Is your horse getting a balanced range of essential and non-essential amino acids from their feeding program? Your horse needs adequate amino acids in their diet to make proteins.
Proteins are complex molecules that are required for almost every physiological function including muscle contraction, neural communication, metabolism of sugars and fats, immune responses and more.
Suboptimal protein or amino acid levels in the diet can cause a broad range of symptoms in horses including:
- Loss of muscle mass
- Poor growth
- Slow recovery from illness
- Poor performance
- Rough coat
- Weak hooves
These signs are not exclusive to protein deficiency and could also occur when energy needs are not met or with vitamin and mineral deficiencies. A diet evaluation complete with a hay analysis is the best way to determine whether your horse is getting adequate amino acids from their feeding program.
Does Your Horse Need More Amino Acids?
Young, growing animals are most susceptible to amino acid deficiency because they have a higher demand for protein to support their rapid growth. Protein requirements are also higher in mares during late gestation and early lactation to support optimal fetal growth and milk production.
Horses, like all animals, can not store excess amino acids to use at a later time. Protein must be continuously supplied by the diet. However, feeding too much protein is not only expensive, but can cause unnecessary strain on the liver and kidneys.
Senior horses, those under heavy exercise, and horses with metabolic concerns should have their protein intake carefully assessed to avoid oversupply.
Some amino acid supplements might be useful to horses if their diet is lacking in that particular amino acid. Lysine, threonine and methionine are the most commonly deficient amino acids in equine diets.
Ensuring their requirements are met will support optimal protein synthesis for overall health of the horse.
Amino Acid Nutrition in Horses
When an equine nutritionist balances the protein content of your horseâ€™s diet, what they are mostly concerned about is meeting individual amino acid requirements.
Horses do not absorb intact proteins from the diet. Instead, the proteins in forages, grasses, and grains, are broken down by enzymes in the small intestine.
The individual amino acids or peptides (short chains of 2-3 amino acids) are subsequently absorbed into the blood. These are used by all cells of the body to make the proteins the horse requires.
Proteins can only be made if all the necessary amino acids are available. If not, the body will breakdown other proteins to supply the required amino acids which can have negative health consequences.
Types of Amino Acids
There are 21 amino acids that are used to make proteins in horses. These all have a similar chemical structure, but differ in the arrangement of atoms in a part of the molecule referred to as the amino acid side chain.
Amino acids can be broadly divided into three categories:
- Essential: 10 amino acids that must be provided in the diet because they can not be made in the body (endogenously).
- Non-essential: Amino acids that can be made from amino acids or other compounds in the body and do not need to be supplied by the diet.
- Conditionally essential: Amino acids that might be necessary in the diet because their supply can not keep up with demand under certain circumstances such as rapid growth or illness.
Below we will review the roles, sources, symptoms of deficiency and excess, and requirements for each amino acid. We also evaluate the amino acid profile of various protein sources.
Before making changes to your feeding program, you can submit your horse’s diet for analysis online and one of our equine nutritionists will help you review your horse’s needs.
Essential Amino Acids
The 10 amino acids that must be supplied by the horse’s diet are:
All essential amino acids are used to make proteins. Some proteins will require more of a specific amino acid so that the protein can fold into the correct shape to perform its function.
Amino acids can also be converted to other molecules that have specific roles in the body.
Lysine is typically considered the first rate limiting amino acid in equine diets. It is the amino acid that is most commonly deficient to the point of limiting protein synthesis in the horse.
- Is converted to carnitine, a vitamin-like compound that supports key enzymes involved in breaking down fat for energy.
- Increases calcium levels in the body by increasing calcium absorption and minimizing calcium loss in urine.
- Is involved in making collagen and elastin, important proteins found in high levels in skin and connective tissue including tendons, ligaments and cartilage.
- Lysine is a critical component of the muscle proteins actin and myosin that interact to facilitate muscle contraction.
- Supports the immune system by helping fight viral and bacterial infections.
Sources: Legumes like soybeans and soybean meal are high in lysine. Canola meal can also provide good levels of lysine. We also carry supplements that supply L-lysine alone or in combination with threonine and methionine to supply these limiting amino acids in the correct balance.
Deficiency: Even with adequate protein intake, horses are likely to be low in lysine, especially if they have limited forage or fresh grasses in their diet.
Low levels of lysine in the diet can result in a variety of symptoms reflective of suboptimal protein synthesis, including poor exercise performance, muscle loss, rough coat and weak hoof structure.
Excess: Lysine competes with the amino acid arginine for uptake into cells. Very high levels of lysine could interfere with how arginine is used in the body and affect nitric oxide production which influences blood flow. This is unlikely to occur with levels typical in equine diets.
Threonine is often considered the second most limiting amino acid in equine diets after lysine. Low levels of threonine in the diet can affect gut health and protein synthesis in all cells of the body.
- Supports gut health and optimal nutrient absorption. It is involved in making mucin proteins which form a protective mucous barrier between the acidic environment of the gut and cells of the stomach and intestine.
- Is converted to another amino acid called glycine which is required to make creatine, a high energy compound naturally found in muscle tissue.
- Can be used to make glucose in a process called gluconeogenesis in the liver and can be broken down for energy.
- Threonine in proteins is often modified through cell signaling networks to change how the protein functions in response to signals from outside the cell.
- Support a healthy body condition by turning on genes involved in burning fat and turning off genes involved in storing fat.
Sources: Threonine is found in most plant and animal proteins. It is highest in potato and pea proteins, soybean meal and alfalfa. It is low in cereal grains like wheat and oats. We carry threonine as a single ingredient supplement for horses, or in a 5:3:2 ratio with lysine and methionine.
Deficiency: When there are low levels of threonine in the diet, most of this amino acid is used for making mucins in the gut. This causes low levels of threonine in other tissues which could manifest as low energy levels and loss of muscle mass.
Excess: No specific consequences of excess threonine intake have been reported in horses.
Methionine is a sulfur-containing amino acid that can be converted to the non-essential amino acid cysteine. It is also used to make several compounds that have important biological functions in the body.
- Cysteine, derived from methionine, is important for making keratin proteins found in high levels in hoof and hair. The sulfur in cysteine molecules forms bonds which help give hooves and hair a strong structure.
- Methionine is converted to s-adenosyl methionine (SAMe) which is a methyl donor involved in regulating gene expression and protein function.
- Is converted to adenosine, the key component of adenosine triphosphate (ATP), the main energy currency of the cell.
- Is used to make taurine, an amino acid that is not used for synthesizing proteins but supports cells of the nervous system.
- Is important for making phosphatidylcholine, a phospholipid found in cell membranes.
Sources: Methionine is high in animal proteins, soybean meal, alfalfa protein and canola meal. It is low in cereal grains and grasses. DL-methionine can be fed as a single ingredient supplement for horses or with lysine and threonine.
Deficiency: Low levels of methionine in the diet can contribute to rough coat and weak hooves because deficiency will result in low sulfur levels.
Excess: No specific consequences of excess methionine intake have been reported in horses. Experiments in rats have shown that high methionine intake can increase plaque formation in arteries, but this is unlikely to occur in horses under normal dietary regimens. 
Tryptophan is often marketed as an equine supplement that has a calming effect on nervous horses. However, the evidence to support this claim is not clear.
- Required to make the neurotransmitter serotonin in the brain which is associated with appetite regulation, decreased anxiety, aggression and fearfulness. Although tryptophan is often marketed as a calming agent for nervous horses, this has not been reliably demonstrated in horses. In fact, research that looked at behavioural responses following tryptophan supplementation have shown no calming effect in horses.   
- Required to synthesize the hormone melatonin which is critical for sleep onset in horses and other animals.
- Used to make vitamin B3 (niacin) in the liver which is important for blood flow, nutrient metabolism, skin health and many other biological functions.
- Converted into kynurenine, a pro-inflammatory compound that is generated in response to oxidative stress.
- Helps proteins such as hormone receptors “anchor” into the cell membrane so they can stay in the correct position for cells to respond to hormones appropriately.
Sources: Soybeans, oats, sunflower seeds, spirulina, animal proteins.
Deficiency: Tryptophan deficiency might be related to changes in mood including excitability.
Excess: In experimental studies, high doses of tryptophan were associated with lower stamina in endurance exercise training. Too much tryptophan is also associated with hemolytic anemia and respiratory distress in horses and ponies.   These side effects are unlikely to occur with tryptophan levels commonly found in protein or amino acid supplements.
Leucine is one of the three branched-chain amino acids (BCAAs), along with isoleucine and valine. In human nutrition, BCAAs are often used for muscle building. Similar to their role in human physiology, leucine and lysine are the most abundant amino acids in the horseâ€™s muscle.
Based on recommendations from Dr. Eleanor Kellon, performance horses might benefit from 10 grams of L-leucine along with a sugar source after exercise to help with exercise recovery and rebuilding glycogen stores. This is especially recommended for horses with poor topline and frequent muscle soreness. 
- Leucine is high in skeletal muscle where it can be used to make new proteins or burned as an energy source.
- Leucine activates the enzyme mTOR which stimulates protein synthesis, helping to build and repair muscle tissue.
- Leucine itself is not gluconeogenic but it can be converted into the amino acid alanine which can be used to make glucose in the liver.
- Involved in making hemoglobin â€“ a protein found in red blood cells that binds oxygen to deliver it to various tissues of the body, including muscle.
- Is part of enkepalins which are opioid-like compounds that can diminish the perception of pain.
- Helps maintain blood glucose levels during exercise to support muscle endurance. 
- Stimulates insulin secretion when given after exercise which might help restore muscle glycogen levels that are depleted in exercise. 
- Is converted to HMG-coA (B-Hydroxy B-methylglutaryl-CoA) â€“ a precursor for cholesterol which is important for maintaining healthy cell membranes. HMG-coA also forms ketone bodies that can be broken down for energy.
Sources: Leucine is widely available in horse feeds. Soybeans, hemp, oats, and corn are good sources of leucine.
Deficiency: Deficiency in the branched chain amino acids (leucine, isoleucine, valine) can limit exercise capacity and lead to early fatigue in exercise.
Excess: Excess leucine can interfere with niacin (vitamin B3) production in the liver.
Isoleucine is a branched-chain amino acid, as are valine and leucine. These are often considered “muscle building” amino acids because they can stimulate protein synthesis in muscle which promotes muscle growth and tissue repair.
- Isoleucine can be converted into propionyl-CoA that can be used to make glucose for energy.
- Can be converted into acetyl-CoA which enters the Krebâ€™s cycle and generates ATP, the main energy source for cells. Acetyl-CoA can also form ketone bodies which can be used for energy, therefore isoleucine is considered “ketogenic”.
Sources: High in legumes including soybeans and alfalfa.
Deficiency: No specific symptoms related to isoleucine deficiency have been reported in horses.
Excess: No issues linked to too much isoleucine have been reported in horses.
Valine is a branched-chain amino acid, along with isoleucine and leucine.
- Valine is needed for muscle coordination and proper muscle contraction.
- It can be broken down for energy in all cells of the body. It is converted to succinyl-CoA which enters the Krebâ€™s cycle to generate adenosine triphosphate (ATP).
- Valine is a “glucogenic” amino acid, meaning it can be used to make glucose which can be used immediately for energy or stored as glycogen to be burned for energy later.
Sources: High in legumes such as soybeans and alfalfa. It is also found in oats.
Deficiency: No specific issues due to valine deficiency have been reported in horses. In other animals, severe valine deficiency can cause neurological symptoms such as poor coordination. 
Excess: No specific issues due to excess valine intake have been reported in horses.
Histidine is incorporated into various proteins in the body. It is also converted into other compounds that have important physiological roles, especially histamine which is important for the immune system, and carnosine – a possible neurotransmitter and acid-buffering molecule in muscle.
Histidine is converted to histamine which has several important functions in the body:
- It is released from immune system cells called mast cells to fight infections.
- Histamine release by immune cells causes blood vessels in the area to expand (vasodilation) which helps bring other immune cells to the area.
- Histamine release during allergic reactions causes itchiness, sneezing and swelling. Antihistamine drugs can be used to treat these symptoms in horses.
- It is a neurotransmitter in the brain where is involved in regulating sleep/wake cycles, memory and learning, anxiety, movement, feeding and drinking, and release of hormones.
- Histamine stimulates gastric acid secretion by parietal cells in the stomach. Ranitidine is a histamine H2 receptor antagonist that has been used to treat gastric ulcers in horses by blocking the effects of histamine.
- Histamine is involved in sexual arousal in males. In stallions, it affects contraction of blood vessels in erectile tissue. 
- Histamine negatively affects the protective layer that surrounds nerves called myelin. Antihistamines have been investigated as a treatment for multiple sclerosis. Whether this is relevant to horses is unknown.
Histidine is also used to make carnosine, a dipeptide of beta-alanine and histidine. Carnosine is found mostly in muscle and brain where it acts as an antioxidant that can protect cells from oxidative damage.
In the brain, it might act as a neurotransmitter and might protect against cognitive impairments associated with aging. In muscle, it acts as a buffer to neutralize acids created during exercise and improve exercise endurance. 
Sources: Histidine is high in alfalfa and soybean proteins, canola meal, and cottonseed meal.
Deficiency: No specific issues due to histidine deficiency have been reported in horses.
Excess: No specific issues due to histidine excess have been reported in horses. Over-supplementation in rodents can reduce food intake causing weight loss and poor growth. It can also lead to high cholesterol levels. 
Phenylalanine is the third-most abundant amino acid in equine diets. It is found at high levels in most forages and grains.
- Phenylalanine is converted into the nonessential amino acid tyrosine which is used to make the neurotransmitter dopamine.Dopamine is important for signaling rewards and pleasure in the brain, control of movement, focus, and executive functions. Changes in how dopamine works in reward and motivation centres of the brain are associated with stereotypic behaviours in horses including crib-biting, weaving and box-walking. 
- Phenylalanine is converted to epinephrine and norepinephrine, also known as adrenaline and noradrenaline. Epinephrine is a hormone released by adrenal glands in response to stress. Norepinephrine is a neurotransmitter produced by the brain to activate the sympathetic nervous system. These are important factors in the fight-or-flight stress response that increases heart rate, breathing, and blood sugars. In horses, chronic disease, laminitis, and abdominal pain can also raise levels of these hormones and create a stress response. 
- Phenylalanine eases pain by preventing degradation of endorphins in the nervous system. Phenylalanine is sometimes included in joint health supplements for horses to help lessen the pain associated with arthritis and other joint problems. 
Sources: Phenylalanine is abundant in the horseâ€™s diet. It is high in soybean meal, alfalfa, pea, and potato proteins.
Deficiency: No specific issues due to phenylalanine deficiency have been reported in horses.
Excess: In excess amounts, this amino acid can interfere with serotonin production in the brain because it uses the same transporter as tryptophan to pass the blood-brain barrier (BBB). If phenylalanine is very high in the blood it could prevent tryptophan from reaching the brain to make serotonin. This is unlikely to be an issue in typical equine diets.
Some sources classify arginine as a conditionally essential amino acid for horses because of its role in supporting the immune system. Arginine deficiency might impair the immune system and cause illness to persist for longer than it otherwise would. For this reason, arginine is considered conditionally essential during times of illness. However, other sources suggest arginine should always be considered an essential amino acid for horses.
- Used to make nitric oxide which dilates blood vessels (vasodilation) to increase blood flow. L-arginine supplementation might benefit exercising horses by increasing blood flow to the lungs and muscles.
- Supports reproductive health by improving blood flow to the uterus. L-arginine supplementation in pregnant and postpartum mares decreased uterine fluid levels which might improve conception rates for a subsequent pregnancy. 
- Supports immune cells, especially T cells that are important for protecting against viruses, bacteria, and cancer cells. 
- Is used to make creatine, along with metabolites of the amino acids glycine and methionine. Creatine is considered an energy boosting compound because it is involved in regenerating ATP, the main energy currency of the cell.
Sources: Arginine is high in soybean meal and linseed meal. Arginine alpha-ketoglutarate (AAKG) is sometimes used as a supplement to add arginine to the diet.
Deficiency: During periods of infection or disease, an arginine deficiency might impair the immune response and prolong the illness.
Excess: Excess arginine can affect lysine absorption and use by cells. This can exacerbate symptoms of protein deficiency by decreasing lysine availability in the body, which is usually already the most deficient amino acid in equine diets.
Conditionally Essential Amino Acids
The remaining 11 amino acids are considered non-essential because they can be made within the body and do not need to be provided by the diet. As such, deficiency is generally not as much of a concern with these amino acids because they can be made endogenously.
Some amino acids are considered “conditionally essential” because the endogenous production does not meet demands under certain circumstances.
During periods of high demand, including rapid growth, stress or heavy work load the following amino acids can become essential for horses:
Cysteine is synthesized from methionine and serine. When diets are low in methionine there may not be enough for adequate cysteine synthesis. This could impair keratin protein synthesis in hooves leading to weak, brittle hooves over time.
Glutamine and serine are conditionally essential under intensive training conditions because they are lost in sweat and broken down at faster rates than they are synthesized. During the exertion and recovery period, they become temporarily conditional and must be supplied by absorption from the gut. 
Glycine and proline are conditionally essential during periods of rapid growth. Collagen protein found abundantly in cartilage of joints is high in glycine, proline and lysine. During growth, the high rate of collagen synthesis outpaces the endogenous supply of glycine and proline, therefore they must be supplied in the diet of weanlings.
Tyrosine is made from the essential amino acid phenylalanine. Tyrosine is used to make neurotransmitters and the stress hormones epinephrine (adrenaline) and norepinephrine (noradrenaline). Prolonged stress that induces ongoing production of these hormones can deplete tyrosine levels, making it conditionally essential.
Non-Essential Amino Acids
The remaining amino acids are always considered non-essentialÂ in all mammals:
- Aspartic acid (Aspartate)
- Glutamic acid (Glutamate)
For these amino acids, the endogenous supply and dietary levels are sufficient to meet the needs for protein synthesis even during periods of high demand.
Even though these are non-essential, they have some interesting characteristics in equine physiology that are worth noting.
Alanine is part of the glucose-alanine cycle (Cahill cycle) that is important for supplying energy to the exercising muscle. In muscle, glucose is a major source of energy during exercise.
When it gets broken down for energy, lactate and alanine are produced. These travel to the liver and are re-formed into glucose which can travel back to the muscle to provide energy.
Beta-alanine is a naturally occurring amino acid that is different from alanine. It is not incorporated into proteins. Instead, it is used to make carnosine, a dipeptide of beta-alanine and histidine.
Carnosine is an antioxidant found at high levels in muscle. It can buffer the acid produced during exercise and support exercise endurance. 
Asparagine can be made from other amino acids, namely glutamate, glutamine and aspartate.
In horses with laminitis, changes in the gut microbiome were associated with increased asparagine, but it remains to be seen what role, if any, asparagine plays in laminitis. 
Aspartic Acid (Aspartate)
The enzyme aspartate aminotransferase (AST) is found mostly in the liver and muscle. It is often measured in blood tests as an indicator of liver damage. When liver cells are damaged, they release AST into the bloodstream, leading to increased levels in the test.
In horses, high levels of AST can indicate acute liver damage due to infections related to gut issues, such as enteritis. It could also indicate chronic liver damage due to high iron intake.
High levels of AST could also reflect muscle damage due to “tying-up” or exertional rhabdomyolysis. It might also identify horses that experience rhabdomyolysis without showing clinical signs such as muscle stiffness and pain. 
Glutamic Acid (Glutamate)
Glutamate is very similar in structure to the amino acid glutamine.
It is an important excitatory neurotransmitter, a chemical that nerves use to send signals to each other. It is found in over 90% of connections (synapses) between nerves.
The enzyme glutamate dehydrogenase (GLDH) is found in the liver. Similar to AST, it is measured in blood tests and high levels indicate possible liver damage.
Selenocysteine is a form of cysteine that contains selenium in place of sulfur. It is an important for making several so-called “selenoproteins”.
Selenoproteins include the antioxidant enzymes glutathione peroxidase and thioredoxin reductase.
The selenoprotein iodothyronine 5-deiodinase is important for making the thyroid hormone T3 which circulates in the blood. It converts the thyroid hormone T4 to T3 by removing an iodine atom from T4.
Conditionally-essential and non-essential amino acids are not as much of a concern for diet formulation as essential amino acids because they can be made within the body. However, under certain circumstances it might be worth considering their roles in equine physiology.
For example, exercising horses might benefit from beta-alanine supplementation to provide the buffering capacity of carnosine.
Growing animals or those undergoing intense exercise with frequent exposure to stressors like trailering and competition might benefit from amino acid supplementation.
Our equine nutritionists can evaluate your horse’s diet to provide recommendations regarding amino acid nutrition during these times.
Amino Acid Requirements
Of the 10 essential amino acids, only the lysine requirement has been carefully determined through scientific experiments in horses.
Mature horses at maintenance with a bodyweight of approximately 500 kg (1100 lb) require a minimum of 18 grams per day of lysine to prevent deficiency. However, the recommended level to support optimal protein synthesis rates is 27 grams per day of lysine.
The general recommendation is that lysine accounts for 4.3% of crude protein in the diet.
Because skeletal muscle is the largest protein reserve in the body, researchers evaluated the amino acid content of muscle in horses and used this as a guide to determine optimal dietary levels of the essential amino acids, compared to the known requirement for lysine. 
If the lysine requirement is met it can be assumed that all essential amino acids are at adequate levels to support protein synthesis in the body.
Table: Essential Amino Acid Requirements for Horses at Maintenance
|Amino Acid||Proportion of Lysine||Requirement (g / day)|
Common Protein Sources for Horses
The best protein sources have the 10 essential amino acids in proportions that are close to recommended ratios. Practical considerations like palatability, cost, consistency of the product, and availability are also important factors to consider when choosing a protein source.
Alfalfa is nutrient-dense legume forage commonly fed to horses. It is a good source of fibre, protein and digestible energy and has low levels of non-structural carbohydrates (NSC) which include sugar and starches. Alfalfa hays are a good choice for horses that have high workloads or are hard keepers.
Protein content: Typically composed of 17 â€“ 25% crude protein (on a dry matter basis).
Amino acid composition: Low levels of lysine, threonine and methionine.
Pros and cons: Alfalfa is a readily available, nutrient-dense forage that can support gastric health by keeping the stomach full for longer and providing calcium as a buffer to stomach acid.
However, high levels of alfalfa in the diet can provide too many calories to sedentary horses and contribute to obesity, equine metabolic syndrome, and insulin resistance.
Some horses have a sensitivity to proteins in alfalfa hay and can experience an allergic reaction. Symptoms of alfalfa intolerance can include itchiness, excitability, irritable behaviour, weight loss, changes in skin or hoof quality, diarrhea and other digestive complaints.
Soybean meal is commonly added to equine diets because it is readily available and provides high levels of essential amino acids. Raw soybeans should not be fed to horses because they contain antinutritive factors. These proteins are mostly destroyed during the oil extraction process which leaves soybean meal as a healthful by-product.
Protein content: Typically contains 44-48% crude protein, on an as-fed basis.
Amino acid composition: High in lysine (up to 30g/kg), threonine (up to 15g/kg) and arginine (up to 39g/kg). Low levels of methionine (up to 4g/kg).
Pros and cons: Soybean meal can easily meet the horseâ€™s lysine requirement at typical inclusion rates. Anecdotal reports suggest some horses can develop allergies to soy proteins, however there is no clear evidence supporting this notion.
Canola meal is commonly used for horses because it has a similar profile to soybean meal and is relatively inexpensive. It is a by-product of canola oil production which is extracted either by mechanical or solvent extraction.
Protein content: 36-41% crude protein (on a dry-matter basis).
Amino acid composition: Lysine (up to 25 g/kg), threonine (up to 18 g/kg), methionine (up to 5 g/kg). Also high in leucine (up to 30 g/kg) and arginine (up to 25 g/kg).
Pros and cons: A widely available source of protein that has a similar nutritional profile to soybean meal and is highly palatable. Should not be confused with rapeseed (an older variety of canola) that contains glucosinolate which might interfere with thyroid function and should not be fed to horses.
Freshly ground flax (also known as linseed) is a good source of energy for horses and supplies alpha linoleic acid (ALA) â€“ an important omega-3 fatty acid that is required in the horseâ€™s diet. It is also a good source of protein, but has lower levels of lysine than soybean meal or canola meal.
Protein content: Ground flax provides 26% crude protein, on a dry-matter basis.
Amino acid composition: Lysine (10g/kg), threonine (8g/kg), methionine (4g/kg). High in arginine (21g/kg).
Pros and cons: Cost-effective, readily available feed that provides a balance of fats, amino acids, and minerals. To avoid rancidity, stabilized product should be purchased or it should be ground fresh before feeding.
Corn gluten meal
Corn gluten meal is a by-product of cornstarch and corn syrup production that is commonly added to commercial equine feeds. Although the protein content is high, it does not provide high levels of lysine and should not be considered a primary protein source for horses.
Protein content: Approximately 65% on a dry-matter basis.
Amino acid composition: Low in lysine (7 g/kg), moderate levels of threonine (17 g/kg) and methionine (13 g/kg).
Pros and cons: Although readily available, it can vary greatly in energy content and has undesirable ratios of phosphorus to calcium.
Dried Distillerâ€™s Grains
Dried distillerâ€™s grains are a by-product from distilleries and are derived from a variety of grains. They are a protein and energy source for horses and contain considerably less starch and sugars than the original grain. However, the overall composition can vary between batches and feeding this product can cause vitamin/mineral imbalances if not part of a carefully formulated diet.
Protein content: Typically around 25% crude protein (on a dry-matter basis).
Amino acid composition: Amino acid content varies depending on the type of grain, but cereal grains are generally low in lysine.
Pros and cons: Readily available, highly palatable and inexpensive in areas near distilleries. Not to be confused with Dried Distillerâ€™s Grains with Solubles (DDGS), the main by-product of ethanol production, which should not be fed to horses. DDGS can contain high amounts of mold and has high levels of phosphorus, potentially leading to imbalances with calcium.
Spirulina is dried blue-green algae that is used as an equine supplement to support immune and respiratory health in horses. It is not typically used as a primary protein source but can boost the protein content of the horseâ€™s diet while providing significant levels of vitamins and minerals.
Protein content: Approximately 52% crude protein (on a dry matter basis).
Amino acid composition: Complete protein that provides all the essential amino acids but is low in lysine.
Pros and cons: Spirulina is considered an equine “superfood” because it provides all the essential amino acids, as well as vitamins and minerals that provide antioxidant and anti-inflammatory benefits.
Some horses may be hesitant to consume this powder at first due to its unique smell. Spirulina can be high in iron which should be considered when feeding this to horses with equine metabolic syndrome or Cushingâ€™s/PPID.
Potential Equine Protein Sources
Protein is the most expensive macronutrient in the equine diet. The competing demand for common protein sources such as soybean meal and canola meal across all livestock sectors drives up their market price and the overall cost of feeding horses. There are also environmental sustainability concerns with many common protein sources as demand increases.
Insect-based proteins are emerging as an environmentally sustainable, high quality alternative to these common sources. Insect protein may seem unappealing at first glance, but considering that horses likely consume many bugs and larvae while grazing these are not an unnatural protein source for them.
Of the many options for insect-based proteins, black soldier fly larvae and crickets have the greatest potential for large-scale production and are already available for pet and human consumption.
Black Soldier Fly (BSF) Larvae
The black soldier fly (Hermetia illucens) is the typical household fly found all over North America and Europe. The larvae can convert organic waste products from food manufacturing into high quality proteins.
Protein content: The crude protein content varies depending on what they are fed, typically ranging between 30 to 45%. 
Amino acid composition: BSF larvae are high in lysine (22 g/kg) and methionine (9 g/kg).
Pros and cons: As production of BSF larvae continues to expand, this feed source will become more widely available and offer a sustainable high-quality protein source. There have not been any published studies on inclusion level, palatability, or digestibility of BSF in horses. For monogastric animals, it is estimated that ground BSF larvae could replace approximately 50% of soybean meal. 
Crickets are another viable option for sourcing insect protein. Ground cricket powder is high in protein and is considered a complete protein because it provides all essential amino acids. It is also a good source of the trace minerals copper, zinc, and manganese. 
Protein content: Cricket powders contain 42 â€“ 46% crude protein.
Amino acid composition: Cricket protein contains 3.5% lysine, 3% threonine, and 1% methionine. It is also a good source of leucine.
Pros and cons: Similar to BSF larvae, as production expands cricket powder will become more widely available. Studies in horses are lacking, but research in goats and chickens suggest it can replace up to 50% of soybean meal without any adverse effects and might actually improve some measures of reproductive health. 
Given all the options for protein sources and their relative expense compared to other ingredients, it is always recommended to consult with an equine nutritionist to optimize your horseâ€™s diet. This includes ensuring essential amino acid requirements are met to optimize protein synthesis and overall health and to minimize excess protein intake.
You can submit your horseâ€™s diet and Mad Barnâ€™s equine nutritionists will provide a complementary review and specific recommendations.
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