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Butyrate is key in supporting intestinal cell health whereas acetate and propionate are energy precursors (Geor 2013). The intensity of these processes depends on the amount and temporal influx of fermentable material arriving from the small intestine (Hintz et al. 1971; Argenzio et al. 1974; Glinsky et al. 1976; Clarke et al. 1988; Merritt and Julliand 2013). Increasing proportions of nonstructural carbohydrates (NSC: starch and water soluble sugars) favour the production of lactate and propionate at the expense of acetate. These volatile fatty acids are absorbed by nonionic passive diffusion and a monocarboxylate/H+ symporter in association with NaCl absorption and H2CO3 secretion; water then passively follows the VFA and Na absorption (Shirazy-Beechey 2008; Nedjadi et al. 2014). The ileocaecal flow of dry matter, its composition and the rate of production and absorption of organic acids have a marked effect on water balance in the hindgut and consequently plasma volume through the direction of the transmucosal movement of water (Clarke et al. 1988, 1990a, b; Merritt and Julliand 2013). Ingestion of excessive levels of starch may exceed the
relatively limited amylolytic capacity of the equine foregut and the undigested starch, together with any resistant starch, will pass into the large intestine where it will be fermented, yielding less net energy than if absorbed as glucose (Geor 2013). Several studies have reported decreased caecal and/ or colonic pH in horses or ponies fed high starch diets (e.g. Willard et al. 1977; Radicke et al. 1991; Medina et al. 2002). Intracaecal lactate was significantly higher when rolled barley as opposed to micronised or extruded barley was fed to ponies as 50% of the ration (McLean et al. 2000). Dietary composition will also influence the composition of the large intestinal microbiota significantly (Medina et al. 2002; Merritt and Julliand 2013; Dougal et al. 2014). The small core microfloral population in the horse is thought to increase the risk of digestive upsets with dietary change (Dougal et al. 2013, 2014). The contribution of hindgut protein to the amino acid
pool of the horse is a matter of some controversy (Woodward et al. 2010; Merritt and Julliand 2013) and currently precaecal protein availability is considered to be key. The majority of minerals and trace elements given to
horses are absorbed in the SI, as are most of the dietary vitamins (Coenen 2013; Zeyner and Harris 2013). Phosphorus, however, is absorbed mainly in the hindgut and high phosphorus intakes, especially in the form of plant phytates, may interfere with calcium absorption in the SI. Most of the water soluble B vitamins are synthesised in the large intestine of a healthy horse through the action of resident microflora (Zeyner and Harris 2013).
What does this mean in practice for veterinarians?
Translating the science of digestion outlined above together with knowledge of feeds/nutrients and their interactions is key in developing rations that support optimal health and performance. Nutritional advice is available for horse owners from various sources, but in particular from within the commercial feed sector. However, many owners seek the advice of their veterinarian to present an independent viewpoint within a clinical context. In this section of the article, the authors hope to offer some essential practical information to help the veterinarian support their clients with
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nutrition related issues. Obviously clients are more likely to seek nutrition advice from their veterinarian where a perceived problem exists, which may vary from a simple ‘failure to thrive’, through to loss of performance, or may encompass a specific clinical issue. Regardless, an in context critical evaluation of the existing components of the diet is essential (Dunnett 2013). Whilst all aspects that should be considered cannot be covered in this article, a few key areas will be discussed.
Forage
Forage/roughages are high fibre (structural carbohydrate) feedstuffs and should be the foundation of all horse diets. For many horses no other energy providing feedstuff will be required, even for some high performance horses (Harris and Geor 2014). However, as they are typically not nutritionally complete, additional vitamins, minerals and trace elements will be required to be fed, plus for most nonlegume forages additional amino acids in order to provide an optimal intake of all nutrients (i.e. some form of a forage balancer see below). Practically it is important to check the actual weight and type of forage offered and consumed daily, as this can often be severely over or underestimated by owners or trainers.
How much is enough? Forage intake is important not only for its nutritional contribution to the diet but also its role in gastrointestinal health and species specific intake behaviour i.e. free-ranging horses perform 10–15
individual feed bouts/day and forage for 12.5 2.5 h/day (Ellis 2010). In the literature, the recommended quantity of forage for a variety of circumstances is commonly described on a dry matter intake basis due to the low fibre content of some very immature hays (especially alfalfa) and haylages/silages. Recent guidelines (Dugdale et al. 2010; Harris et al. 2013; Harris and Geor 2014) suggest that to ensure gastrointestinal and behavioural health, under most circumstances forage (grass and preserved forages) should be provided at a minimum of 15 g DM/kg bwt/day (and more ideally at 20 g DM/kg bwt/ day), even for horses with high energy requirements (in which case young, less mature high energy content forages should be considered). The minimal level suggested for animals undergoing restriction for weight loss purposes under veterinary supervision is 10 g DM/kg bwt/day and a suggested target minimum for elite level high intensity exercising athletic horses is 12.5 g DM/kg bwt/day (1.25% bwt/day). To facilitate this, ideally all forage should be sold with a basic analysis (including DM, protein and nonstructural carbohydrate [NSC]). In addition, owners/trainers, especially in large establishments, should be encouraged to routinely analyse any home produced forage so that their diet can be appropriately balanced for their forage (Table 2). However, in practice, the majority of owners do not have a detailed analysis of their forage and therefore simplified guidance can be useful based on an ‘as fed’ basis for the average hay or haylage (partially wilted grass preserved utilising airtight conditions, but often without significant lactic acid fermentation and therefore little/variable reduction in water soluble carbohydrate [WSC] content from that of the parent grass) (Table 1). However, it needs to be appreciated that there will be variance due to differences in actual dry matter. An indication of the energy value of forage can be estimated according to its maturity, grass species etc., as well as the bodyweight and condition of the individual animal it is
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