Physiology and feed formulation: the proper role of carbohydrates in the equine diet
Published: February 1, 2007
By: STEPHEN DUREN and KATHRYN WATTS - Performance Horse Nutrition LLC/Rocky Mountain Research and Consulting (Courtesy of Alltech Inc.)
The statement, “I want a low carbohydrate feed for my horse”, has become popular in recent years. The reasons, or potential reasons, for wanting a low carbohydrate horse feed could be many. One possibility is a simple carry over from the current human dietary trend of reducing carbohydrate intake.
The Atkins Diet is one popular means of reducing carbohydrate intake in human meals. With millions of dollars spent on media advertisements for restaurants with ‘Atkins Friendly’ menus, each of us is at least reasonably familiar with the low carbohydrate concept stressed in this diet. It follows then that with an estimated 64% of Americans overweight and at any given time 29% of men and 44% of women trying to lose weight, one could conceive that popular food culture may be influencing the horse owner’s thought processes when buying horse feed.
However, there may be more legitimate reasons for a horse owner to seek a low carbohydrate feed. Included in the list of reasons are the desires to influence or modify behavior, or sensitivity to socalled carbohydrate diseases including tying-up, Cushing’s Disease, laminitis, insulin resistance (IR), obesity and osteochondrosis dissecans (OCD). But before we recommend that our clients reduce or eliminate carbohydrate from the equine diet, we should understand more about carbohydrates and their proper role in horse diets.
Classification of carbohydrates
The equine digestive tract is anatomically classified as that of a non-ruminant herbivore (Frape, 1986).
This digestive arrangement allows feedstuffs to be broken down with enzyme digestion in the small intestine and microbial fermentation in the cecum and colon, commonly referred to as the hindgut. Plant material (hay/pasture), cereal grain and commercial grain concentrate, the cornerstones of modern equine diets, consist of a wide array of carbohydrates and may contain up to 75% carbohydrate (Pagan, 1998).
However, not all of this carbohydrate is digested or absorbed in the same manner within the equine digestive tract.
On a simplistic basis, carbohydrates found in the equine diet can be crudely divided into two types, structural and non-structural. Structural carbohydrates are typically found in the cell wall of the plant and are often referred to as fiber. The major carbohydrates associated with the cell wall are cellulose, hemicellulose and lignin. These carbohydrates are represented on a laboratory analysis report as neutral detergent fiber (NDF). Baled hay, mature pasture grass, beet pulp and soybean seed coats are good sources of structural (fibrous) carbohydrate (NRC, 1989). Structural carbohydrates are resistant to enzyme digestion in the small intestine and must be fermented by bacteria in the horse’s hindgut (Frape, 1986). Bacterial fermentation of fiber yields volatile fatty acids (VFAs). VFAs are absorbed from the hindgut and are transported to the liver where they are converted to energy substrates for the horse. The overall digestibility of fibrous carbohydrate is quite variable, depending on the distribution of cellulose, hemicellulose and lignin in the carbohydrate fraction.
Since lignin is non-digestible by bacterial fermentation (Frape, 1986) the higher the degree of lignin present the lower the overall digestibility. Thus, as plants mature and increase in lignin content, digestibility is decreased. The overall digestibility of NDF in good quality forages by horses varies from 40-50% (Pagan, 1998).
Non-structural carbohydrate (NSC) is carbohydrate associated with the inner portion of the plant cell, or plant cell contents. The plant cell includes NSC along with protein, lipids, organic acids and soluble ash. NSC is made up of sugars, disaccharides, starches and fructans. In warm season grasses (C4 plants), starch is the primary storage carbohydrate, whereas in cool season grasses (C3 plants) fructan is the primary storage carbohydrate. As a practical point, commonly fed legumes such as clover and alfalfa do not contain fructan, and store carbohydrate as starch.
Enzymes in the horse’s small intestine break down sugars and starch to monosaccharides (simple sugars) that are absorbed and circulate in the blood as glucose.
Fructans are resistant to mammalian enzyme digestion and must be fermented by bacteria in the horse’s hindgut (Suzuki and Chatterton, 1993). Sugar and starch are highly digestible, greater than 95%, within the length of equine digestive tract. Bacteria located in the hindgut ferment any starch or sugar that is not digested by enzymes in the small intestine.
Unfortunately, fermentation of sugar, starch and fructan by hindgut microorganisms can produce lactic acid, and the resulting acidosis can destroy the environment within the hindgut leading to death of the microorganisms and health concerns such as colic and laminitis (Richards et al., 2003).
The extent to which NSC is digested in the small intestine is dependent on the source of the NSC, processing, level and rate of intake, time and frequency of forage feeding and individual horse digestive characteristics (Meyer et al., 1993). Briefly, starch originating from oat grain is more digestible within the small intestine compared to starch from corn or barley. This difference is related to the microscopic structure of the starch granule, and thus the surface area available for enzyme digestion.
Processing (rolling, crimping, grinding, etc.) generally increases the digestion of starch in the small intestine, again increasing the surface area for enzyme digestion. As level of starch intake increases, the small intestine digestibility decreases due to rate of passage of feed material. The feeding of forage following a meal rich in sugar and starch will increase rate of passage and decrease small intestine digestibility of sugar and starch. Finally, differences in the ability of individual horses to either digest or absorb sugars from the small intestine has been reported (Richards et al., 2003).
The NSC content of feeds can be determined with modern laboratory methods (e.g. Dairy One, Forage Analysis Laboratory, Ithaca, New York, USA). Feeds with large amounts of NSC include commercial grain concentrates containing large amounts of oats, corn and barley and any product containing high amounts of molasses. Forages such as pasture and baled hay also contain NSC, but the amount is quite variable, ranging from 1 to 40% of dry matter. The NSC content of forages is influenced by type of plant, environmental temperature, light intensity, drought stress, plant fertility, and rate of drying during the curing process (Smith, 1973).
Briefly, C3 (cool season) grasses are generally higher in NSC than C4 (warm season) grasses. Cold stress, especially in C3 grasses, increases the NSC content. Photosynthetic capacity, and hence production of sugar, is directly correlated to light intensity and duration. NSC concentration will be lowest in early morning if the night was warm enough to allow the sugars produced the previous day to be utilized by respiration. Further, shading or cloud cover of grass plants reduces NSC content. Drought stress is a stimulus for NSC accumulation in grass. Drought limits plant respiration causing the accumulation of NSC. It is widely accepted by forage researchers that nitrogen and phosphorus deficiency causes an increase in concentration of NSC in both grass and legumes.
Finally, the faster plants dry after being cut for hay production, the higher the NSC content. Plants will continue to respire and burn off sugars until plant moisture content is below approximately 40%. The only method to determine the NSC content of a feed ingredient is laboratory analysis. Visual characteristics and type of hay are not predicable indicators of NSC content.
Carbohydrate-related diseases
In the last ten years, many equine disease treatment protocols have investigated the role of dietary carbohydrates. Several disease states may be precipitated or exacerbated when high levels of carbohydrate are being fed. The following is a brief description of several disease conditions and the role that carbohydrates may play in the treatment or prevention of disease.
Tying-up is a condition associated with the equine musculature system. Tying-up is also known by the following names: Azoturia, Monday Morning Disease, and Exertional Rhabdomyolysis. It is expressed clinically as firm, painful muscles over the lumbar, sacral and gluteal regions, along with excessive sweating, increased heart rate and elevated respiration (Valberg and McKenzie, 2002). Horses experiencing a bout of tying-up often are in extreme pain to the point they are reluctant to move or are recumbent.
Tying-up is confirmed with elevated serum levels of muscle enzymes including creatine kinase (CK) and aspartate transaminase (AST). Horses that suffer frequent or chronic bouts of tying-up have been studied to determine possible dietary treatments.
Dietary modification that both reduces the grain or starch intake and increases the intake of fat has been shown to be effective in managing this condition (Valberg and McKenzie, 2002).
Cushing’s Disease is a result of a pituitary gland tumor causing an overproduction of pituitary hormones. This disease is thought to be principally associated with older horses (greater than 15 years of age), but has also been described in horses as young as seven (Geor, 2001). The clinical signs of this disease include hirsutism (excessively long and curly hair coat), weight loss, muscle wasting, lethargy, increased sweating, bulging of the orbit due to an increase in fat deposition around the eyes, polydipsia (increased thirst) and polyuria (increased urination).
The complications of this disease are laminitis and diabetes mellitus, with both complications made worse with high starch diets (Geor, 2001).
Laminitis is a disease condition associated with the hoof and its attachment to the bony structure of the equine foot. In a bout of laminitis, inflammation of the laminae alters tissue blood flow to the sensitive living tissues that attach the coffin bone to the hoof wall (Redden, 2001). This causes separation of the non-sensitive hoof wall and the laminae such that the coffin bone becomes detached from the hoof wall and sinks toward the ground. This disease is extremely painful for the animal and often debilitating.
Laminitis is associated with a number of risk factors, with overeating of grain (starch) the best known cause. Grain overload is commonly used as a method to induce laminitis in research animals.
Obesity is a very common problem for modern horses. According to Dr. Philip Johnson from the University of Missouri, evolution equipped the equine metabolism for survival based on the seasonally variable availability of forage (grass). The temporary development of additional body fat (relative obesity) at times when forage is plentiful provides a survival adaptation for time when conditions are harsh and forage is scarce. During periods in which forage is relatively unavailable, the body fat stores, which were never intended to become excessive, are depleted in order to provide energy for survival.
Under many modern horse management systems, the combination of feeding starch-rich rations over many years and protracted periods of stall confinement or lack of adequate exercise tend to lead to the acquisition and maintenance of substantial body fat in the domesticated horse. The development of obesity in both humans and horses directly causes insulin insensitivity (Johnson, 2002). Insulin insensitivity is known to lead to endothelial cell dysfunction, which is involved in the pathogenesis of vascular complications in humans. Insulin insensitivity is currently being studied as a cause of laminitis in horses, and future feeding recommendations may require these horses to eat low carbohydrate (low glycemic) diets.
Osteochondrosis is one of a number of growth anomalies that affect the skeleton of young horses. Osteochondrosis can be very serious in that it often results in debilitating lameness that in many instances reduces or eliminates athletic performance (Jackson, 2003). The cause of osteochondrosis is multifactorial. The proposed causes of osteochondrosis include genetic predisposition, rapid growth rate, mechanical stress and trauma, nutrition excess, mineral imbalances and endocrine factors (McIlwraith, 1996).
New research is re-focusing on hyperglycemia and/or hyperinsulinemia as a cause of osteochondrosis (Glade et al., 1984; Ralston, 1995; Pagan 2001). Specifically, scientists are looking to determine if certain foals that experience an exaggerated and sustained increase in blood glucose and insulin in response to a carbohydrate meal may be predisposed to development of osteochondrosis (Pagan, 2001). Hyperinsulinemia has been reported to be a potential endocrine factor that contributes to equine osteochondrosis (Henson et al., 1997). If this does prove to be a predictable cause of osteochondrosis, then feeding foals carbohydrate-rich diets may be contraindicated.
Minimizing carbohydrate intake
From the available information on several diseases thought to be sensitive to carbohydrate in the diet, the desire to create a low carbohydrate diet is really a need to control the amount of NSC (sugar, starch and fructan) the horse consumes. Structural carbohydrate (fiber) should not be a target for elimination since fiber is essential for proper function and motility of the horse’s digestive system.
Practicing equine nutritionists commonly recommend that horses receive a minimum of 1.5% of body weight per day in dry forage to provide essential fiber.
However, some work is now being done to correlate a more predictable fiber value, neutral detergent fiber (NDF), to voluntary dry matter intake of horses (St. Lawrence et al., 2001).
If it is necessary to reduce the NSC intake in an equine diet, the following steps will help accomplish that goal. The first step in minimizing the amount of NSC (sugar and starch) in a horse’s diet is to determine the NSC content of the feed. Unfortunately, this is not information that can be found on the feed tag. A quality lab for determination of NSC is the Dairy One Forage Lab, Ithaca, New York. The cost of analysis is approximately $15 for a combination of sugar, starch and fructan analyses and approximately $26 for a more complete nutrient analysis including minerals and full carbohydrate fractions. Table 1 lists the average NSC content of feed ingredients in the US as published in the Dairy One Ingredient Library.
The reader should note that NSC content of the ingredients can be quite variable due to growing conditions previously outlined. With a range of up to 35% of dry matter, NSC content of forages certainly requires that we not rely on ‘book’ values for average NSC content.
Table 1. Non-structural carbohydrate content of feed ingredients (dry matter basis)1.
1Dairy One Forage Laboratory, Ithaca, New York.
The second step in minimizing the amount of NSC in the diet is to only feed the amount of grain or supplement necessary to maintain body condition and ensure proper dietary fortification of protein, vitamins and minerals. Since grains and grain concentrates with molasses contain high levels of NSC, minimizing the amounts of these products included in the diet will minimize total NSC intake.
Unfortunately, many horse owners are quick to eliminate all grain products from the diet and therefore also eliminate the source of essential vitamins and minerals that are associated with the grain concentrate portion of the diet. If grain is to be eliminated from the diet, a low-intake protein, vitamin and mineral supplement should be added to provide these essential nutrients. If horses do require large amounts of grain, it should be recommended that clients feed grain concentrates with moderate to low levels of NSC. The use of dietary fat, either in the feed or top-dressed onto the feed, has been shown to slow the rate at which the stomach empties and to control the surge of glucose into the blood following a meal (Pagan et al., 1995). Therefore, feeding high fat feeds or adding fat to the diet may help to eliminate many carbohydrate problems. However, long term feeding of fat may lead to impaired glucose tolerance in ponies (Schmidt et al., 2001), and is contraindicated in obese horses.
The final step in reducing NSC intake is to consider the forage portion of the diet. Overgrazed, stressed pasture grass can contain an abundance of NSC and should be avoided with horses known to have laminitis, episodes of tying-up, insulin resistance, Cushing’s Disease or obesity. Pasture grass should be replaced with baled hay or other fibrous carbohydrate sources such as hay cubes or hay pellets, appropriately tested for NSC content.
Since visual characteristics and type of hay are not reliable indicators of carbohydrate content, new research is focusing on methods to reduce the carbohydrate load of hay after it has already been baled. Watts (2003) has reported on one method to reduce the watersoluble carbohydrate (sugar) content of hay. In that research, Watts soaked fifteen samples of various species of hay in water. She reported the average amounts of sugar reduction after 30 and 60 minutes of soaking in cold water were 18.9% and 30.7%, respectively. The average amount of sugar removed in 30 minutes of soaking in hot water was 29%. This research offers the first step in methods to practically limit the sugar content of baled hay.
Summary
In summary, there are health concerns that dictate the need to feed low carbohydrate diets to certain horses. Since there is an abundance of different carbohydrates in horse feed, the choice of which carbohydrates to limit is critical. Carbohydrates found in equine feeds can be roughly divided into two types, structural and non-structural. Structural carbohydrates are often referred to as fibrous carbohydrates and should not be eliminated from the diet. Non-structural carbohydrates (NSC) are commonly referred to as sugars, starch and fructans and can be associated with several so called carbohydrate diseases. NSC is the carbohydrate that can be minimized in the equine diet. Methods to minimize NSC content of the diet include analysis of feed ingredients to determine NSC content, reduction in intake of ingredients with high NSC content, and modification of forage intake including elimination of stressed pasture and the soaking in water of baled hay to rinse water-soluble carbohydrate away.
References
The Atkins Diet is one popular means of reducing carbohydrate intake in human meals. With millions of dollars spent on media advertisements for restaurants with ‘Atkins Friendly’ menus, each of us is at least reasonably familiar with the low carbohydrate concept stressed in this diet. It follows then that with an estimated 64% of Americans overweight and at any given time 29% of men and 44% of women trying to lose weight, one could conceive that popular food culture may be influencing the horse owner’s thought processes when buying horse feed.
However, there may be more legitimate reasons for a horse owner to seek a low carbohydrate feed. Included in the list of reasons are the desires to influence or modify behavior, or sensitivity to socalled carbohydrate diseases including tying-up, Cushing’s Disease, laminitis, insulin resistance (IR), obesity and osteochondrosis dissecans (OCD). But before we recommend that our clients reduce or eliminate carbohydrate from the equine diet, we should understand more about carbohydrates and their proper role in horse diets.
Classification of carbohydrates
The equine digestive tract is anatomically classified as that of a non-ruminant herbivore (Frape, 1986).
This digestive arrangement allows feedstuffs to be broken down with enzyme digestion in the small intestine and microbial fermentation in the cecum and colon, commonly referred to as the hindgut. Plant material (hay/pasture), cereal grain and commercial grain concentrate, the cornerstones of modern equine diets, consist of a wide array of carbohydrates and may contain up to 75% carbohydrate (Pagan, 1998).
However, not all of this carbohydrate is digested or absorbed in the same manner within the equine digestive tract.
On a simplistic basis, carbohydrates found in the equine diet can be crudely divided into two types, structural and non-structural. Structural carbohydrates are typically found in the cell wall of the plant and are often referred to as fiber. The major carbohydrates associated with the cell wall are cellulose, hemicellulose and lignin. These carbohydrates are represented on a laboratory analysis report as neutral detergent fiber (NDF). Baled hay, mature pasture grass, beet pulp and soybean seed coats are good sources of structural (fibrous) carbohydrate (NRC, 1989). Structural carbohydrates are resistant to enzyme digestion in the small intestine and must be fermented by bacteria in the horse’s hindgut (Frape, 1986). Bacterial fermentation of fiber yields volatile fatty acids (VFAs). VFAs are absorbed from the hindgut and are transported to the liver where they are converted to energy substrates for the horse. The overall digestibility of fibrous carbohydrate is quite variable, depending on the distribution of cellulose, hemicellulose and lignin in the carbohydrate fraction.
Since lignin is non-digestible by bacterial fermentation (Frape, 1986) the higher the degree of lignin present the lower the overall digestibility. Thus, as plants mature and increase in lignin content, digestibility is decreased. The overall digestibility of NDF in good quality forages by horses varies from 40-50% (Pagan, 1998).
Non-structural carbohydrate (NSC) is carbohydrate associated with the inner portion of the plant cell, or plant cell contents. The plant cell includes NSC along with protein, lipids, organic acids and soluble ash. NSC is made up of sugars, disaccharides, starches and fructans. In warm season grasses (C4 plants), starch is the primary storage carbohydrate, whereas in cool season grasses (C3 plants) fructan is the primary storage carbohydrate. As a practical point, commonly fed legumes such as clover and alfalfa do not contain fructan, and store carbohydrate as starch.
Enzymes in the horse’s small intestine break down sugars and starch to monosaccharides (simple sugars) that are absorbed and circulate in the blood as glucose.
Fructans are resistant to mammalian enzyme digestion and must be fermented by bacteria in the horse’s hindgut (Suzuki and Chatterton, 1993). Sugar and starch are highly digestible, greater than 95%, within the length of equine digestive tract. Bacteria located in the hindgut ferment any starch or sugar that is not digested by enzymes in the small intestine.
Unfortunately, fermentation of sugar, starch and fructan by hindgut microorganisms can produce lactic acid, and the resulting acidosis can destroy the environment within the hindgut leading to death of the microorganisms and health concerns such as colic and laminitis (Richards et al., 2003).
The extent to which NSC is digested in the small intestine is dependent on the source of the NSC, processing, level and rate of intake, time and frequency of forage feeding and individual horse digestive characteristics (Meyer et al., 1993). Briefly, starch originating from oat grain is more digestible within the small intestine compared to starch from corn or barley. This difference is related to the microscopic structure of the starch granule, and thus the surface area available for enzyme digestion.
Processing (rolling, crimping, grinding, etc.) generally increases the digestion of starch in the small intestine, again increasing the surface area for enzyme digestion. As level of starch intake increases, the small intestine digestibility decreases due to rate of passage of feed material. The feeding of forage following a meal rich in sugar and starch will increase rate of passage and decrease small intestine digestibility of sugar and starch. Finally, differences in the ability of individual horses to either digest or absorb sugars from the small intestine has been reported (Richards et al., 2003).
The NSC content of feeds can be determined with modern laboratory methods (e.g. Dairy One, Forage Analysis Laboratory, Ithaca, New York, USA). Feeds with large amounts of NSC include commercial grain concentrates containing large amounts of oats, corn and barley and any product containing high amounts of molasses. Forages such as pasture and baled hay also contain NSC, but the amount is quite variable, ranging from 1 to 40% of dry matter. The NSC content of forages is influenced by type of plant, environmental temperature, light intensity, drought stress, plant fertility, and rate of drying during the curing process (Smith, 1973).
Briefly, C3 (cool season) grasses are generally higher in NSC than C4 (warm season) grasses. Cold stress, especially in C3 grasses, increases the NSC content. Photosynthetic capacity, and hence production of sugar, is directly correlated to light intensity and duration. NSC concentration will be lowest in early morning if the night was warm enough to allow the sugars produced the previous day to be utilized by respiration. Further, shading or cloud cover of grass plants reduces NSC content. Drought stress is a stimulus for NSC accumulation in grass. Drought limits plant respiration causing the accumulation of NSC. It is widely accepted by forage researchers that nitrogen and phosphorus deficiency causes an increase in concentration of NSC in both grass and legumes.
Finally, the faster plants dry after being cut for hay production, the higher the NSC content. Plants will continue to respire and burn off sugars until plant moisture content is below approximately 40%. The only method to determine the NSC content of a feed ingredient is laboratory analysis. Visual characteristics and type of hay are not predicable indicators of NSC content.
Carbohydrate-related diseases
In the last ten years, many equine disease treatment protocols have investigated the role of dietary carbohydrates. Several disease states may be precipitated or exacerbated when high levels of carbohydrate are being fed. The following is a brief description of several disease conditions and the role that carbohydrates may play in the treatment or prevention of disease.
Tying-up is a condition associated with the equine musculature system. Tying-up is also known by the following names: Azoturia, Monday Morning Disease, and Exertional Rhabdomyolysis. It is expressed clinically as firm, painful muscles over the lumbar, sacral and gluteal regions, along with excessive sweating, increased heart rate and elevated respiration (Valberg and McKenzie, 2002). Horses experiencing a bout of tying-up often are in extreme pain to the point they are reluctant to move or are recumbent.
Tying-up is confirmed with elevated serum levels of muscle enzymes including creatine kinase (CK) and aspartate transaminase (AST). Horses that suffer frequent or chronic bouts of tying-up have been studied to determine possible dietary treatments.
Dietary modification that both reduces the grain or starch intake and increases the intake of fat has been shown to be effective in managing this condition (Valberg and McKenzie, 2002).
Cushing’s Disease is a result of a pituitary gland tumor causing an overproduction of pituitary hormones. This disease is thought to be principally associated with older horses (greater than 15 years of age), but has also been described in horses as young as seven (Geor, 2001). The clinical signs of this disease include hirsutism (excessively long and curly hair coat), weight loss, muscle wasting, lethargy, increased sweating, bulging of the orbit due to an increase in fat deposition around the eyes, polydipsia (increased thirst) and polyuria (increased urination).
The complications of this disease are laminitis and diabetes mellitus, with both complications made worse with high starch diets (Geor, 2001).
Laminitis is a disease condition associated with the hoof and its attachment to the bony structure of the equine foot. In a bout of laminitis, inflammation of the laminae alters tissue blood flow to the sensitive living tissues that attach the coffin bone to the hoof wall (Redden, 2001). This causes separation of the non-sensitive hoof wall and the laminae such that the coffin bone becomes detached from the hoof wall and sinks toward the ground. This disease is extremely painful for the animal and often debilitating.
Laminitis is associated with a number of risk factors, with overeating of grain (starch) the best known cause. Grain overload is commonly used as a method to induce laminitis in research animals.
Obesity is a very common problem for modern horses. According to Dr. Philip Johnson from the University of Missouri, evolution equipped the equine metabolism for survival based on the seasonally variable availability of forage (grass). The temporary development of additional body fat (relative obesity) at times when forage is plentiful provides a survival adaptation for time when conditions are harsh and forage is scarce. During periods in which forage is relatively unavailable, the body fat stores, which were never intended to become excessive, are depleted in order to provide energy for survival.
Under many modern horse management systems, the combination of feeding starch-rich rations over many years and protracted periods of stall confinement or lack of adequate exercise tend to lead to the acquisition and maintenance of substantial body fat in the domesticated horse. The development of obesity in both humans and horses directly causes insulin insensitivity (Johnson, 2002). Insulin insensitivity is known to lead to endothelial cell dysfunction, which is involved in the pathogenesis of vascular complications in humans. Insulin insensitivity is currently being studied as a cause of laminitis in horses, and future feeding recommendations may require these horses to eat low carbohydrate (low glycemic) diets.
Osteochondrosis is one of a number of growth anomalies that affect the skeleton of young horses. Osteochondrosis can be very serious in that it often results in debilitating lameness that in many instances reduces or eliminates athletic performance (Jackson, 2003). The cause of osteochondrosis is multifactorial. The proposed causes of osteochondrosis include genetic predisposition, rapid growth rate, mechanical stress and trauma, nutrition excess, mineral imbalances and endocrine factors (McIlwraith, 1996).
New research is re-focusing on hyperglycemia and/or hyperinsulinemia as a cause of osteochondrosis (Glade et al., 1984; Ralston, 1995; Pagan 2001). Specifically, scientists are looking to determine if certain foals that experience an exaggerated and sustained increase in blood glucose and insulin in response to a carbohydrate meal may be predisposed to development of osteochondrosis (Pagan, 2001). Hyperinsulinemia has been reported to be a potential endocrine factor that contributes to equine osteochondrosis (Henson et al., 1997). If this does prove to be a predictable cause of osteochondrosis, then feeding foals carbohydrate-rich diets may be contraindicated.
Minimizing carbohydrate intake
From the available information on several diseases thought to be sensitive to carbohydrate in the diet, the desire to create a low carbohydrate diet is really a need to control the amount of NSC (sugar, starch and fructan) the horse consumes. Structural carbohydrate (fiber) should not be a target for elimination since fiber is essential for proper function and motility of the horse’s digestive system.
Practicing equine nutritionists commonly recommend that horses receive a minimum of 1.5% of body weight per day in dry forage to provide essential fiber.
However, some work is now being done to correlate a more predictable fiber value, neutral detergent fiber (NDF), to voluntary dry matter intake of horses (St. Lawrence et al., 2001).
If it is necessary to reduce the NSC intake in an equine diet, the following steps will help accomplish that goal. The first step in minimizing the amount of NSC (sugar and starch) in a horse’s diet is to determine the NSC content of the feed. Unfortunately, this is not information that can be found on the feed tag. A quality lab for determination of NSC is the Dairy One Forage Lab, Ithaca, New York. The cost of analysis is approximately $15 for a combination of sugar, starch and fructan analyses and approximately $26 for a more complete nutrient analysis including minerals and full carbohydrate fractions. Table 1 lists the average NSC content of feed ingredients in the US as published in the Dairy One Ingredient Library.
The reader should note that NSC content of the ingredients can be quite variable due to growing conditions previously outlined. With a range of up to 35% of dry matter, NSC content of forages certainly requires that we not rely on ‘book’ values for average NSC content.
Table 1. Non-structural carbohydrate content of feed ingredients (dry matter basis)1.
1Dairy One Forage Laboratory, Ithaca, New York.
The second step in minimizing the amount of NSC in the diet is to only feed the amount of grain or supplement necessary to maintain body condition and ensure proper dietary fortification of protein, vitamins and minerals. Since grains and grain concentrates with molasses contain high levels of NSC, minimizing the amounts of these products included in the diet will minimize total NSC intake.
Unfortunately, many horse owners are quick to eliminate all grain products from the diet and therefore also eliminate the source of essential vitamins and minerals that are associated with the grain concentrate portion of the diet. If grain is to be eliminated from the diet, a low-intake protein, vitamin and mineral supplement should be added to provide these essential nutrients. If horses do require large amounts of grain, it should be recommended that clients feed grain concentrates with moderate to low levels of NSC. The use of dietary fat, either in the feed or top-dressed onto the feed, has been shown to slow the rate at which the stomach empties and to control the surge of glucose into the blood following a meal (Pagan et al., 1995). Therefore, feeding high fat feeds or adding fat to the diet may help to eliminate many carbohydrate problems. However, long term feeding of fat may lead to impaired glucose tolerance in ponies (Schmidt et al., 2001), and is contraindicated in obese horses.
The final step in reducing NSC intake is to consider the forage portion of the diet. Overgrazed, stressed pasture grass can contain an abundance of NSC and should be avoided with horses known to have laminitis, episodes of tying-up, insulin resistance, Cushing’s Disease or obesity. Pasture grass should be replaced with baled hay or other fibrous carbohydrate sources such as hay cubes or hay pellets, appropriately tested for NSC content.
Since visual characteristics and type of hay are not reliable indicators of carbohydrate content, new research is focusing on methods to reduce the carbohydrate load of hay after it has already been baled. Watts (2003) has reported on one method to reduce the watersoluble carbohydrate (sugar) content of hay. In that research, Watts soaked fifteen samples of various species of hay in water. She reported the average amounts of sugar reduction after 30 and 60 minutes of soaking in cold water were 18.9% and 30.7%, respectively. The average amount of sugar removed in 30 minutes of soaking in hot water was 29%. This research offers the first step in methods to practically limit the sugar content of baled hay.
Summary
In summary, there are health concerns that dictate the need to feed low carbohydrate diets to certain horses. Since there is an abundance of different carbohydrates in horse feed, the choice of which carbohydrates to limit is critical. Carbohydrates found in equine feeds can be roughly divided into two types, structural and non-structural. Structural carbohydrates are often referred to as fibrous carbohydrates and should not be eliminated from the diet. Non-structural carbohydrates (NSC) are commonly referred to as sugars, starch and fructans and can be associated with several so called carbohydrate diseases. NSC is the carbohydrate that can be minimized in the equine diet. Methods to minimize NSC content of the diet include analysis of feed ingredients to determine NSC content, reduction in intake of ingredients with high NSC content, and modification of forage intake including elimination of stressed pasture and the soaking in water of baled hay to rinse water-soluble carbohydrate away.
References
Frape, D.L. 1986. Equine Nutrition and Feeding. Longman Press, London. UK.
Geor, R. 2001. Cushing’s Disease and other problems of the older horse. In: Proceedings of the 2001 Equine Nutrition Conference for Feed Manufacturers.
Kentucky Equine Research, Versailles, KY. Glade, M.J., S. Gupta, T.J. Reimers. 1984. Hormonal responses to high and low planes of nutrition in weanling Thoroughbreds. J. Anim. Sci. 59(3):658- 665.
Henson, F.M., C. Davenport and L. Butler. 1997. Effects of insulin and insulin-like growth factors I and II on the growth of equine fetal and neonatal chondrocytes. Equine Vet. J. 156(3):177-192.
Jackson, S.G. 2003. Nutrition and equine productivity: practical problems related to nutrition. In: Nutritional Biotechnology in the Feed and Food Industries, Proceedings of Alltech’s 19th Annual Symposium (T.P. Lyons and K.A. Jacques, eds). Nottingham University Press, UK.
Johnson, P.J. 2002. Peripheral cushingoid syndrome (‘equine metabolic syndrome’). In: Current Therapy in Equine Medicine, 5th Edition. (N.E. Robinson, ed). W.B. Suanders, Philadelphia, PA.
McIlwraith, C.W. 1996. The equine skeleton – how does bone grow and how do abnormalities in the development process affect soundness? In: Proceedings of the 1996 Short Course Equine Nutrition Conference for Feed Manufacturers. Kentucky Equine Research, Versailles, KY.
Meyer, H., S. Radicke, E. Kiengle. S. Wilke and D. Kleffken. 1993. Investigations on preileal digestion of oats, corn and barley starch in relation to grain processing. In: Proceedings of 13th Equine Nutrition and Physiology Conference, Gainesville, FL.
National Research Council. 1989. Nutrient Requirements of Horses. 5th Edition NRC-NAS. Washington, D.C.
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Authors: STEPHEN DUREN1 and KATHRYN WATTS2
1 Performance Horse Nutrition, LLC, Weiser, Idaho, USA
2 Rocky Mountain Research and Consulting, Center, Colorado, USA
1 Performance Horse Nutrition, LLC, Weiser, Idaho, USA
2 Rocky Mountain Research and Consulting, Center, Colorado, USA
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