486


EQUINE VETERINARY EDUCATION


Equine vet. Educ. (2018) 30 (9) 486-496 doi: 10.1111/eve.12657


Review Article Nutritional tips for veterinarians


P. Harris†* and C. Dunnett‡ †Equine Studies Group, WALTHAM Centre for Pet Nutrition, Melton Mowbray, Leicestershire; and ‡Independent Equine Nutrition, Bury St Edmunds, Suffolk, UK. *Corresponding author email: pat.harris@effem.com


Keywords: horse; digestion; forage; quality; analysis; supplements


Summary As a herd-living, nonruminant, hindgut fermenting, primarily grazing herbivore the horse has evolved with a specialised gastrointestinal tract capable of utilising a wide range of plant species, which are hydrolysed and/or fermented to yield energy and nutrients for bodily processes. Domestication has resulted in the horse often being fed and managed to suit human requirements rather than their own, which can lead to digestive, behavioural and clinical issues. Appropriate nutrition not only reduces the risk of a nutritional component being a limiting factor to performance, but it also supports the maintenance of health and appropriate behaviour. A basic understanding of the digestive processes therefore can be very valuable when deciding how to feed horses optimally for both health and activity. This article provides a general overview of the gastrointestinal tract from a nutritional perspective and points out a few key practical areas where knowledge of feeding practices can be helpful for veterinarians.


Introduction


Providing appropriate and adequate nutrition is a fundamental component of managing horses and ponies. For the veterinarian, knowledge of the key principles of equine nutrition is essential especially when dealing with clinical cases. This is particularly important where nutrition has been shown to have an impact on either the disease process or recovery. However, it is also important where veterinarians are involved with the management of horses at their different life stages and in particular when they are being used for competitive purposes. The veterinarian is also often called upon to provide advice on the feeding and management of horses in their care and in particular the use of specific ‘supplementary complementary’ feeds colloquially referred to as supplements. As discussed in Geor and Harris (2013), appropriate


nutrition and feeding management is both a science and an art. The science takes in feeding behaviour, the physiology of nutrient digestion and utilisation as well as the nutrient requirements of animals at various physiological stages and the composition, digestibility and safety of feedstuffs available for use in horse rations. The art recognises that there is considerable variability between horses and therefore there is not one single way to feed horses appropriately; feeding management needs to be tailored to the individual animal as well as the circumstances in which it is kept. This article is one in a series of articles dealing with


nutrition in clinical practice. It is not intended to provide a comprehensive review of equine nutrition, nor to discuss feed


© 2016 EVJ Ltd


requirements, as these areas are well covered by books such as the National Research Council's Nutrient Requirements for


Horses (Anon 2007; Geor and Harris 2013), but it will try to give firstly an overview of the gastrointestinal tract from a nutritional perspective and then secondly provide pointers on the key aspects that need to be considered in practice under field conditions. It will concentrate on the performance horse/pony although many of the principles will apply to all healthy animals as well as those with clinical issues.


Basic principles of gastrointestinal physiology as it influences nutritional practices


Mouth The mobile equine lips enable selective grazing and, in some horses, the ability to sift out small particles in feed such as vitamin and mineral pellets. Horses only produce saliva (which is >99% water) when they chew and it contains virtually no digestive enzymes (e.g. amylase 0.44 u/mL compared with 98 u/mL in pigs: Varloud 2006). Adult horses can secrete up to 35–40 L/day of saliva which has a pH of 8.6–9.1 and its role is primarily as a lubricant and a gastric buffer. The dry matter (DM) content of any swallowed bolus depends on the physical composition of the feed and in particular the time taken for adequate mastication (Meyer et al. 1985, 1986). As rate of intake depends in part on the density and ease of comminution of the feed (Bonin et al. 2007; Cuddeford 2013) horses take longer to chew roughage and therefore the DM of the swallowed bolus is much lower (~11–15% DM) than when, for example, a pelleted complementary feed is fed (~35% DM). The chewing cycle is complex (Bonin et al. 2007) and helps not only to reduce particle size, thereby supporting optimal digestion, but also to release soluble nutrient components that can then be digested precaecally (Merritt and Julliand 2013).


Stomach The stomach of an adult 500 kg horse has a capacity of 8– 15 L (Nickel et al. 1979) and naturally, in a trickle feeder foraging for the majority of the day, would never be completely empty, with feed being retained for 85–300 min postprandially. Horses, however, secrete hydrochloric acid at a variable rate even when the stomach is empty (Merritt et al. 2003) which increases the risk of gastric ulceration in this species especially if not regularly provided with forage (Luthersson et al. 2009). The size and composition of a meal is thought to influence the amount of saliva produced, as well as the amount of gastric acid secreted plus the nature and stratification of the feed material within the stomach, thereby affecting the pH gradient within the stomach (Merritt and


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