Fermentation in the hindgut of the horse is similar to that in the rumen, resulting in the production of short-chain volatile fatty acids mainly acetic, propionic and butyric acids. The proportions of these acids are influenced by the availability and type of substrate, composition of the microbial community and the hindgut physiologic conditions.
Fermentation in the hindgut of the horse is similar to that in the rumen, resulting in the production of short-chain volatile fatty acids mainly acetic, propionic and butyric acids. The proportions of these acids are influenced by the availability and type of substrate, composition of the microbial community and the hindgut physiologic conditions. The microbial community of the hindgut of the horse, particularly the fibre degrading bacteria is far less understood than that of the ruminant's compartmental stomach. Volatile fatty acids are absorbed across the hindgut wall, transported by blood into different tissues and used as an energy source. Horses are less efficient than ruminants in the digestion of fibre and therefore have a lower survival rate under severe drought conditions than ruminants. In addition to volatile fatty acids, a large volume of gases is produced and removed dorsally. Under normal feeding conditions, horses spend approximately10-12 hours eating a day. Such feeding patterns allow them to maintain a full stomach and continuous supply of nutrients both to the host animal and the microbial community residing the hindgut. This feeding behaviour will help the horse to overcome the problem of having a relatively small stomach compared with other herbivore species of a similar body size such as the cow. The feeding behaviour of this kind is important with respect to health and meeting their nutritional needs, especially energy. In contrast, racehorses are fed grain rich diets twice daily and withheld from feed for extended periods before exercise. The increase in gastric acid production during exercise, the reduction in saliva production due to low fibre content of the diet and periods of feed deprivation between meals lead to prolonged periods of exposure of the unprotected non-glandular region of the stomach to acid. This combined with typical indoor confinement and the stress of intense exercise is the likely cause of stomach ulcers in race and performance horses. All of these conditions currently occur in the performance horse. The ingestion of high starch or non structural carbohydrate rich diets is also associated with diseases such as fermentative acidosis, Equine Metabolic Syndrome, Equine Cushing's disease, laminitis and colic. Maintaining health of horses under conditions of concentrate feeding is a real challenge facing nutritionists, veterinarians and owners. Such challenge requires better understanding of the anatomical, physiological and functional features of the horse gastrointestinal tract. Understanding the complexity of the microbial ecosystem, interaction among the vast numbers and diverse microbial community, between microbes and the host animal, and the dietary influences on the microbes are equally important. This paper will focus on the feeding conditions that maintain good health and dietary changes associated with diseases mainly of the intensively managed horses such as fermentative acidosis, laminitis and colic.
Horses are best described as hindgut fermenters with an enlarged cecum and colon that harbors a complex microbial community. These microbes contribute to the digestion processes, which enable the horse to extract energy from dietary components that otherwise would be wasted. Horses are subject to nutritional disorders and diseases common to intensively managed animals such as acidosis, colic, and laminitis. The digestive system of the horse has special anatomical and physiological features that need to be considered when deciding what to feed horses. Improved pastures that have been developed to maximize microbial protein synthesis in the rumen of cattle and sheep (Merry et al. 2006) and milk yield (Miller et al. 2001) may be deleterious to horses due to their high content of non structural carbohydrates and crude protein. The effect of improved pastures may be similar to that experienced by grain or concentrate feeding for performance or racehorses (Longland and Byrd 2006).
The large intestine consists of the cecum, the colon and the rectum, which harbor a complex microbial population. The conditions of the large intestine match the description of a continuous chemical reactor, where the input of nutrients is continuous, mixing is adequate, products are removed and reactants are in contact with enzymes, whether these enzymes are produced by the animal body or the microbes. Mixing of contents allows control over the temperature, pH and the removal of gasses. The large intestine comprises approximately 64% of the volume of the equine GI tract, but only 30% of its total length (Kohnke et al., 1999). It is a voluminous but short organ, holding approximately 95 to 112L of liquid, approximately the size of the compartmental stomach of an adult cow, and similar to the rumen in having a fermentative microbial community inhabiting the organ. The walls of the large intestine lack villi, and the epithelial cells lack microvilli. However, they contain glands for secreting mucus. The walls of the large intestine also contain longitudinal and circular muscle fibers. Unlike other herbivores the small intestine in the horse opens directly into the cecum, the first segment of the large intestine, through a muscular valve. Relatively close to this is a second valve where digesta passes from the cecum and into the right ventral colon.
The cecum is a large blind-ending, comma-shaped structure with a capacity of between 25-35L and of approximately 1m in length. It extends from the pelvic inlet to the abdominal floor within the right flank area of the horse. The base of the cecum extends forward to lie under the cover of the last few ribs, with its tip just behind the diaphragm. The large intestine in the horse does not have a longitudinal muscle layer. Instead, it has bands of muscle that follow the length of the cecum, called taenia, which gather the caecal wall into sacculations called haustra. The haustra acts like buckets, delay the transport of chyme through the large intestine and contribute to the mixing of the colonic contents.
In the horse the ascending colon is called the large colon, which consists of a ventral colon and a dorsal colon (Reece 1997). The first segment of the colon, the ventral colon, can hold a volume of approximately 150 litres, has a diameter of 250-300mm and is about 2-4 meters long. The transition between the ventral colon and dorsal colon is narrow. This narrow segment will delay the transport of large particles from the ventral to the dorsal colon, increase their retention time and increase fermentability. The diameter varies significantly between regions however it is largest in the right dorsal colon, forming sacculations with a diameter of up to 500mm. The large colon is folded together forming a double loop consisting of the right and left ventral colons and the left and right dorsal colons. The four parts of the large colon are connected by structures called the sternal, pelvic and diaphragmatic flexures. Their significance probably lies in differences in microbial populations and functions between the segments (Frape 1998). The small or descending colon joins the dorsal colon to the rectum and is approximately 3m in length. The rectum is approximately 30 cm long and is the continuation of the small colon, into the pelvic inlet where it ends at the anus. Due to the size, location and importance in digestion, the equine large intestine may be referred to as the hindgut.
The microbial community of the equine gastro-intestinal tract has received very little attention despite its importance to the health of the animal and digestion of feed. Available information has dealt mainly with fermentation processes in the hindgut, especially those related to fibre digestion (Lin and Stahl, 1995; Julliand et al., 1999; Daly et al., 2001, Al Jassim et al. 2005a), lactic acidosis (Al Jassim and Rowe, 1999, Al Jassim et al. 2005b) and laminitis (Milinovich et al. 2006, 2007). Early work by Mackie and Wilkins (1988), reported a higher bacterial population in the distal parts of the GI tract from the duodenum to the colon. Horses that were fed a grass diet had a bacterial population of 2.9×106 per g wet weight in the duodenum, 29.0×106 in the jejunum, 38.4×106 in the ileum, 2.05×109 in the cecum and 1.26×109 in the colon. In their study Mackie and Wilkins, (1988) reported a relationship between mucosal and luminal counts. Bacteria associated with the mucosa were 73, 22, and 24% of the luminal counts in the duodenum, jejunum and ileum. Using similar culturing techniques we often obtain counts for total anaerobes around 4.3×108 in the stomach, 4.2×108 in the cecum, 1.0×108 in the proximal colon, 6.7×107 in the distal colon and 1.7×108 in the rectum. Early work focused on bacterial groups and relied mainly on culture dependent methods that had their limitations. However, molecular techniques have revealed the bacterial diversity within the equine large intestine (Daly et al. 2001, Al Jassim et al. 2005b) and the stomach (Al Jassim et al. 2006).
Many bacteria species in the hindgut produce D-Lactic acidosis in response to carbohydrates. Most of the work in the past focused on S.bovis and S.equinus due to their established role in acid build up and their involvement in the sequence of events leading to the development of laminitis. In recent work Milinovich et al. (2007) monitored changes in the bacterial community of the cecum of horses using the florescence in situ hybridization (FISH) technique. Laminitis was induced in all horses following oral administration of oligofructose at a relatively high rate of 10g per kg BW. All horses lost their appetite by 8 hr post oligofructose administration (POA) and developed profuse, watery diarrhea by 16h. Clinical signs of laminitis were observed in these horses (e.g. lameness and shifting of weight from one foot to another) from 20 hours POA onwards. The population of the Gram positive bacteria changed from <20% at 0 time to >70% between 4 and 8h POA and remained at high level up to 32h POA. This was confirmed by FISH results which showed the relative abundance of S.bovis /S.equinus to be >50% by 16h POA in all samples. These results confirm the association of S.bovis/S.equinus with acid build up and sequential events leading to the development of laminitis in horses.
Under normal feeding conditions horses consume diets consisting mainly of cell wall polysaccharides including cellulose, hemicellulose and pectin, the main structural constituents of the plant cell wall. Such diets are often bulky and low in energy content. In order for the horses to meet their requirements for energy and other nutrients they graze continually and spend approximately 10-12 hours eating during a 24-hour period, in sessions of 0.5 to 3 hours under free range conditions (Sjaastad et al. 2003). Another report noted that a free range horse spend 16-18 hours or more out of 24-hours searching for and selecting feed to meet their requirements (Kohnke et al. 1999). The fact that horses feed continuously, ensures they maintain a full stomach and continuous buffering from saliva protects the non-glandular stomach from acid injuries.
The digestive processes begin with acid hydrolysis in the stomach and enzymatic digestion by the host in the small intestine followed by extensive fermentation in the cecum and colon.
In horses fed grass or hay, fermentation of structural carbohydrates by the action of microbial enzymes in the cecum and the colon produces VFA, which are readily absorbed and utilized as an energy source. The contribution of VFA to the energy requirements of the horse could be as high as 70-80%. Under these conditions the large intestine's environment remains buffered and supports the proliferation of major groups of the microbial community. One disadvantage of the system is that considerable amounts of fermentation products such as vitamins and amino acids are retained by the microbes, hence these products are not utilized by the horse and appear in the feces. One way for the horse to benefit from products that are retained by bacteria is to consume its own feces. The incidence of coprophagy appears to be related to the quality of the horse's diet. In the other hand, feeding starch rich diets, which is particularly a problem in intensively managed racehorses, disrupts the microbial ecosystem of the GI tract, particularly the large intestine.
Similar condition may results from feeding grasses rich in the oligosaccharides fructans. Fructans are polymers of fructose molecules and there are 3 types of fructans present in temperate grasses as follows: Inulin, a linear fructans generally linked by β (2→1) glycosidic bonds; Levan, linear fructans generally linked by β (2→6) glycosidic bonds; and Graminan, branched fructans linked by both β (2→1) and β (2→6) glycosidic bonds (Chatterton and Harrison 1997). Accumulation of fructans in grasses is influenced by grass type, environmental conditions, and stage of maturity (Chatterton et al. 2006). Although, fructans are water soluble the horse does not produce enzymes capable of digesting them and thus they pass through the small intestine and enter the hind gut of the horse where they undergo fermentation process by the hind gut microbes. Excessive intake of fructans can lead to acid build up and metabolic disorders similar to that experienced under grain overload conditions (Longland and Byrd 2006). However, little is known about the interaction between the different bacterial species and the level of intake of fructans.
Dietary supplementation is essential to maintain good health and to satisfy the nutritional needs for growth and performance of various equine classes. Grain and mixed feeds contain large amounts of non structural carbohydrates mainly as starch. The choice of cereal grain is based on its safety, palatability, energy value and cost. Oat grain is palatable and relatively safe because it is lower in energy density and higher in fibre than other grains.
Starch is not well digested in the small intestine of the horse and variable proportions of ingested starch that escape enzymatic digestion in the small intestine enter the large intestine and are rapidly fermented, increasing VFA and lactate production (Figures 2, 3) and lowering the pH. An acidic environment favors the rapid proliferation of lactic acid producing bacteria (LAB), resulting in increased lactic acid production and a further decline in the pH. Lactic acidosis is often accompanied by laminitis.
Accumulation of lactic acid in the hind gut of the horse results from the rapid fermentation by bacteria of starch that enters the cecum and colon. This occurs when the supply of starch by the diet exceeds the digestive capacity of the small intestine. The upper limit of 3.5–4.0 g starch per kg BW per meal was earlier suggested by Potter et al. (1992), but studies on variations in the digestion of starch with increasing starch intake (Potter et al. 1992; Kienzle, 1994) suggest that the upper limit is less than 3.5. Differences can partly be explained by the source of starch. An increase in the population of lactic acid producing and utilizing bacteria and a decrease in concentration of cellulolytic bacteria in the caecal contents of horses were measured in horses fed high starch diet supplying 3.4 g starch per kg BW per meal (Medina et al., 2002). The lowest pH and highest lactic acid values were measured at 5 and 6 h post-feeding in the cecum (6.45; 532 mg/L) and the colon (6.38; 582 mg/L), respectively. A recent report by Hussein et al. (2004) showed a decrease in fecal pH level from 7.04 to an average of 6.74 and increase in the concentration of lactate in horses fed Alfalfa cubes supplemented with different grains (e.g. barley, corn, naked oats, and oats). Lactate concentration was notably high for the barley supplemented diet (2.82 μmol/g versus 0.96μmol/g of DM). This supports earlier results reported by Meyer et al. (1993), which showed a very low preileal digestibility for rolled barley starch in horses (e.g. 0.21). The source of starch and the processing method of the cereal grain were also emphasized by McLean et al. (2000) who observed unfavorable intra-caecal fermentation changes in ponies fed hay and rolled barley diet (50:50) that was formulated to supply 2.1 g per kg BW per meal. These unfavorable changes were not observed with micronized or extruded barley. Processing of grain to improve the extent of enzymatic hydrolysis in the small intestine should reduce the amount of starch reaching the large intestine (Meyer et al., 1993). However, work by de Fombelle et al. (2003) indicated the presence of significant numbers of lactic acid producing and utilizing bacteria in the horse stomach suggests that such grain processing might have the undesirable effect of increasing starch fermentation, and hence lactic acid production in the stomach. Such a situation could contribute to lactic academia, a condition associated with laminitis (Garner et al., 1977; Rowe et al., 1994; Pollitt and Davies, 1998). Fermentation in the stomach was also shown to be extensive under concentrate feeding conditions, particularly in the non-glandular region of the stomach (NG-S). Horses fed 4 kg grass hay and 4 kg dry-rolled or steam-flaked sorghum per head per day, had a lactic acid concentration in the NG-S greater than 40mmol/L (Al Jassim, 2006). Another important feature of the fermentation process in the stomach is the high proportion of the D-lactate isomer because in other anaerobic fermentations, the L-isomer is the more prominent. In addition to the two isomers of lactate, which are the main products, small amounts of VFA are also produced. The study also showed that lactic acid is absorbed from the gastrointestinal tract before the digesta reaches the cecum.
It is widely acknowledged by veterinarians, horse owners and keepers that colic is a serious medical problem that affects horses worldwide and could be life threatening. Among the several potential causes of the problem: internal parasites, change of diet, dietary management and housing conditions of the horse, lack of access to pasture and water, increasing exercise and transport are the most common (Archer and Proudman, 2006). Factors that cause rapid fermentation and excessive build up of gasses could lead to distention of the digestive tract and consequently cause colic. Inadequate supply of dietary fibre and high concentrate/less frequent feeding are man-made problems of modern horse husbandry systems. Bad teeth can also contribute to colic through ineffective chewing and ingestion of long feed particles, which could cause disruption of the digesta passage.
The effect of seasons has also been documented, with the equine grass sickness, which is considered as acute colic, being a good example. An increase in the incidence of colic during the months of spring and autumn were reported in the UK Proudman (1991). Spring pastures are known to be rich in soluble carbohydrates, non protein nitrogenous compounds and sometimes have unbalanced mineral contents. If recovered in the hindgut, soluble carbohydrates will undergo rapid fermentation, which leads to a buildup of gases and acids. Little is known about the link between green spring pastures and the incidence of colic and there is a real need to investigate this experimentally. Most of the available information is derived from epidemiological studies based on statistical analysis of veterinary records that often reference nutrition. Although the majority of reports emphasize the dietary factor and its implication in the development of colic, these reports lack solid experimental evidence derived from well-defined nutritional trials with controls. As a result these remain as speculations that need to be verified.
It is important to note here that the advancements in veterinary care that led to the development of more effective parasitic control methods (Nieto, 2006), have not been matched achievements in the nutrition of the horse to reduce the risk of modern feeding of the captive and domesticated horse.
A review of the relevant literature on the incidence of colic by Archer and Proudman (2006) showed that colic episodes could be as high as 10.7% per year and vary considerably within any horse population. They also indicated that the majority of cases (69-72%) were of the spasmodic/gas colic or colic of unknown nature and only a small portion of these cases (7-9%) required surgery. In the light of the available literature and apart from the well documented parasitic causes, all other factors are observation based and not experimentally confirmed. Interesting evidence from studies on horses diagnosed with acute equine grass sickness in the UK suggested that toxin produced by the bacterium Clostridium botulinum type C is involved (McCarthy et al. 2004). This bacterium is soil-born, Gram positive and spore forming that is capable of surviving the acidic conditions of the stomach by producing spores that, when leaving the stomach, vegetate and colonize the intestine (Al Jassim et al. 2006). Under favorable conditions that could be brought up by abrupt dietary changes, this bacterium becomes one of the predominant bacteria; it establishes its population and starts producing toxins. The type C complex toxoid toxin produced by this bacterium, affects the nervous system of the horse and negatively impacts on the motility of the GI tract causing intestinal dysfunction. This is an important development that emphasizes the importance of a well balanced microbial ecosystem of the GI tract of the horse and the need to adhere to the belief of gradually introducing horses to new types or amounts of feed. Gradual introduction will allow the non-pathogenic commensal bacteria to adjust to the new environment and establish a new balance. A balanced bacterial community helps to encounter pathogenic bacteria such as C. botulinum from colonizing the intestine and establishing an effective population.
Horses are important herbivore animals that have a unique gastrointestinal tract most suited for the digestion and utilization of fibre rich diets. In order to maintain health and derive adequate nutrients to meet their requirements, horses need to graze continually and feed on pastures that are not too rich in non-structural carbohydrates and water soluble carbohydrates that ferment in the GI tract, alter the conditions, and adversely affect the balanced microbial population. Feeding concentrate diets high in energy in less frequent meals is a practice that leads to complications and diseases such as acidosis, laminitis, colic and stomach ulcers. Just like other living things, violation the laws of nature that are a products of a complex and a very long evolution process will inevitably leads to complications and problems that may not be easily solved.