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Julliand 2013). Meal size and composition also influence the rate of gastric emptying: in general, although more work is needed in this area, liquids empty more rapidly than solids, larger meals have a greater rate of emptying than smaller meals and the higher the starch and oil content the longer the T1/2 value regardless of meal size (Geor et al. 2001; Wyse et al. 2001; Sutton et al. 2003; Metayer et al. 2004). There is a large diverse bacterial community present in the stomach even of fasted animals (Al Jassim et al. 2005; Varloud 2006, Varloud et al. 2007), although their main influence appears to be on starch digestion due to the high proportion of amylolytic bacteria; lactic acid then acetate being the main end products (Varloud et al. 2007; Merritt and Julliand 2013). The variation in dry matter, pH gradient, etc. obviously influences the intensity and effectiveness of intragastric fermentation. Pepsin and lipase are secreted by the stomach, but there is no information as to their specific role in equine digestion, although they are potentially important to protein and fatty acid digestion (Merritt and Julliand 2013). It is also possible that fructan can either be hydrolysed to some extent by gastric acid (Van Soest 1994; Ince et al. 2014) or fermented in the stomach. Whilst treatments are available to reduce gastric acid
output and promote the healing of squamous and glandular gastric disease, in particular the proton pump inhibitor omeprazole (Sykes et al. 2015), the use of such agents may not be without risk as there are potential adverse effects of gastric acid suppression, for example, on barrier function against bacterial colonisation (Javsicas and Sanchez 2009), as well as calcium balance and fracture risk. A recent study (Caston et al. 2015), however, showed no significant effect in horses of 60 days of once a day compounded omeprazole administration on total/ionised serum calcium, bone density or bone mineral content (measured using peripheral quantitative computed tomography). Longer term effects as well as effects of potentially more prolonged/pronounced gastric acid suppression have not been evaluated. Although significantly lower gastric acid output and
increased gastric juice prostaglandin E2 concentrations were reported when ponies were given 45 mL corn oil per os once a day (Cargile et al. 2004); corn oil, refined rice bran oil and crude rice bran oil (240 mL, once daily, mixed in grain) failed to improve squamous ulcer scores under experimental squamous ulcer inducing conditions in adult mares (Frank et al. 2005).
Small intestine Relative to its body size the horse has a rather short small intestine and the rate of passage of ingesta is therefore relatively quick and is influenced by the amount of food entering. The positioning of the bile and primary pancreatic duct means that there is an increased chance of refluxing back into the stomach, especially an empty stomach (Kitchen et al. 2000) although any role of bile acids in gastric ulceration has not been elucidated (Widenhouse et al. 2002). Due to the absence of a gall bladder, bile is continually produced and bile salt excretion, important for fat digestion, depends on an intact enterohepatic circulation. Pancreatic secretion, (~pH 8 due to high bicarbonate content) is also apparently continuous (up to 25 L/day in an average sized horse [Kitchen et al. 2000; ]). The basic digestive processes are similar to those of other monogastric animals with dietary nonstructural carbohydrates and proteins being hydrolysed
by the pancreatic and intestinal brush border enzymes (Kienzle and Radicke 1993; Dyer et al. 2002, 2009; Richards et al. 2004; Merritt and Julliand 2013). The transport mechanisms for glucose and fructose are also believed to be similar to those in other species, although the equid small intestine has a limited capacity to transport glucose compared with an omnivorous mammal such as the pig (Merediz et al. 2004; Woodward et al. 2013). Less is known about protein and fat assimilation (Cehak et al. 2013), although interestingly equine pancreatic tissue contains relatively high concentrations of lipase (Lorenzo-Figueras et al. 2007). The concentrations of amylase and trypsinogen within the
pancreatic juice are, however, very low compared with other species (Kienzle et al. 1994). The horse, therefore, has a limited capacity to digest starch in the small intestine (due to digestive enzyme activity, transit time etc.). The exact limit does seem to vary with the individual and although feeding high starch diets may result in increased levels of amylase, the increases are not marked and do take time (Kienzle et al. 1994; Dyer et al. 2009). The amount of starch fed in a meal will, therefore, influence precaecal starch digestion leading to recommendations that intakes of not more than 2 g and more recently 1 g starch/kg bwt/meal are fed (Potter et al. 1992; de Fombelle et al. 2004; Julliand et al. 2006; Harris et al. 2013). In addition, the extent of any nonstructural carbohydrate digestion in the small intestine will also depend on the feedstuff itself, availability to the digestive enzymes (e.g. the extent to which any outer husk or hull has been broken down), the ratio of amylose to amylopectin within the starch granule, nature of the starch granule as well as the effect of any processing, plus type and amount of forage in the diet (Meyer et al. 1993, 1995; Kienzle 1994; Kienzle et al. 1997, 1998; de Fombelle et al. 2004; Harris et al. 2005a,b; Julliand et al. 2006). Thermal treatment of certain grains (e.g. corn and barley) in order to swell and gelatinise the starch is necessary to improve foregut starch digestibility (Granfeldt et al. 1994; McLean et al. 2000; Julliand et al. 2006; Vervuert et al. 2007). The duodenal microbiota is largely composed of
Lactobacillus ssp. and Streptococcus spp. (Costa et al. 2015). The microbiota of the ileum is significantly less diverse than that of any region of the large intestine and compared with other compartments, has a greater relative abundance of Proteobacteria (Dougal et al. 2012), although the precise role of the microbiota in digestion within the small intestine is not clear (Merritt and Julliand 2013) it is likely they play a role.
Hindgut The large intestine does not have mucosal enzymes and does not have any significant active transport mechanisms for hexose sugars. Digestion and absorption of residual carbohydrates relies instead on microbial action and then absorption of the end products of microbial fermentation undertaken by a complex microbiota comprising mainly bacteria but also fungi and to a lesser extent protozoa (Daly and Shirazi-Beechey 2003; Dougal et al. 2014). Unlike the ruminant, this microbial fermentation obviously occurs after the ‘monogastric-like’ section rather than before, which has a
great impact on how we should feed a horse. There is intense fibrolytic activity in the hindgut producing short chain or volatile fatty acids (VFA), predominantly acetate, then propionate and then butyrate (de Fombelle et al. 2003).
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