15 Oct 2018
Jamie Prutton
Job Title
Image: pimmimemom / Adobe Stock
Equine nutritional advice is a topic often marred by a number of preconceptions and misunderstandings, and even defining a horse as obese, underweight or appropriate can be difficult.
A gradual trend has seen the overweight horse being considered normal when, in fact, as with the human population, this is putting greater strain on the animal’s musculoskeletal and endocrinological systems, with a resultant increased requirement for veterinary intervention. The majority of this article is going to look at controlling weight in the obese horse – the most frequent problem faced by vets – but underweight and geriatric patients and how to maintain them will also be covered.
The equine patient poses a relatively unique nutritional requirement. As a hindgut fermenting, primarily grazing herbivore, the horse is able to use a number of plant species, which are fermented in the hindgut to produce energy and the nutritional necessities for homeostasis. A good understanding of the principles involved in equine nutrition is essential to guiding owners and nursing teams in clinical cases.
As a grazing animal, the horse is designed to be consuming feed throughout most of the day and will forage for 12.5h/day ± 2.5h/day (Ellis et al, 2010) – doing so produces large amounts of saliva, which is basic with a pH of 8.6 to 9.1 (Meyer, 1985). This plays a pivotal role in lubrication and some digestion (although this is limited in equids), but also acts as a buffer for gastric fluids. This, along with the relatively delayed emptying up of the stomach (85 to 300 minutes), reduces the risk of gastric ulcers.
For the hindgut to function normally, the horse requires a diverse bacterial community, not only in the caecum and colon, but also in the stomach to start the digestive processes. The small intestine will start the starch digestion, but a limited concentration of amylase and trypsinogen exists in pancreatic juices; therefore, if a horse is fed a starch heavy diet (more than 1g starch/kg bodyweight/meal) a higher concentration of starches will reach the caecum and alter its pH negatively.
The hindgut itself does not possess mucosal enzymes and, therefore, is completely reliant on the microbial population for digestion. It is also unable to absorb significant amounts of hexose sugars and requires the end products of bacterial fermentation for energy. Increasing the amount of non-structural carbohydrates (NSCs) will lead to an increased production of lactate and propionate at the expense of acetate production. This shift leads to increased absorption of NaCl and loss of H2CO3, leading to a shift in the water balance and pH of the hindgut and plasma.
Any change in this micriobiota – be it in the stomach, small intestine or large intestine – can lead to digestive upsets and an increased risk of diarrhoea. Therefore, trying to ameliorate this risk is often key to diet changes and plans.
Forage is the backbone of nutrition in the horse and will often satisfy all energy requirements, but can be lacking in some vitamins, minerals and trace elements, dependent on the quality of the forage.
In some cases, these deficits can be easily resolved with the addition of appropriate supplements. Ideally, all forage should be analysed for nutritional content, and feed given to the horse should be weighed beforehand to ensure accurate feeding is achieved. These additions can easily be supplied with a balancer that is low in carbohydrates, therefore not increasing the NSC intake of the horse, but it is important to assess any balancer prior to administration to ensure it is appropriate.
NSCs – a combination of starch and water-soluble carbohydrates (WSC; simple sugars and fructans) – are an essential part of a forage analysis, particularly when feeding the laminitic patient. The recommended NSC intake is less than 2g/kg bodyweight/day for most horses (Harris et al, 2013), while a strict protocol of NSC less than 10% should be present in the forage of laminitic horses. When considering supplements they should be assessed critically for the NSC content, as they can range up to 35%, which will increase the risk of an insulin spike and laminitis risk.
Most horses, even with increased energy demands, would require 1.5% to 2% dry matter (DM)/kg bodyweight/day. Most recommendations are as DM to try to reduce the variation between hay, haylage and so on. At a basic level, the absolute minimum a horse can be fed under dieting conditions is 1% DM/kg bodyweight/day, but this should only be done under veterinary supervision. When under strict restriction of feed, especially if the forage is being soaked, a balancer must be added to the diet to ensure the horse does not become deficient in any essential minerals/vitamins. This can normally be provided at the recommended amount per product, but will often be around 100g/100kg bodyweight/day.
Soaking of hay, especially in temperatures of more than 16°C, will reduce the WSC content (Longland et al, 2014), but it is important to note the exact reduction in NSC is variable and this technique should only be used as an adjunct to an appropriate diet.
Hay hygiene can play an important role as it can reduce palatability and total DM intake in the horse. More importantly, contamination can occur with various toxic plants or mycotoxins secondary to fungal growth. The exact pathogenesis of many of the mycotoxins is unknown, but research showed an association between fumonisin B1 and liver pathology. Therefore, if any concern exists regarding the hygiene of the hay, it should not be fed and another source found.
The supplement and nutraceutical industry is enormous and often difficult to get to grips with. As mentioned, most horses do not need much more than their forage source and a basic balancer, but supplements can be added for a number of reasons.
Whenever considering if a horse requires the supplement and whether it is appropriate some basic questions should be asked, including:
Some examples of where these questions easily fall include the likes of the efficacy of products such as glucosamine and chondroitin, the bioavailability of different preparations of vitamin E and the negative impact of Vitex agnus-castus on insulin regulation when it is recommended for patients with pituitary pars intermedia dysfunction (PPID) and equine metabolic syndrome (EMS).
With laminitis having a proposed incidence of between 1.5% to 34% in the general equine population, reducing its incidence via appropriate management is integral to equine health.
Laminitis has been associated with prolonged, abnormal elevations in insulin following ingestion of feeds with elevated concentrations of NSCs. Concurrently, the number of obese equids with an accompanying insulin dysregulation (ID) has increased. The combination of risk factors for laminitis lead to EMS, which encapsulates the aforementioned metabolic abnormalities, as well as changes in adipokine production.
Diet, therefore, plays a significant role in reducing the risk of hyperinsulinaemia and subsequently can reduce the risk of laminitis. Bodyweight management is the cornerstone to controlling obesity and EMS. Ideally, horses should be weighed on a regular basis, but this is unlikely to happen in reality. Instead, training the owners to measure the body condition score and aiming at five out of nine can be a useful guide. Owners should be alerted to the fact a BCS is not an accurate reflection of complete obesity values, nor are weight tapes.
To reduce weight we must induce a negative energy balance and the first thing to be done is to remove the horse from pasture. It is impossible to know the amount of grass a horse will eat during a day and also the NSC content of that grass. During dieting there is no reason to give the horse anything but forage and a balancer – unless medications are required, in which case a low NSC feed could be given.
As a starting point, the NSC content of hay should be less than 12%, but can be lower than 10% in some cases. As mentioned previously, soaking can reduce the NSC and be used as an adjunct to the main diet, but a balancer must be given throughout the diet period.
Initial feed amounts should be aimed at 1.5% bodyweight/day of DM, remembering that soaking hay will reduce the DM content of hay and, therefore, the forage should be weighed prior to soaking. The author found it beneficial to weigh the horse’s feed prior to changes as if the drop in feed is dramatic (more than 1%), then the horses will infrequently tolerate this change. Ideally, the diet should lead to a 0.5% to 1% drop in bodyweight weekly, but if this is not being achieved then the feed ration can be dropped further to 1.2% bodyweight/day of DM. Further reductions should only be undertaken under the close supervision of the vet.
Although exercise has not been shown to improve insulin dysregulation directly, it will help with weight loss and, therefore, should be undertaken as much as possible. Frequently, the cases are laminitic, so this is not appropriate. Horses can be turned if there is a school or woodchip/dirt enclosure with no grass.
If removal from pasture is not possible, or the owner unwilling to do it, grazing muzzles can reduce intake dramatically. Care should be taken when using these to ensure the horse is able to drink and the sward length is long enough to enter through the muzzle. Owners should also be warned of the risks of rubbing and appropriate fitting of the muzzle.
The geriatric patient provides a whole different combination of problems, including difficulty in maintaining muscle mass, weight, poor mastication and prehension. Most studies have found very little difference between older (more than 20 years old) and younger horses’ ability to absorb nutrients.
Most owner questionnaire research has found the percentage of older horses within a good bodyweight did not differ compared to younger horses, but 17% of owners felt their geriatric horses had lost some condition (Ireland et al, 2011). Therefore, any sudden weight loss should be fully investigated, as it might be caused by an underlying medical, dental or social problem.
Assuming no underlying medical issues are present, dentition should be the first concern. If the horse is found to have expiring or no teeth, a transition should be made from long fibre feeds (such as hay) to short fibre or pelleted feeds. With multiple complete pelleted feeds available, these can offer an excellent way to maintain the geriatric patient, but come at a cost, both financially and in time. Soaking of these feeds will increase the volume the horse has to eat, therefore increasing consumption time and water intake.
Feeding the gastric ulcer patient is fraught with complications as a number of studies have shown different outcomes. The often-held belief that increased turn out and fibre-based food will reduce the risk of gastric ulcers is not scientifically clear cut. That said, the use of increased amounts of starch and infrequent meals did increase the likelihood of gastric ulceration.
Significantly reduced gastric acid output, alongside increased prostaglandin E2 concentrations, was noted following administration of 45ml of corn oil by mouth, which could aid in mucosal health (Cargile et al, 2004). This could not be confirmed with repeat studies looking at various oils. The author generally recommends the use of corn oil in gastric ulcer cases, unless a concern about obesity exists.
Considering the aforementioned fact, nutrition recommendations are difficult and, as outlined in the European College of Equine Internal Medicine consensus statement (Sykes et al, 2015), basic management should include:
The microbiota of the equid patient is highly complex and unique in each horse. The predominant phylum in horse faeces in Firmicutes with lower numbers of Bacteroidetes, Proteobacteria, Verrucomicrobia, Actinobacteria and Spirochaetes. A shift has been seen in horses with gastrointestinal disease away from a predominance of Actinobacteria and Spirochaetes to Fusobacteria (Costa et al, 2012).
No definitive protocol exists to guide the vet and owner on how to institute feed changes. Sensible advice is to change the forage source over approximately two weeks with a 25% change in the feed every four days. An example would be, on days 0 to 4, 75% old hay and 25% new hay, then gradually altering these every 4 days. Any supplementary feed should also be gradually increased as not to overburden the gastrointestinal tract.
Feeding the equine patient is often complicated, not by the horse, but by the myriad of available supplement, forages and feed options. Frequently, the author will remove all supplements from a horse’s diet and only gradually add those back deemed essential by either the attending vet or the owner.
The use of a weigh scale cannot be overemphasised. Frequently, horses will receive a ”section” of hay, but with no quality control it is impossible to know its weight. Therefore, all feeds should be weighed out prior to feeding the horse.
