrence, Laurie M. (2013) Amino Acids and Protein. In: Geor, R., Harris, P., Coenen, M., eds., (2013) Equine Applied and Clinical Nutrition. Saunders Elsevier.
2. Nutrient Requirements of the Horse. 6^th Revised Edition. 2007 National Research Council. The National Academies Press. Washington, DC.

By Peggy Auwerda
Equine Extension and Outreach Specialist