How to feed a horse is one of the most popular questions that I get as a veterinarian.  There are many things to consider in the horse diet, mainly being macro- and micro- nutrient provisions, which include proteins, fats, vitamins, and minerals. Protein is essential for the body in a variety of ways, from building muscle mass to regulation of metabolic function.  How much protein is needed is dependent on each horse and the demands present.  Exact recommendations are hard to make, generally speaking, but in most cases, we have to look at each individual horse, demands, existing health problems and dietary sources.

Collection of Protein Sources for Horse

Collection of Protein Sources For Horse

Protein is defined by a quick Google search as “any class of nitrogenous organic compounds which are composed of long chains of amino acids, being an essential part of all living organisms especially as structural components of body tissues including muscle, hair, collagen and as enzymes and antibodies.”

Before we get into details regarding intake requirements and overall needs for the horse, we must get some basic housekeeping out of the way so we have a better understanding.

Protein Composition and Amino Acids

There are 21 main amino acids that are required by most mammals, which are then broken down into three subclasses; essential, non-essential and conditionally essential. The essential amino acids are required to be consumed via the diet as the body does not have cellular pathways to produce them or the pathways are insufficient to meet demands.  Non-essential amino acids are those that the body can produce due to existing cellular pathways, which do generally meet demand.  Conditionally essential amino acids are those that the body does possess capabilities to produce but in certain situations, such as disease or injury, those means may not be enough, thus creating a deficit resulting in higher needs via the body.

Essential amino acids include: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan and Valine.

Non-Essential amino acids include: Alanine, Asparagine, Aspartate, Glutamate, Serine and Selenocysteine

Conditionally Essential amino acids include: Arginine, Tyrosine, Cysteine, Glycine, Proline and Glutamine.

Crude protein is needed by the body for several functions:

  1. Structure/function (muscle/supporting structures)
  2. Nutrient transport into cells
  3. Regulation of metabolic processes (enzymes/peptide hormones)
  4. Immune function and production of antibodies (immunoglobulins)
  5. Buffers to regulate pH of the body

Other functions of the amino acids are production of nitrogen containing compounds which include:

  1. Glutathione (naturally occuring antioxidant) produced via Cysteine, Glycine and Glutamine
  2. Creatine (involved in cellular energy production) produced via Arginine, Glycine and Methionine
  3. Carnitine (involved in transport of fats for energy production) produced via Lysine and Methionine

Some amino acids actually have specific functions and values to the body which include:

  1. Arginine: Involved in the Urea cycle for nitrogen metabolism, also involved in nitric oxide production which impacts blood flow and blood vessel dilation
  2. Glycine: Involved in the synthesis of Porphyrin in hemoglobin which binds/transports oxygen in red blood cells
  3. Branched Chain Amino Acids (Leucine, Isoleucine, Valine) involved in energy or ATP production in muscle cells
  4. Glutamine: Involed in energy/ATP production in gastrointestinal and immune cells
  5. Alanine: Produced by muscle during exercise, then converted in the liver to glucose, which is then used by the muscle cells for energy (“Alanine Cycle”)
  6. Neurotransmitters including Acetylcholine, Serotonin, Epinephrine, Norepinephrine, Melanin and Thyroid hormones are all produced from individual amino acids including Serine, Tryptophan and Tyrosine.

Overall, we can see that protein is necessary for the body in more way than one. All to often, when the word “protein” is tossed around, we immediately think of muscle mass or weight gain, while in fact that is true, protein is needed for many other cellular process as well.  So, we need to take this into consideration dependent again on the individual needs of the horse and the specific situation.

Sources of Protein and Protein Quality for the Horse

All foods in the horse diet contain protein to some degree, the question is what is the quality of that protein, meaning how well are the individual amino acids represented.  This is tough to actually put a score to and in some measurements, they are comparing food amino acid sources to those amino acid levels naturally found in skeletal muscle, which is not seen as a completely reliable method.  In other situations, they measure rate of gain when using various protein sources and determine success or failure as being a measurement of quality.  In the end, there are no tried and true methods to measure protein quality, as it can be arbitrary or relative.  Overall, the specific requirements for essential amino acids in the horse, aside from Lysine, have not been adequately determined at this time.  

In the equine diet, we essentially are dealing with forages/hays and grains as the main sources of food and protein.

Grains are generally used as pure energy sources, but do serve as protein and even macro/micronutrient sources.  The amount of protein per a particular grain is generally seen as standard and determined, however, in today’s world we have new grain varieties that are being produced with a grain species that do have altered amounts of various amino acids. Overall, grains are generally seen as having moderate to low quality protein, implying that the amino acids ratios are off or some are not present.  Grain byproducts which include wheat middlings, wheat bran, corn gluten and even rice bran generally contain higher levels of protein that the whole cereal grains, but the protein quality is often the same or even lower.  As for definition, we often see the word ‘meal’ used, as in rice bran meal or flax seed meal.  This is a byproduct created after the extraction of oils from the seed, which then after extraction are often high in protein.

Forages are generally broken down into Legumes or Grasses.  Legumes include Alfalfa, clover and other varieties, while grasses include anything from fescue to Timothy to Orchard grass or other varieties. Legumes generally have a higher overall protein content averaging above 14%, while grass hays are generally lower unless well maintained and fertilized. The protein content found within the legumes or grass hays is dependent on the stage of maturity when cut and baled.  The more active the growth, the higher the protein content in general.   The vegetative states can have protein contents from 16-20% or higher, while the full bloom or headed (late maturity) can contain from 6-9% protein.  So, a hay that is cut late stage or full maturity, especially if it has begun to wilt and lose color prior to baling, is going to have a much lower content as compared to one that is baled prior to fully heading out.

Average Protein Levels of Grains and Hays

Grass Pasture    26%     Lysine 35 mg/gram
Legume Pasture    26%     Lysine 52 mg/gram
Grass Hay Mid Maturity    13%     Lysine 35 mg/gram
Legume Hay Mid Maturity    20%     Lysine 51 mg/gram
Mix Legume Hay Mid Maturity    18%     Lysine 39 mg/gram
Oats    13%     Lysine 42 mg/gram
Barley    12%     Lysine 36 mg/gram
Corn    9%     Lysine 29 mg/gram
Flax Seed Meal   32%     Lysine 37 mg/gram
Rice Bran   15%     Lysine 47 mg/gram

 

 

 

 

 

 

 

 

 

Average Protein Requirements for the Horse along with Lysine Recommendations

Maintenance 690 grams protein        30 grams Lysine
Light Work 760 grams protein        33 grams Lysine
Medium Work 840 grams protein        36 grams Lysine
Heavy Work 940 grams protein        40 grams Lysine

 

 

 

 

In order for protein synthesis to occur by the body, all amino acids must be present in specific ratios.  If one amino acid is present in levels lower than that required for protein synthesis to occur, then the total rate of protein synthesis will be limited by the intake of that limiting amino acid.  In most food sources for the horse, the rate limiting amino acid is considered to be Lysine first and Threonine second. The reason is that Lysine is generally in the lowest levels in those food sources, thus considered to be the limiting amino acid in the diet.  This is why we often refer to Lysine with importance and also use Lysine levels in relation to other amino acids. Threonine levels can vary quite a bit between food sources, thus not considered to be a huge limiting factor as often levels are sufficient.

If we look at general recommendations to feed forages at 2% of bodyweight, that would equate to 20 lbs of hay per 1000 lbs of body weight. If we then look at forage types, we can see the differences in protein that they provide overall.

If we fed a grass hay, mid-maturity, 13% crude protein at 2% of bodyweight, it would equate to 1170 grams of protein and 318 grams of Lysine.

If we fed a legume hay, mid-maturity, 20% crude protein at 2% bodyweight, it would equate to 1800 grams protein and 459 grams of Lysine.

We then have to take grains into consideration, their feeding levels and associated protein.  As you can see, if we feed high enough quality hay at sufficient levels, then protein requirements are often met with no problem.  The issue that I see is the quality of forage that is fed by most.  In many cases, we can look at the hay, determine quality knowing at what stage it was cut and gauge protein levels.  In other cases, the hay looks good, but protein levels after analysis are often very low.  This is reflective of maturity and also pasture management on the grower’s part.  In my experience, most hays in our region are less than 10% protein, usually of a Fescue variety and cut late stage. As a vet, I have seen fescue hail baled when it is brown and withering in the pasture, almost looking like straw.  Here, as a case in point, protein level, not to mention overall nutritional value is sub par.

Protein Deficiency in the Horse Diet

In reality, a protein deficiency can occur due to inadequate levels of intake of one or more amino acids, but it can also occur due to inadequate intake of total nitrogen which leads to decreased non-essential amino acid production by the body.  In most respects, nutritionists will say that if enough ‘energy’ is consumed via the diet, then generally nitrogen intake is sufficient, so it really boils down to decreased essential amino acid intake.

Clinical signs of protein deficiency include: general unthriftness, decreased appetite, weight loss and poor hair/hoof conditioning.  These are general signs associated with overall decreased protein synthesis by the body but may also be reflective of specific amino acid deficiencies.  One thing to keep in mind is with the equine athlete in training, which is developing lean muscle mass and increasing demand for protein.  In many of these cases, the protein needs are not kept up with as the body’s demands increase, which equates to a relative protein deficiency and can create weight/muscle loss, poor performance and increased nitrogen excretion by the body.

Protein Excess In the Horse Diet

An excess protein intake is relative to total body functions and needs or essentially demand.  When there is an excess intake of amino acids, they cannot be stored and are often catabolized to Carbon Dioxide (CO2) and Urea in the liver, which then results in increased nitrogen excretion in the feces and urine. In many cases, this is detected by high levels of ammonia in the stall within the urine, as the nitrogen is then transformed to ammonia, which actually can be detrimental to overall health to the groom and impacts soil pH as well.

The production of Urea by the liver has been termed ‘metabolically expensive’ as it requires a good bit of energy in the form of to create, so an excess intake of protein can actually deplete the body of energy, in theory, impacting growth gains and even performance.  The high production of urea also requires high volumes of water to ‘flush’ the system and eliminate it in the urine.  High protein diets have also been shown to actually create an acidic body pH, which when combined with an already lowered pH due to lactic acid production during exercise, could actually contribute to further health problems, acidosis and poor performance.

High protein diets have not been connected with liver or kidney disease, but those animals with pre-existing kidney or liver damage may be at risk with high protein diets due to formation and elimination of excess urea.

Concluding Thoughts on Protein Intake and the Horse Diet

Overall, protein is an essential part of the equine diet, serving many functions and purposes.  As the work load and demands on that horse are increased, likewise, protein and other nutrient requirements will rise.  One interesting note revolves around the limiting amino acid, Lysine.  In one study, it was noted that if horses were fed a lower protein diet in addition to added Lysine and Threonine, to compensate for their limiting rates, the growth in that group of horses was no different and they actualy seemed to utilize the protein in their diet more efficiently.

As a veterinarian, I have fought with the concept of protein in our patients.  Obviously, in critically ill or injured patient, we had higher protein requirements to aid in recovery as demands were increased.  However, in other classes such as metabolic patients, we often deprive these patients of protein due to using much lower quality forages and overall diets. We have to remember that disease comes in all different forms, from a septic colic to a metabolic patient.  In all those situations, there is an ongoing inflammatory condition, which is creating increased demands on the body, which often are not met.  In human medicine and research, as a case example, they are finding that increased protein intake, specifically Glutamine, helps aid in recovery of various health conditions impacting the GI tract and leaky gut conditions.

Our goal in metabolic cases is to reduce carbohydrate intake, but on the same side of the coin, we are also reducing protein and other micronutrients, which is then reflective in their overall appearance down the road.  Is it possible, due to increased demands metabolically and also due to concurrent inflammation, that our needs are actually higher than what we originally perceive?  Could it be possible that by ‘depriving’ these patients, dry lotting them and feeding much lower quality hays that we are actually inducing more harm?  In human research, a higher protein diet is associated with improved weight loss and metabolic function. The theory is to create more lean muscle, while impacting metabolic rate and thus burning fat. There is no answer to this from a research point of view at least in horses, but only one from experience.  My experience tells me that if we improve the quality of the diet, even increasing the levels of protein, that these patients actually lose weight and their condition becomes more manageable.  Of course, carbohydrate intake needs to be monitored, but even then, without getting too excessive regarding intake, I find that these patients actually still do well without much intervention regarding carbohydrates.  Could it be that proper nutrition, actually using whole foods and all of the goodness that comes with them, could be the answer or the missing piece of the puzzle?

This is part of a concept that we have explored recently with the creation of our EQ Rejuvenate formula.  We have used this formula in our TB rehab patients for some time, helping to provide another source of protein along with nutrients in their natural form, mainly providing support for tendon and hoof conditioning.  Recently, we also explored the use of EQ Rejuvenate in metabolic patients with laminitis, in which case, it did seem to help provide additional relief for those patients, even in the short term.  Many are also using the formula in Cushing’s patients, which have a tendency towards metabolic problems, reporting improved hair coat and even weight loss.  Could it be the added protein?  Is it the naturally occurring nutrients?

We briefly discussed increased nitrogen excretion in those horses receiving too much protein in the diet, noting increased ammonia smell to the stalls. This is common and can be an indicator of excess protein in the diet in relation to demand, but on the opposite side of logic, it could also be an indicator of too little protein or possibly more due to a low level of Lysine or other rate limiting amino acid, which then impacts overall protein utilization.  This is something we have played with in our rehab horses, which are on roughly 2% bodyweight of an alfalfa mix hay along with whole grains.  In some, there was an increased ammonia smell to the stall, which would indicate too much protein, but these horses needed the protein for weight gain, hoof conditioning and recovery.  In those horses, recently, we have added pure Lysine to their diet, which in 3 days time has reduced the ammonia smell in the stall by over 50%, while still maintaining the prior diet.  Could it be in actuality, they were not receiving too much protein, but actually too little of the rate limiting amino acid Lysine, which was impacting overall protein utilization?

These are all just observations from my perspective, applying some research theories and testing the waters.  This area is somewhat void of concrete information in the horse and there are really only guides.  The rest we have to gain based on observation.  There is no ‘cookie cutter’ approach and each horse is different in their requirements.

Everything in moderation, I suppose, but time will tell. So far, things look promising.

I hope this article helps to guide you.

For more information on Carbohydrates and the Horse

For more information on Fats and the Horse

Tom Schell, D.V.M.

Nouvelle Research, Inc.

www.nouvelleresearch.com

 

References:

Geor, RJ et al. Equine Applied and Clinical Nutrition. Saunders. 2013, 113-132.

Graham-Thiers PM, Kronfeld DS. Amino acid supplementation improves muscle mass in aged and young horses. J Anim Sci. 2005 Dec;83(12):2783-8

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