A Study On Equine Digestive Anatomy Biology Essay

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The horse has evolved as a grazing and browsing animal, with a natural feed choice of grass, as well as other sources of roughage such as leaves, bark and branches (Thomas, 2004). They are monogastric (have a single, simple stomach) (Konke, Kelleher, & Trevor-Jones, 1999) and are designed to digest a high-fibre diet, and the digestive tract works most effectively when there are small amounts of roughage constantly passing through (Brega, 2005). The horse is very unique in the design of their digestive system, especially when compared to ruminants, not only in the anatomy and physiology of the digestive tract, but also the digestion processes for carbohydrates, proteins and lipids (Frandson, Wilke, & Fails, 2006).

Equine Digestive Anatomy

The digestion process in the horse begins in the mouth, or oral cavity (Frandson, Wilke, & Fails, Anatomy and Physiology of Farm Animals, 2006) which is used primarily for holding, grinding and mixing food with saliva to form a bolus through the use of the teeth, tongue, lips, cheeks and their appropriate muscles (Frandson, Wilke, & Fails, 2006) (Konke, Kelleher, & Trevor-Jones, 1999).

The pharynx is the common passage way for both food and air intake (Frandson, Wilke, & Fails, 2006) . The pharynx directs food down the esophagus by muscular contractions (Frandson, Wilke, & Fails, 2006) and is a tube that begins at the pharynx and extends down into the stomach (Konke, Kelleher, & Trevor-Jones, 1999).

The horse has a 'simple stomach' which is used primarily for the storage of food, as well as the beginning of digestion (Frandson, Wilke, & Fails, 2006) and is divided into the cardia (the entrance of the stomach), the fundus, the body, and the pyloric region (the exit of the stomach) (Frandson, Wilke, & Fails, 2006). The stomach is responsible for mixing of the food, as well as some protein digestion (Konke, Kelleher, & Trevor-Jones, 1999). It has a relatively small volume (7.5 - 15L) and has a retention time of approximately 30 mins - 12 hours for food (Konke, Kelleher, & Trevor-Jones, 1999). The upper area of the equine stomach is non-glandular, while the lower area is glandular, and this is separated by the margo plicatus (Frandson, Wilke, & Fails, 2006).

The small intestine consists of three sections; the duodenum, the jejunum and the ileum (Frape, 2004). The duodenum begins at the pylorus of the stomach, and receives ducts from the pancreas and liver (Frandson, Wilke, & Fails, 2006). The jejunum is the longest portion of the small intestine and is similar in function to the duodenum (Frandson, Wilke, & Fails, 2006). The ileum is the short last portion of the small intestine and is lined with many mucous cells (Frandson, Wilke, & Fails, 2006). The small intestine has a volume of approximately 40 - 50L and is responsible for the digestion of proteins and lipids, as well as the absorption of many vitamins and minerals (Konke, Kelleher, & Trevor-Jones, 1999). It also assists in the digestions of some carbohydrates (Frape, 2004).

The first portion of the large intestine is the caecum, with a volume of 25 - 30 L (Konke, Kelleher, & Trevor-Jones, 1999). It is a large comma shaped structure and is the primary site of microbial fermentation in the horse, turning carbohydrates into volatile fatty acids (Frandson, Wilke, & Fails, 2006). It also assists in the fermentation of additional protein, and has a retention time of about 6 - 12 hours (Konke, Kelleher, & Trevor-Jones, 1999). The second portion of the large intestine is the large, or great colon, with a volume of 50 - 60 L (Konke, Kelleher, & Trevor-Jones, 1999). It ferments fibre into volatile fatty acids and also ferments any leftover starch. It is also responsible for water uptake (Konke, Kelleher, & Trevor-Jones, 1999).

The final portion of the large intestine is the small or descending colon, which also ferments fibre, and predominantly is responsible for water uptake (Konke, Kelleher, & Trevor-Jones, 1999). It terminates at the rectum, where further water uptake takes place, as well as the storage of faeces, which are excreted via the anus (Frandson, Wilke, & Fails, 2006).

Ruminant Digestion

Ruminant digestion begins in the oral cavity, where forage is obtained by wrapping their tongues around and tearing (Parish, Rivera, & Boland, 2009). Unlike equines, they do not have any incisors, instead using their hard and soft palate, which the lower incisors work, crushing and grinding their food. (Frandson, Wilke, & Fails, 2006). Saliva assists with lubrication and mastication (Frandson, Wilke, & Fails, 2006) and contains enzymes that help begin the digestive process (Parish, Rivera, & Boland, 2009). The bolus then travels down the esophagus, which allows muscle contractions in both directions, aiding in rumination or "chewing the cud" (Frandson, Wilke, & Fails, 2006). This leads to the first two of four compartments in the stomach; the rumen and the reticulum (Parish, Rivera, & Boland, 2009). These are often referred to as the ruminoreticulum, and contains the bacteria, protozoa and fungi involved in the fermentation (Frandson, Wilke, & Fails, 2006) (Parish, Rivera, & Boland, 2009). The reticulum has a honeycomb appearance, and catches any foreign materials ingested, as well as assisting in moving food through to the rumen and omasum (Parish, Rivera, & Boland, 2009). The rumen is lined with papillae for nutrient absorption, and its main functions are to store, soak and break down food (Frandson, Wilke, & Fails, 2006). The omasum reduces particle sizes and absorbs small amounts of water, and has many folds (Parish, Rivera, & Boland, 2009). The abomasum is the true stomach, and is quite similar to the stomach of the horse. It contains acids and enzymes to aid in digestion (Parish, Rivera, & Boland, 2009). The small and large intestine follow on from the abomasum, and mixes digesta with secretions from the pancreas and liver, including bile from the gall bladder (Parish, Rivera, & Boland, 2009). Their main functions are absorption of nutrients and water, and the rest is excreted from the body via the rectum and anus (Frandson, Wilke, & Fails, 2006).

Protein Digestion

Protein is a major component of most tissues of the body, and is made up of chains of amino acids (National Research Council (US) Subcommittee on Horse Nutrition, 2009) which they must be broken down into in order to be absorbed and utilised (Konke, Kelleher, & Trevor-Jones, 1999). A small amount of protein digestion occurs in the stomach, by the enzyme pepsin, though the majority occurs in the small intestine, where protein is broken down into dipeptide and amino acid components (Konke, Kelleher, & Trevor-Jones, 1999) (Frape, 2004). The enzymes amino peptidase and carboxypeptidase are secreted by the walls of the small intestine (Frape, 2004) and assist in this digestion. The smaller components are then absorbed through the walls of the small intestine, into the bloodstream, where it can be utilised by the horse (Frape, 2004). Excess protein that is not digested in the small intestine will enter the large intestine, where it will be available for degradation and synthesis of microbial protein (Konke, Kelleher, & Trevor-Jones, 1999). The death and breakdown of the microorganisms in the large intestine also releases proteins and amino acids. Only a small amount of this can actually be used by the horse, and the rest is excreted as urea (Frape, 2004).

In ruminants, protein is obtained from both feed, and can also microbial protein (Parish, Rivera, & Boland, 2009). All ingested protein is classified as either degradable intake protein, which is digested by the microbes in the rumen of the ruminant, into ammonia and peptides (Frandson, Wilke, & Fails, 2006) and are used for growth and reproduction by the microbes (Parish, Rivera, & Boland, 2009). Any excess ammonia is absorbed through the wall of the rumen, and is carried to the liver by the blood stream to be converted into urea (Parish, Rivera, & Boland, 2009). The other group is undegradable intake protein, which is digested in the abomasum, where it is used as a protein source, along with any microorganisms which have been 'washed out' of the rumen (Parish, Rivera, & Boland, 2009). This is absorbed through the walls, into the bloodstream, to be used by the ruminant (Frandson, Wilke, & Fails, 2006).

Carbohydrate Digestion

Carbohydrates are the primary source of energy for the horse, and consist of monosaccharides, disaccharides, polysaccharides and oligosaccharides (National Research Council (US) Subcommittee on Horse Nutrition, 2009). As monosaccharides are the only form of carbohydrate that can be absorbed and utilised by the horse, the role of digestion is to break down the more complex forms (Pagan, 1998). Carbohydrates can be defined as either non-structural or structural carbohydrates. Non structural carbohydrates occur as simple sugars or can be easily broken down by the horse's digestive enzymes (Pagan, 1998). They are hydrolyzed into simple sugars in the small intestine by the enzymes α-amylase, α-glucosidases and β-galactosidase, with the help of the gastric acids in the stomach (Hoffman, 2003). The simple sugars that result can be directly absorbed into the blood stream (Hoffman, 2003). Structural carbohydrates, in contrast, are resistant to the horse's digestive enzymes and are therefore fermented by microbial digestion in the horse's hindgut (Pagan, 1998). Carbohydrates that cannot be hydrolyzed in the small intestine pass through to the cecum and large colon to be digested by the microorganism of the large intestine (Hoffman, 2003). They are turned into volatile fatty acids (VFA's), in particular acetate, propionate, butyrate and to a lesser degree, lactate and valerate (Hoffman, 2003) (Frape, 2004). The VFA's are then absorbed into the bloodstream, to be utilised by the body as energy (Frape, 2004). The rest is excreted from the horse's body (Hoffman, 2003).

Carbohydrate digestion in ruminants is different depending on whether it is feed a high forage or a high-grain/concentrate diet (Parish, Rivera, & Boland, 2009). On a high forage diet, ruminates "chew the cud" or regurgitate ingested forage, allowing them to reduce the size of the food particles, also increasing palatability (Parish, Rivera, & Boland, 2009). These carbohydrates are fermented in the rumen by the microbes to produce volatile fatty acids (Frandson, Wilke, & Fails, 2006). These are absorbed directly into the bloodstream to be utilised as a major energy source (Frandson, Wilke, & Fails, 2006). On a high concentrate diet, digestion occurs in a very similar way. There tends to be less ruminating, and they carbohydrates tend to be more digestible (Parish, Rivera, & Boland, 2009). This is more rapidly digested, creating more volatile fatty acids, which are absorbed in the same way (Frandson, Wilke, & Fails, 2006).

Lipid Digestion

Fats and fat oils are generally added to a horse's diet to increase energy density, but also serves to improve energy efficiency, and to enhance body and coat condition as well as to provide the fat-soluble vitamins (National Research Council (US) Subcommittee on Horse Nutrition, 2009). Lipids (or fats) and the long chain fatty acids are digested in the small intestine (Frape, 2004). Bile continuously flows from the liver increases the fat-water interface of the lipid, allowing the enzyme lipase to hydrolyse them into fatty acids and glycerol (Frape, 2004) (Konke, Kelleher, & Trevor-Jones, 1999). These are very easily absorbed into the bloodstream. In addition, some is also absorbed into the lymphatic system, and is then transported to the liver and cells as lipoprotein (Frape, 2004) (Konke, Kelleher, & Trevor-Jones, 1999). The fat soluble vitamins A, D, E and K are also released into the bloodstream during digestion (Konke, Kelleher, & Trevor-Jones, 1999).

In ruminants, digestion of lipids begins with the enzymes in the saliva (salivory lipase) (Parish, Rivera, & Boland, 2009). Many of the dietary lipids are split apart and fermented by the microbes in the rumen (Drackley, 2007). However, highly saturated lipids cannot be digested this way, and so are passed through to the abomasum, where pancreatic lipase begins breaking down the fats (Parish, Rivera, & Boland, 2009), and then the small intestine, where bile from the gall bladder emulsifies the lipids (Parish, Rivera, & Boland, 2009) (Drackley, 2007). This is where the lipids are primarily absorbed into the bloodstream for use (Parish, Rivera, & Boland, 2009).


While at a glance, monogastrics and ruminants appear similar, in reality it is quite to the contrary. Though both ingest primarily large amounts of fibre, their digestive systems vary greatly (Frandson, Wilke, & Fails, 2006). The major difference is obviously the rumen in ruminants, which functions as the fermentation chamber, rather than the large intestine in monogastrics (Frandson, Wilke, & Fails, 2006). Because of this, the majority of a ruminants digesta will have already been fermented by the microbes in the rumen before it reaches the stomach for enzymatic digestion (Frandson, Wilke, & Fails, 2006). Ruminants also "chew the cud", aiding in their easy digestion of fibre (Frandson, Wilke, & Fails, 2006).

Due to these anatomical and physiological variations, monogastrics (horses) should be fed in an unique way (Konke, Kelleher, & Trevor-Jones, 1999). They should be fed smaller meals regularly (Jackson, 1998), as they have only a relatively small stomach, and feeding should be done with the aim of maintaining the important hindgut fermentation (Jackson, 1998). This includes maximising the amount of forage and adequate fibre intake, and not too much readily fermentable carbohydrates (Jackson, 1998). This, as well as feeding quality feeds and providing a good supply of fresh water, is essential for the health and wellbeing of the horse (Konke, Kelleher, & Trevor-Jones, 1999).