Equine Carbohydrate Disorders Part 3: Metabolic Syndrome

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Written By Kris Hiney
Imagine a bright spring day. You excitedly turn your horse out to indulge in the fresh spring grass as a special treat. You return in a few hours to collect your companion, but instead are met by an unhappy painful horse, slowly limping its way back to the gate.
Sound familiar? Unfortunately for some owners, this is an all too real scenario. Many horses suffer from carbohydrate sensitivities, or metabolic syndrome, which make them extremely susceptible to changes in carbohydrates in the diet.  One may also hear these horses referred to as insulin resistant, almost like Type II diabetes in humans.  In recent years there has been an upsurge in the number of studies and articles written about metabolic syndrome in horses. While awareness in the general public has increased, many horsemen still wonder if their horse is, indeed, one of these individuals. Should they be paying strict attention to every type of carbohydrate their horse consumes? Should horses no longer consume grass? Does their horse need medication? How do you know if your horse truly has metabolic syndrome?
Classically, horses with metabolic syndrome are described by a certain appearance. They are typically obese horses which gain weight readily, and are considered “easy keepers”.   Breeds with a higher prevalence of metabolic syndrome include the traditional easy keepers such as ponies, Morgans, and Paso Finos. However, metabolic syndrome  can be seen in a wide spectrum of breeds including Quarter Horses, Arabians and Thoroughbreds.  Beyond just being obese, metabolic horses tend to have regional adiposity, or specific fat deposits on the crest of their neck, over their tailhead, the sides of their abdomen and also in the scrotal or mammary area.  The size of the crest of the neck is often the best physical predictor of metabolic syndrome. The thicker the crest, the more likely the horse truly fits into this category. However, it is important to note that it is possible for leaner horses to also suffer from metabolic syndrome. Despite being lean these horses still demonstrate regional adiposity, along with a susceptibility to pasture associated laminitis, as well as insulin resistance. Therefore, if your horse shows symptoms, it may be wise to have it tested, despite it not being overly obese.
Unfortunately the most common way horses are diagnosed with metabolic syndrome is the frequency of laminitic bouts. Usually this is seen following grazing on pasture, especially in the spring or fall.    These horses may be young or middle aged, which sets them apart from horses who suffer from Cushings disease. However, horses who suffer from metabolic syndrome early in life are certainly more likely to develop Cushings later on. Cushing horses are also distinct in the prevalence of hair coat which does not shed or long curly hair while the metabolic horse has a normal hair coat.
Physiologically, these horses demonstrate insulin resistance.   Essentially they must secrete larger amounts of insulin compared to a normal horse, in order to stabilize their blood glucose levels. Therefore, their insulin levels remain higher in their bloodstream, which can have a cascade of effects on their body. They also present with elevations in blood lipids, as well as an increase in leptin. Leptin is a hormone secreted by fat cells or adipocytes, that normally helps in the feeling of satiety (or fullness). However, increased concentrations of leptin may contribute to inflammation in the body. Metabolic horses also have a lower resting thyroxine levels (T4) then their normal counterparts. However, the low level of T4 does not cause insulin resistance and metabolic syndrome, but rather is merely a consequence of altered metabolic profiles.
So why are these horses so susceptible to laminitis? What could insulin resistance possibly have to do with painful feet? One of the commonalities between the myriad of disorders that can result in laminitis in horses is a disruption of the circulation to the hoof. Insulin is most commonly recognized for its role in glucose disposal, but it is a hormone with systemic effects. It is presumed that sustained hyperinsulinemia promotes vasoconstriction. It is already known that carbohydrate overload induces laminitis by creating vasoconstriction in the hoof, so the hyperinsulinemic horse may be even more susceptible to shifts in carbohydrate intake. This disruption of blood flow to the foot results in hypoxia and tissue damage to the sensitive laminae. Severe bouts may render the hoof wall unstable and allow the coffin bone to rotate downwards within the foot. This may lead to permanent alterations of the hoof structure.
Testing for metabolic syndrome frequently involves blood sampling after a short period of fasting (typically 6 hours). Blood is analyzed for glucose and insulin levels that are above normal. The presence of altered adrenocorticortropin releasing hormone can also be tested if Cushings is suspected in an older horse.   Further testing can be done if horse’s insulin levels are within the normal range, but metabolic syndrome is suspected. Horses are again removed from feed, and a standard blood sample is taken. Horses are then given a bolus of glucose and then insulin to determine how the body metabolizes these compounds. This provides a more dynamic picture of the horse’s metabolic response to carbohydrates.
If your horse has been diagnosed with metabolic syndrome, or has show signs of pasture associated laminitis, it is important to start them on a rigorous management protocol. First, as these horses have sensitivities to carbohydrates, concentrates should be removed from the diet. As these horses are typically obese anyhow, there is little need to supply concentrates to them anyhow. If the owner is concerned with mineral and vitamin intake, there are many products which are intended to complement forage only diets. Typically these are pelleted supplements which are fed at very low levels of intake. The obesity issue in the horse should also be addressed. Exercise should be increased to 5 days a week. Not only will this aid in reducing the body weight of the horse, but exercise also enhances glucose clearance from the blood in a non-insulin dependent manner. However, be sure that the horse is not recovering from any laminitic episodes. Pasture intake should also be limited in these horses. Horses should only have access to pasture for a short time or have access to a very small area. If more movement of the horse is desired, a grazing muzzle should be employed to prevent overconsumption of grass. The horse should receive an all forage diet, preferably of grass hay, with intake reduced in order to encourage weight loss. If weight loss is not able to be achieved at an intake of 2% of the body weight, then reduce feed intake to 1.5% of bwt. Unfortunately simple diet restriction may take a long time due to the efficiency of the horses prone to metabolic syndrome. If the horse has greater degrees of insulin resistance, it is advisable to monitor the non-structural carbohydrate composition of the hay, with it ideally below 10%.If horses have persistent issues with metabolic syndrome after calorie restriction, decrease in adiposity, alteration of diet, limitation of pasture intake and exercise have all been employed, then there are medical therapies which can be used. Levothyroxine is effective in improving insulin sensitivity. If all of these measures are followed faithfully, there is no reason that these horses cannot be returned to a metabolically normal state and enjoy a long healthy life.
Next month: We will discuss other strategies that have been employed to assist the metabolic horse.

Equine Carbohydrate Disorders Part 2: Understanding the Terminology

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Written By Dr. Kris Hiney
Last month we began a new series looking at carbohydrates commonly fed to horses, and the different forms they come in. We discussed the differences between simple sugars and polysaccharides and how the bonds which join these sugars have direct implications on how they are digested in the equine. We also discussed a few carbohydrates in particular in which many horse owners have particular interest, including fructans, which may carry health risks, and fructooligosaccharides, which can be used as digestive aids. In the following article I will attempt to define the wide array of terminology that one finds in equine nutrition, such as crude fiber, neutral detergent fibers, and non-structural carbohydrates, among others.
Horses’ diets primarily consist of plants which are largely made up of carbohydrates. Certainly plants also contain protein, lipids, vitamins and minerals, as well as water, but their primary composition is carbohydrates. If we use the carbohydrate definitions from last month, we know that the cell wall of plants is composed of cellulose, hemicellulose, lignin (which is indigestible by horses), beta glucans, gums and pectins. Inside the cell wall we find the more digestible cell contents which contain the simple sugars, starches, oligosaccharides and fructans. Often horse owners need to know what percentage of these compounds appear in a particular feedstuff. We use a variety of analytical methods to partition these carbohydrates into groups.
Crude Fiber
The most commonly used descriptor of carbohydrates on feed tags is usually crude fiber. Crude fiber content of a feed is determined using proximate analysis or the Weende system. Crude fiber is the residue remaining after subtracting water, lipids, proteins and the nitrogen-free extract from the feed. Nitrogen-free extract is said to represent mainly sugars and starches.   It uses a chemical method of solubilizing the feed using acids and bases. One of the problems in using crude fiber as a descriptor is that any lignin, cellulose or hemicellulose which is solubilized in the process is lost and appears in the nitrogen-free extract value for the feed. Thus, crude fiber values may actually underestimate the fiber value of the feed, and over estimate the nitrogen-free extract portion.
NDF and ADF
The Van Soest system of analysis improves on simple values of crude fiber, by separating out different fiber fractions into neutral detergent fiber and acid detergent fiber. These are the most common values which are reported if you have a forage analyzed. Unfortunately this method is most applicable to the value of feedstuffs for ruminants, rather than horses. Regardless, it still does offer the horse owner some important information. The neutral detergent fiber of a feed contains hemicelluloses, cellulose and lignin, although some soluble hemi-cellulose may escape into solution.    It does not provide information about fructans, pectins, gums, or beta glucans which are quite readily fermented by the equine digestive tract. Acid detergent fiber represents the lignin and cellulose content of the feed, as the hemicelluloses have been removed. The amount of hemi-cellulose in a feed, which is easily fermented by horses as well, is represented by the difference in value of the NDF and ADF value for the feed.
Total Dietary Fiber
Total dietary fiber is more frequently used in human nutrition, but may actually offer better information for species which are hind gut fermentors, as is the horse. Total dietary fiber combines many plant values together, and essentially represents the portion of the plant that resists enzymatic digestion which takes place in the small intestine. This includes the traditional fiber components of hemi-cellulose, cellulose and lignin, but also includes pectins, oligosaccharides, mucilages, gums, etc. which are often referred to as soluble fibers in human nutrition. Thus this method recovers more of the true fiber portion of the feed which is susceptible to fermentation in the horse and contributes to their energy supply. Unfortunately this method is not commonly used in commercial feed testing laboratories which serve the animal industry.
Non-Fiber Carbohydrates (NFC)
Now the definitions are going to start getting a little trickier, and the horse owner may encounter many different acronyms. Non-fiber carbohydrates, or NFC, represent the carbohydrates which are not in the cell wall and are not recovered when doing an NDF feed analysis.    The NFC is what remains after subtracting the NDF, protein, fat and ash. It represents the sugars, starches, fructans, galactans, pectins, beta glucans and organic acids. NFC is determined via a mathematical procedure and not a chemical analysis. You may also see values reported as NSC, which stands for non-structural carbohydrates. These values are actually determined chemically and differ from NFC in that NFC values may also include pectins and gums which will not appear when analyzed for NSC.
WSC
Confused? Let’s add some more letters. Water soluble carbohydrates are also used in equine nutrition to determine the most easily digested carbohydrate portion. The monosaccharides, disaccharides, oligosaccharides and some polysaccharides appear in this portion of feed separation. Compared to the NSC value for feeds, it would equal the WSC portion added to the starch content of the feed. Starch content of feed can also be analyzed separately as well.
Finally, some equine specialists have suggested that carbohydrates need to be redefined in terms of horse nutrition. As they are hind gut fermentors, they handle carbohydrates much differently than do ruminants. Certainly, they also get much more use of cell wall components than do simple monogastrics. Many of the particular disorders found related to carbohydrate digestion in the horse may require us to look differently at feeds than is needed in other species. One of the suggested systems created by Rhonda Hoffman (currently of Middle Tennessee State University) is to separate equine carbohydrates into hydrolysable carbohydrates (CHO-H) subject to enzymatic digestion, and fermentable carbohydrates (CHO-F) which undergo fermentation in the hind gut. The fermentable carbohydrate fraction can further be defined as either rapidly fermentable carbohydrates (CHO-FR) or slowly fermentable (CHO-FS). Slowly fermentable carbohydrates would include those seen in NDF values, with the indigestible lignin portion removed from the value. The rapidly fermentable carbohydrates, whose presence can increase the energy content of a feed, include the oligosaccharides, fructans, beta glucans and pectins. Feeds higher in rapidly fermentable carbohydrates can offer more calories to the horse. Alternatively, some horse owners may need to be aware of the fructan content of feeds or forages in horses more prone to developing laminitis.
So what values do you need to know and why should you care?
In general, crude fiber values are listed on most feed tags. Typically the lower the crude fiber value, the higher the energy density of the feed. This is not true of feeds which are designed to have higher fat values, and may include fibers to provide a healthier type of feed for the horse. Owners who have horses with insulin resistance or metabolic syndrome, as well as PSSM horses, should try and choose feeds which are lower in the soluble carbohydrates such as sugars and starches. However, these values are not always provided on feed tags. Owners may also select away from forages which may be higher in sugars and starches as well for these particular types of horses with demonstrated metabolic disorders.
Next month, we delve more deeply into particular carbohydrate disorders seen in horses.

Lipid Nutrition: Part 4, Omega-3 Fatty Acids

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Written By Dr. Kris Hiney
In previous articles we have discussed the many benefits of feeding fats to horses. Typically these fats in feeds are vegetable oils, or even occasionally animal fats. We have not yet discussed specifically the type of fat in the diet. However, researchers in human and animal medicine have much information supporting the idea that specific types of fatty acids can provide numerous health benefits. This month we look at the science behind Omega-3 fatty acids and begin the process of understanding the terminology used.
Omega-3 fatty acids
So what makes Omega-3 (or “n-3”) fatty acids so unique? Quite simply, it’s just the location of the double bonds which occur between the carbons in the fatty acid chain. The location of these bonds are what provide these fatty acids with their naming system.  Omega-3 fatty acids have the last double bond placed three carbons from the methyl end of the carbon chain, which is the opposite end from the attachment to the glycerol backbone in a triglyceride. Compare this to the Omega-6 fatty acids (or “n-6”), which have their last double bond six carbons in from the methyl end. This simple change in location of a double bond can have tremendous impact on the metabolism of these fats in the body.
Essential fatty acids
Previously we mentioned that horses must ingest certain fatty acids in their diet as they do not have the capability of synthesizing them in great enough quantities. These include linoleic acid and linolenic acid. Both of these fatty acids are 18 carbons long but differ in the number and placement of the double bonds.  Linolenic acid has three double bonds with the last one placed three carbons from the methyl end.  Thus, it is an Omega-3 fatty acid.  Linoleic acid has two double bonds, with the last double bond six carbons from the methyl end and is an Omega-6 fatty acid. These two fatty acids represent the essential fatty acids that horses must consume. These fatty acids do occur in forages and concentrates such as corn and oats, just in smaller quantities than we think about in more fat rich feedstuffs. Typically there will be more Omega-3 fatty acids in forages, especially pasture grasses, while grains will contain more Omega-6 fatty acids.
              Linoleic Acid Molecular Diagram               Linolenic Acid Molecular Diagram
The horse, as well as humans, must consume these fatty acids as we both lack the necessary enzymes to build these structures on our own. However, we do possess the enzymes needed to elongate these fatty acids to more complex fatty acid structures.  These elongation enzymes are shared by both linoleic and linolenic acid in their metabolic pathway. Their products in turn can be used to synthesize a whole host of biologically active compounds.  Linolenic acid can be elongated to eicosapentanoic acid or EPA, a twenty carbon fatty acid with five double bonds, and docosahexaenoic acid or DHA, a 22 carbon fatty acid with 6 double bonds, as well as others. Both EPA and DHA are Omega-3 fatty acids, due to their origin from an Omega-3 fatty acid. Linoleic acid is elongated to arachidonic acid, a twenty carbon fatty acid chain with four double bonds which is, of course, an n-6 fatty acid.   These fatty acids can be used to synthesize eicosanoids, which are biologically active lipids.
Eicosanoids have hormone-like activity which is typically mediated locally within a tissue. These include prostaglandins, thromboxanes and leukotrienes.   These compounds differ by their structure and perform a host of activities within the body. All of these compounds are necessary for normal bodily function, but an imbalance can contribute to a disease state. Prostaglandins can effect smooth muscle contraction, vasodilation, inflammation, pain, and fever, as well as gastric acid and mucus secretion. Leukotrienes are released during the inflammatory process and can contribute to inflammation and bronchoconstriction. While their role may be to aid in healing the damaged tissue, overproduction of leukotrienes can contribute to asthma or allergic reactions. Finally, thromboxanes cause the aggregation of platelets and constriction of blood vessels. Again, all of these compounds are part of normal bodily function, but their potent effects can contribute to the diseased state.
So how do Omega-3 fatty acids fit into this story of thromboxanes and leukotrienes? When animals ingest greater quantities of Omega-3 fatty acids, these fatty acids can displace arachadonic acid in the cell membrane. Thus, there is less arachadonic acid available to be released and formed into eicosanoids.   Increased linolenic acid also decreases the amount of linoleic acid which is elongated simply due to a competition for the same enzymes. The elongation products of linolenic acid and subsequently EPA may also directly counter act some of the inflammatory products of arachadonic acid metabolism. Thus increased consumption of Omega-3 fatty acids may aid in decreasing inflammation in the diseased state.
Feeding Omega-3 fatty acids may be helpful for horses which may have chronic pain or inflammation. Traditionally horsemen have used NSAIDS, or Non-Steroidal Anti-Inflammatory DrugS, to manage pain and inflammation. When we provide NSAIDS these compounds act by blocking the activity of enzymes which cause the release of inflammatory agents.  However, NSAIDS are not specific and block the activity of both cox-1 and cox-2. These enzymes are essential in the conversion of arachidonic acid to progstaglandins.  Cyclo-oxygenase 1 unfortunately is also intimately involved with the formation of thromboxane in platelets and in gastromucosal integrity. It is the inhibition of cox-1 which leads to the formation of ulcers in horses which have long term NSAID administration. However, many specific cox-2 inhibitors have been produced (Celebrex® and Vioxx®).
Sources of Omega-3s for horses
Compared to humans, it may be harder to increase the consumption of Omega-3 fatty acids in horses, but not impossible. Typically the greatest concentration of Omega-3 fatty acids is found in marine fish. Certainly fish oils have been fed to horses, but there may be limits in the acceptability of fish oil by the horse. Flaxseed, however, is also an excellent source of Omega-3 fatty acids, has a slightly sweet, nutty, whole-grain flavor and aroma, and is readily accepted by horses. Many flax products are now offered to the horse owner.  Increasing consumption of fresh grass will also boost the Omega-3s in your horses’ diet.
Next month we continue to look at Omega-3 fatty acids in the horse’s diet and examine some of the available literature concerning their effects in the horse.

Lipid Nutrition: Part 1, Feeding Fat to Horses

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Written By Dr. Kris Hiney

This month we begin a series looking at the value of incorporating fat into the diets of our horses. We will discuss how fat is digested and handled in the equine, the types of fats fed to horses, and the many beneficial effects that can be realized through the addition of fat to the diet of our horses.
 
Fat digestion
Feeding fat to horses became more popular in the 1980’s and has continued to see an increase in the share of the feed market. Most feed stores now offer a selection of fat added feeds, or specific fat supplements. While one may not think of horses as a species that routinely consumes fats, horses can handle fats quite well in their digestive system. Lipid digestion occurs primarily in the small intestine, via the production and release of digestive enzymes and bile salts. As the horse does not possess a gall bladder, bile salts are continually released into the intestine. Fats that are added to the diet in the form of oils or fat are very well digested, typically up to 90%. Comparatively, naturally occurring fats in the diet (muchsmaller percentages of fat are actually present in forages and cereal grains) are less well digested, between 40-50% for forages and 50-75% for grains. Addition of fat to the diet does not alter digestibility of other components of the diet, unless the amount of lipid exceeds 22% of the total diet. However, typically this is not a concern, as acceptability and practicality of such diets make them improbable. There are some published studies which do report a lowered fiber digestibility in horses fed soy oil, however, these horses were also rapidly introduced to the fat in the diet. Ideally horses should be gradually transitioned onto a higher fat diet in order to adapt and increase the necessary fat digesting enzymes in their system. This should take place over one to two weeks, depending on how much fat is being added to the diet.
 
Acceptability
Palatability of fat added feeds is quite good, especially if supplied by vegetable oils. Typical vegetable oils include corn oil, soybean oil, canola oil and linseed oil. Horses will consume animal fats and fish oil, but typically not as readily as vegetable sources. The acceptability of fats in the diet is good up to about 15% of the diet. After that consumption rates do drop off.   There are commercially available feeds which have a higher percentage of fat, but these are typically extruded feeds which are more acceptable. Again, these are fed at a smaller percentage of the diet, such that 15% of the total diet is never exceeded. When feeding fat added feeds, it is important to realize that they do have a shorter shelf life than non-fat added feeds. This is due to the peroxidation that takes place, especially in polyunsaturated fats. These feeds then develop an off taste and flavor. If your feed smells rancid, it is best to avoid feeding it. Storing feeds in a cool, dry area will help to preserve their shelf life as well. These feeds often have anti-oxidants added to them to aid in protection against oxidation. Some products, such as Omega Horseshine, specialize in stabilized fats with a prolonged shelf life, up to 12 months.
 
Benefits to feeding fat
The most readily realized benefit to adding fat to the diet is in order to help meet the animals’ caloric needs. Fat is very readily digestible as already stated, and is much more energy dense than other components of the horse’s diet. Compared to proteins and non-structural carbohydrates which contain 4 Mcal/kg, fat is 2.25 times more energy dense at 9 Mcal/kg. Thus inclusion of fat allows a horse to gain weight much more readily or conversely, need to consume less feed to obtain the same amount of calories. Lowering the total amount of feed may be advantageous to horses working in hotter climates as it lowers the total heat production associated with digestion. Furthermore, fat itself is a relatively cool feed, as there is no fermentation and thus heat production associated with its digestion. Replacing high energy cereal grains with fats is an additional benefit, as less digestive risk is associated with feeding fats. Horses fed large amounts of cereal grains over time are at greater risk for ulcer formation, potential development of  stereotypies such as cribbing, laminitis and insulin resistance. This does not mean that starch needs to be eliminated from normal equine diet (the exception are horses with metabolic disorders which render them more sensitive to starch in the diet), but fat can make a very useful substitution. Another benefit to replacing starch in the diet with fats appears to be a calming effect on the horse. Horses fed fat added diets compared to typical sweet feeds have been found to be  less reactive to novel stimuli. Therefore, there is a second reason that fat is a cool feed, not only does it produce less heat during digestion, but it appears to “cool” the hot minded horses. Now obviously it is not a substitute for proper training and exercise!
 
Essential fat and fatty acids
 Horses must also consume some amount of fat for normal body function. Lipids are used in the synthesis of steroid hormones, and  all of the fat soluble vitamins (ADEK) are contained within the fat portion of the feed.  However, the exact amount of fat necessary in the diet of the equine has not been determined. Additionally, the horse, like all other animals, must consume its essential fatty acids, linoleic (18:2 omega 6) and linolenic acid, (18:3, omega 3) from the diet. They lack the enzymes necessary to produce these particular fatty acids within the body. Important sources of these fatty acids include pasture grasses, canola oil and linseed oil or flax seed.
 
Practical guidelines for feeding fat to horses.
As stated previously, most fats in horse feed actually come from vegetable oils. The oils can either be extracted and purified, or the actual oil seed can be fed. Examples of common oilseeds include cottonseeds, soybeans, canola and flaxseeds. If these seeds are referred to as meal, such as cottonseed meal, the fat has already been extracted and then they are being fed typically for their high protein content, not for additional fat. Thus, feeding linseed meal provides a much different percentage of fat compared to feeding flax, despite it being the product of the same plant! Pure vegetable oils can also be fed to horses as a top dressing to their feed. One cup of vegetable oil provides as many calories as 1.5 lbs of oats or 1 lb of corn, allowing you to decrease the amount of cereal grains fed.   If feeding a fat added feed, typically these feeds will allow you to feed less concentrate for a similar work class of horse, due to the increased caloric density of the feed. The benefit of feeding a fat added feed, rather than top dressing, may be in its simplicity, as well as the fact that these rations are rebalanced with the knowledge that the horse may consume total less feed. However, if you are just top dressing fat to existing feeds, and thereby decreasing the total amount of feed, be sure that the total diet still meets the horse’s other nutritional requirements.
 
In the next part, we will discuss the potential for performance enhancing effects of feeding fat beyond merely an easy way to supply calories.

Protein Nutrition IV: Protein for the Working Class

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Written By Dr. Kris Hiney

Last month we learned that meeting a mature idle horse’s protein requirements are surprisingly easy.  If a horse is provided with good quality hay at 2% of its body weight it can easily consume enough protein even without eating concentrate.  However, if forage quality is low, adding a supplemental designed to provide essential amino acids can easily make up the difference.

But my horse works hard!

But what about if your horse has more of a job to do than just stand in his pasture and eat?  Many people automatically reach for a higher protein feed once their horse goes to work, but is that really the right thing to do?  Of course protein requirements of a working horse do go up due to the increased tissue turnover and repair associated with exercise.  Further, horses also lose nitrogen through sweating and increase muscle mass with training.  However, the increase in protein required pales in comparison to the increase in calories needed.  High protein diets increase the need for horses to excrete urea (the form in which excess nitrogen is removed from the body) and may alter their acid base balance. While horses seem to be able to handle the increased need to remove nitrogen from higher protein diets quite easily, it will result in more urine excretion.  Thus more ammonia may build up in poorly ventilated buildings and bedding costs will go up.  In fact, it may be beneficial to feed a lower total protein amount to the horse while providing key amino acids.   In one study, horses fed a lower protein diet but supplemented with lysine and threonine had higher blood pH values after exercising compared to horses on a higher protein diet (Graham-Theirs et al., 2001). When horses exercise intensely they produce lactic acid.  Lactic acid drops the pH in the blood and can contribute to the onset of fatigue. Therefore this lowered protein diet may protect against a drop in blood pH and therefore allow the horses to exercise longer or recover faster.  However it should be noted that the lower protein group was also supplemented with fat as well, clouding interpretation of results.

So how much do they really need?

 Table 3 shows the total amounts of protein needed, while Table 4 again expresses this on a % protein basis.  You can see that most performance horses will do quite well if you select a feed between 10-12% crude protein.  Remember that when selecting a feed, you must consider your forage source first!  For example, if your performance horse was eating a primarily alfalfa hay with a value of 16% crude protein, his protein needs would already be met!  Selecting a concentrate then would primarily serve to supply any additional energy needs the horse may have.

Table 3.  Crude protein requirements for work (g of CP/d).

Wt of horse (lb)

Light

Moderate

Heavy

Very Heavy

900

562

617

693

808

1000

624

685

769

896

1100

687

754

846

986

1200

750

823

924

1077

1300

811

891

1000

1165

Table 4.  Percent total protein required in the diet on an as-fed basis depending on the total consumption of the horse per day.

% of Bwt consumed

Light

Moderate

Heavy

Very Heavy

1.75

9.3

10.3

11.6

13.5

2.0

8.2

9.0

10.1

11.8

2.25

7.3

8.0

9.0

10.5

2.5

6.6

7.2

8.1

9.5

Let’s do math!

Now let’s put this together in a practical problem.  We will feed an 1100 lb horse 2% of his body weight in grass hay.  Our grass hay has 9% crude protein value on a DM basis.  We weigh out 22 lbs of hay for our horse per day but we weigh it on an as-fed basis (meaning what it weighs on a scale that day).

First we will convert our weight of hay to the weight of our hay on a dry matter basis.  We will assume the hay is 85% dry matter.

22 lbs x .85(% dry matter) = 18.7 lbs of hay on a dry matter basis

Then we will convert our lbs to kilograms.

18.7 lbs /2.24 = 8.3 kg of hay

Multiply that by our percentage of protein.

8.3 kg  x .09 = 747 g of CP.

Let’s check this horse’s lysine requirements as well.  Remember that the only value for amino acids required by the horse is for lysine.  The current available knowledge suggests that horses need 4.3% of their protein to come from lysine.  Typically grass hays are fairly low in lysine compared to legume hays.  An average grass hay harvested at a mature stage is 0.38 % lysine.  Again we multiply the amount of hay fed 8.3 kg x .0038 = 31.5 grams of lysine.  Our maintenance horse only needs 32 grams of lysine.  We therefore have met his requirement by feeding this hay.

Even if our 1100 lb horse is in moderate work we are short by only 7 grams of CP.  This can easily be met by any additional concentrate or by simply eating more hay.  However, if we move him up in work, we become much more deficient in protein as well as lysine.  Let’s assume he is now in heavy work and deficient by 100 g of protein.  We want to add 3 lbs of concentrate (which isn’t very much) to his diet.

3 lbs /2.24 = 1.3 kg of feed

We need our 1.3 kg to supply 100 g of CP.  So our feed needs to be 100g/1300 g of feed = 7.7 % CP on a dry matter basis. On an as fed basis, this would be 9% CP.  Almost every commercial feed will contain this level of crude protein.  Hopefully we have now illustrated that there is no need to feed a high protein feed designed for growing horses or broodmares to our exercising horses.

In summary, protein requirements for maintenance horses or even those at work are fairly easy to meet by a normal horse diet.  If feeding a poor quality hay, you may have to supplement your horse’s diet.  If so, then choose a feed that contains legumes (like alfalfa meal) or a concentrate that contains a high quality protein like soybean meal.  While no clear amino acid recommendations are available for working horses, there appears to be some benefits of feeding lower total quantity of protein while supplementing with key amino acids.  This certainly does appear to be the future of equine research concerning protein nutrition.

Next month we will address the protein needs of the groups of horses which need the most attention: the growing horses and the broodmares.

Protein Nutrition III: Determining Protein Requirements for Your Horse

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Written By Dr. Kris Hiney

In the last two articles we discussed the importance of protein quality, not only in terms of site of digestion in the horse, but also the amino acid composition of that feed.  Now we will try to simplify these concepts into selecting appropriate feeds for horses.

The first task in developing a feeding program for your horse is to identify what class of horse you have.  (For a review revisit: Equine Energy Requirements and Energy Requirements for the Working Class)  The horses with the lowest protein requirements relative to their body size are the mature horses not in work, or maintenance horses.  This does not include geriatric horses which may have altered protein needs due to the lowered efficiency of their digestive system.  We also must assume that the horse is receiving enough calories in the diet.  A horse fed a diet sufficient in protein, but low in calories will lose weight (makes sense right?) but a horse which has adequate calories but not enough protein can also lose weight.  We also make some consideration for the activity level of the horse as well.  Remember the difference in energy requirements  between the couch potato Quarter Horse and the active Thoroughbred mare (Equine Energy Requirements)?  Well a similar relationship exists with protein requirements.  Essentially the more active horse would have more lean tissue/muscle  to support than the lazy horse.   Table 1 lists the actual protein requirements for a  maintenance horse depending on activity level and their body weight.  Remember that this does assume a quality protein source. Lower quality protein (less digestible or poor amino acid profile) can adjust these figures upwards.

Table 1.  Crude Protein requirements for maintenance (grams of CP/d) based on average activity level.

Wt of horse (lb)

Couch potato

Average

Active

900

434

507

579

1000

482

562

642

1100

530

619

707

1200

579

675

772

1300

626

731

835

 

What percent protein do you need?

But let’s put these numbers into something more people are familiar with, percent of the diet.  Table 2 provides the percent protein of the total diet a horse would need to consume to meet their protein requirements.   Looking at Table 2 shows how easy it is to meet a maintenance horse’s protein requirement.  You can also see that as total consumption goes up, the percent of the protein needed in the diet goes down.  Conversely, if you fed less you would need to increase the percent protein in the diet.  Horses will usually consume between 1.5 and 2.8% of their body weight per day on a dry matter basis.  Typically you will see horses lower their consumption of less palatable hay which often equals poor quality.  However, this can largely be based on the individual, as some horses compensate by lowered feed quality by increasing intake (Edouard et al., 2008).  It turns out that horses are much more variable in their voluntary intake than other domestic species are!  Just like a horse to always want to be unpredictable. If you notice your horse picking through its hay and leaving a good proportion of the hay untouched it may be wise to select a supplement designed to provide amino acids but not to greatly increase the calorie consumption by the horse.  Alternatively it may be time to find a new hay supplier (see Selecting Forages).

 

Table 2a. Percent total protein required in the diet on a dry matter basis depending on the total consumption of the horse per day.

% of Bwt consumed

Couch potato

Average

Active

1.5

7.2

8.4

9.6

1.75

6.2

7.2

8.2

2.0

5.4

6.3

7.2

2.25

4.8

5.6

6.4

2.5

4.3

5.0

5.6

 

Table 2b. Percent total protein required in the diet expressed on an as fed basis assuming an average dry matter content of 85%. Note: this can change with the feed fed and is only representative of harvested feeds, not pasture or grasses.

% of Bwt consumed

Couch potato

Average

Active

1.5

8.5

9.9

11.3

1.75

7.3

8.5

9.7

2.0

6.4

7.4

8.4

2.25

5.6

6.6

7.5

2.5

5.1

5.9

6.8

 

Protein content of common horse feeds

Now let’s look at some typical protein values for feeds.  To be sure of your own feed ideally have your forage tested as well as examine your feed tag.  Corn ranges between 8-9 % CP on a DM basis, oats 12-13%, soybean meal – 43-49%, grass hays – 10-18%, and legumes between 18-25%.   With the range of protein content in forages, one can see how important it is to have knowledge of your nutrient content prior to selecting your concentrate.   Even with these ranges, most maintenance horses will easily meet their protein requirements by forage alone.  If you look at the range of percent protein needed by the maintenance horse in their total diet, it compares quite well with grass hays.  If you are feeding your horse and he is maintaining weight, he should easily be meeting his protein requirements at the same time.  Remember, we assume  the horse is receiving good quality hay.  If you are worried about the horse meeting its amino acid needs, many feed companies make supplements specifically designed to be fed with a strictly forage diet, rather than greatly increasing the concentrate intake.  For example, many feed companies offer protein supplements in the range of 30-35% crude protein.  These are designed to be fed at a minimal rate (only 1-2 lbs per day) in order to simply balance out any deficiencies from an all forage diet.  Clearly not all horses need the extra calories that come from feeding higher levels of concentrates.  This provides a convenient, easy way to ensure that your horse’s nutritional needs are being met.

Next month we continue with protein nutrition in the exercising horse.

 

Edouard et al.2008. Animal:An international journal of animal biosciences. 102:10:1526-1533.