The horse’s digestive system labelled and explained, with facts, figures and other important information that is relevant to their health.

In this three part series we dissect the equine gastrointestinal (GI) tract to gain a better understanding of the digestive process and learn how we should keep our horses, and what feeding management we should follow to maintain optimal (digestive) health in our horses.

The GI tract of an adult horse (~500kg) is about 30 meters long and has a total volume of approximately 180 litres. The entire tract can be divided into two functional parts; the foregut and the hindgut (see fig 1). In part one and two (March and April issues) we described the digestive process in the foregut of the horse. In this, the last part of the series, we discuss the final stage of food digestion – the large intestine and fermentation process.

The large intestine

The large intestine (hindgut) of the horse has three parts: caecum, colon and rectum (figure 1). Horses have an enlarged caecum, a blind sac at the junction of the small and large intestine and an enlarged and sacculated (large) colon (see fig 1). In the adult horse (500kg) the caecum is about 1 m long and has a capacity of about 30-34 liters. Nearly all of the non-starch polysaccharides (NSP) and undigested soluble carbohydrates in feed passes from the small intestine into the caecum. Together with the colon (large intestine) it contains micro-organisms that hydrolyse (break down with water) much of the fiber and soluble carbohydrates that are ingested. After digestion the nutrients (volatile fatty acids (VFA)), are absorbed from the caecum and colon.

Digestion in the caecum and colon depends almost entirely on the activity of micro organisms. In contrast with the small intestine, the walls of the large intestine contain only mucus-secreting glands, and does not produce digestive enzymes. However, high alkaline phosphatase activity is found in the large intestine. This is not been seen in other species like cats, dogs and man and is known to be related to a high digestive and absorption action.

The colon consists of three parts; ascending, transverse and descending. The first part of the colon has the greatest capacity and is known as the large colon. In contrast, the descending part of the colon is known as the small colon. The large colon is 3 to 3.7 m long and has a capacity of 50 to 60 litres.

The large colon can be divided into four compartments; the right and left segments of the ventral colon and the left and right segments of the dorsal colon (see figure 1). The four parts of the large colon are connected by three flexures (bends) . The diameter of the different segments of the large colon varies abruptly (20 to 25 cm), but reaches a maximum in the right dorsal colon where it forms the large sacculation (sack) with a diameter of up to 50 cm. The small colon is about 3 m long with an average diameter of 7.5 to 10 cm and has a capacity of 18 to 19 liters. Together with the large colon it has a total capacity of 70 to 80 litres. The rectum is located at the end of the large colon and is about 0.3 m long and opens to the exterior at the anus.

Microbial population and fermentation

There appears to be little difference in the biochemisty of fermentation in all hoofed animals studied, whatever the affinities of the animal or the site of fermentation chamber. The taxonomic composition of micro-organisms in the digestive system of all animals is also apparently broadly similar.

Microorganisms (e.g. bacteria, protozoa and fungi) can live in most segments of the animal gut. But the rumen and hindgut provide a unique environment for microorganisms. The anaerobic (without oxygen) system, constant pH conditions and nutrient supply are ideal for the growth of microbes. The pH (6-7) remains relatively constant because fermentation acids are absorbed rapidly across the rumen and hindgut wall or neutralized by saliva.

The flora of the caecum and colon of horses and rumen of cattle consist mainly of bacteria. Protozoa and fungi are present in much lower numbers, because of the lower rates of turnover.

Bacteria make the greatest contribution to metabolic work during fermentation compared to protozoa and fungi, especially the small bacteria. Bacteria that colonize the hindgut or rumen are diverse.

Bacterial species can be divided based on the type of energy source used, type of fermentation products produced, thickness, structure and composition of the cell walls. However, there are only two major types of bacterial cell wall; whether a given cell has one or the other type of wall can be determined by the cell’s reaction to certain dyes (Hans Christian Gram method with violet and iodine staining). Bacteria which retain the dye are called Gram-positive bacteria; those which can be decolorized are called Gram-negative bacteria.

The majority of the bacteria in the rumen and hindgut are Gram-negative. The number of Gram-positive bacteria tends to increase if animals are fed a high-energy diet containing abundant carbohydrates. This is an important concept that is studied a lot in relation to digestive disorders such as hindgut acidosis and laminitis (see the full series on laminitis in the lameness section of the website.)

Herbivores, because of the symbiotic relationship with microorganisms, are able to gain energy indirectly from fibrous materials i.e. non-starch polysaccharides (NSP).The microorganisms are able to break down the plant polymers to monomers and oligomers primarily by exogenous (generated outside a system) microbial enzymes. The products of this hydrolysis process are engulfed by microbes and converted to pyruvate in intracellular metabolism. Pyruvate is converted into volatile fatty acids (VFA’s: propionate, butyrate and acetate), CO2 and methane. The microbes cannot fully utilize these products. However the host animals, are able to absorb and gain energy from VFAs. The absorption of VFAs takes place in the enlarged colon of the horse and in the rumen, reticulum and omasum of cattle. However as hindgut fermenters, horses have the advantage to first digest and absorb the simple nutritional compounds, such as starches to glucose in the small intestine (which we described in detail in part 2 of this series). This system is more metabolically efficient for energy utilisation than fermentation to VFAs, which is obligatory in the ruminant.

Today most horses are used for sports and a grass/plant-only diet is usually not adequate to cover energy demands. Many commercial manufactured pelleted feeds are added to equine diets to supply the horse with energy, protein and micronutrients, but are based on cereal grains containing abundant starch. Starch plays a minor role in the natural diet of the horse; as they are grazing/browing herbivores and receive most carbohydrates in the form of fructans and NSP.

Grain supplements may cause starch overload and can affect the physiology of the horse due to the lack of amylase to hydrolyse all the ingested starch. If starch is not well digested in the small intestine of the horse a proportion of the ingested starch will reach the hindgut and be rapidly fermented. The microbial degradation rate of starches is much faster than that of NPS. A change in the ratio of starch to fiber in the diet has a rapid impact on VFA yields.

With increasing starch in the diet (e.g. grains) more propionate and lactate are produced within a short time compared to that of other NSP’s, which leads to reduced digesta pH. Several studies associate excess feeding of grain concentrates with a number of digestive and metabolic disorders, including, acidosis, laminitis, gastric ulcers, developmental orthopedic disease and some forms of exertional rhabdomyolysis. However, it should also be stated that excess intake of lush pastures with high levels of fructans can also cause these digestive and metabolic disorders.

Digestive and metabolic disorders are very common in the domestic horse population all over the world and as you now understand the majority of the cases it can be traced back to the way we manage and feed our horses! The horse is designed to eat large quantities of fibre on a continuous basis. The concentrate diet should be mixed with a fibre source such as (low non-structural carbohydrate; NSC) chaff or super fibres. You can also offer roughage before the feeding of the concentrate diet to slow down the passage rate and facilitate fiber digestion. The main aim is that we maximise the fiber intake and minimise the NSC intake so that we promote healthy functioning of the digestive system of our horses.

Read Part 1: From the mouth to the stomack here.

Read Part 2: The small intestine here.

Learn more about nutrition at MB Equine Services