The gastrointestinal tract – The key to feeding

Ask any zoo based nutritionist and they will tell you that understanding a particular animal’s gastrointestinal physiology is the key to understanding what and how to feed them. The shape, size and structure of an animal’s gut reflects what their natural diet consists of, and horses are no exception.

We know that horses are herbivores and we know that they graze for long periods each day if they have pasture available. You will also be familiar with the advice to feed ‘little and often’ and to base your horse’s diet on fibre. The reason why you should do these things lies in the way their gut is structured.

Monogastric Hindgut Fermenter

The horse can be classified as a monogastric (or single stomached) hindgut fermenter whose gastrointestinal tract consists of the mouth, stomach, small intestine, caecum, large colon, small colon and rectum (Figure 1).

Figure 1: The structure of the gastrointestinal tract of the horse (Drawing L. Ferguson)

The Mouth

Horses use their mobile upper lip and incisors to select and shear forage from its base or to select and pick up hay or concentrate feeds they are fed. Horses are incredibly clever with their lips and teeth and are able to carefully select feeds and forages that they either wish or do not wish to eat (often much to the frustration of their owners).

Once the feed is in the mouth it is pushed by the tongue to the molars at the back of the mouth where it is chewed extensively. During chewing, the particle size of the feed is reduced to facilitate digestion further down the gastrointestinal tract and, importantly, it is mixed with saliva. The saliva lubricates the feed, making it easy to swallow (thus preventing choke), and also provides some buffering to protect the upper regions of the stomach from the acids produced in the lower regions of the stomach. Once the feed is chewed sufficiently it is swallowed and travels down the oesophagus into the stomach.

The Stomach

The stomach of the horse is a “J” shaped organ with a capacity of approximately 5 – 15 litres meaning the stomach comprises around only 10% of the total volume of the horse’s digestive tract. The stomach has two major functions. These are:

  • The storage and controlled release of feed into the small intestine; and
  • The initiation of protein digestion.

The stomach of the horse is unique in comparison to other monogastrics like dogs and humans (both meal feeders) for two reasons: the first is that acidic gastric juices are constantly secreted into the stomach (dogs and humans only secrete gastric juices when they see or start to eat food); and the second is that feed tends to pass rapidly through the stomach (unlike in carnivorous monogastrics like dogs where food spends a large amount of time in the stomach).

While both of these features are well suited to the grazing horse consuming a high fibre diet, it is likely that these same design features play a role in the incidence of gastric ulcers in horses. For more information on gastric ulcers in horses, read our FeedXL Newsletter #8 – Equine Gastric Ulcers.

Once feed is released from the stomach it enters the small intestine.

The Small Intestine

The small intestine is where a majority of protein, fats and non-structural carbohydrates (starch and simple sugars) are digested and absorbed. The small intestine of a 500 kg (1100 lb) horse is approximately 20 to 27 meters long and has a capacity of 55 to 70 litres.

When feeds enter the small intestine they are mixed with digestive enzymes. These enzymes include proteases that digest protein, lipases that digest fats and glycanases, that digest non-structural carbohydrates. These enzymes act much like scissors, cutting the large protein, fat and carbohydrate molecules into very small pieces that can be absorbed from the small intestine into the horse’s bloodstream or lymph for transportation around the body.

The passage rate of feed through the small intestine is relatively rapid, with feed passing through the entire small intestine in as little as 45 minutes. Given that the feed can move so quickly it is very important that everything entering the small intestine is easy to digest. Horse have a natural ability to extensively digest fats and proteins, however starch from raw cereal grains is very difficult for horses to digest in the small intestine. For this reason, cereal grains must be cooked to improve its digestibility in the small intestine (read FeedXL Newsletter #18 Feed Cooked Grains).

At the end of the small intestine nearly all of the fat, simple carbohydrate and a majority of the protein components should have been digested, leaving only the structural carbohydrate or fibre components to continue on and enter the hindgut.

The Hindgut

In the horse the caecum, large colon, small colon and rectum are collectively referred to as the ‘hindgut’. The hindgut is a specialised structure the horse has developed to enable them to digest high fibre forages.

Monogastric animals do not possess the enzymes necessary to digest fibre. In order to extract the energy from fibrous feeds the horse houses billions of bacteria in its hindgut. These bacteria do posses the enzymes necessary for fibre digestion and they digest the fibre that enters the horse’s hindgut in a process known as fermentation. As bacteria ferment the fibre in the horse’s hindgut they produce volatile fatty acids (VFAs). These VFAs are absorbed and used by the horse as a source of energy. In fact, for horses consuming pasture or hay as the major component of their diet, these VFAs are their main source of energy.

The fermentation of fibre is a lengthy process. So the passage rate of feed through the horses hindgut is slowed dramatically, with feed taking from 50 hours to several days to travel from the end of the small intestine to the rectum where it is excreted as faeces, ensuring there is plenty of time for extensive fermentation to take place. This passage rate will vary however depending on how much feed a horse is eating, speeding up if large amounts of hay or pasture are consumed.

Any starch that is left undigested as it passes through the small intestine is also fermented in the hindgut. However, unlike the steady fermentation of fibre, the fermentation of starch is a rapid process. During this rapid fermentation VFAs are produced in such large amounts that the ability of the horse to absorb them is overwhelmed. Lactic acid is also produced in large quantities and the accumulation of these acids in the horse’s hindgut causes a condition known as hindgut acidosis. Hindgut acidosis can cause serious diseases including laminitis and colic as well as loss of appetite, reduced production of vitamins, changes in behaviour and a reduced ability to digest fibre. Preventing starch from entering the hindgut must be a priority when feeding all horses. To read more about starch digestion in detail and about feeding grain, read our FeedXL Newsletter #18 – Feeding Cooked Grains.

The hindgut’s second most important function is to reabsorb and conserve electrolytes and water that have been secreted from the body into the gastrointestinal tract during the digestion process in order to prevent dehydration and electrolyte deficiency.

It is absolutely critical to keep the hindgut healthy. For more information on the hindgut and how to keep it healthy, read our FeedXL Newsletter #15 – Keeping the Hindgut Healthy.

Understand the gut and you will understand how to feed

Because of the structure of the horse’s gut we know:

  • Horses should be fed little and often (to make sure their stomach which is continuously secreting acid is never empty),
  • Non-structural carbohydrates must be easy to digest in the small intestine and
  • The diet should always be based on large amounts of forage to keep their hindgut full and healthy.

When putting together your horse’s diet and daily feeding routine you should always be asking yourself, what impact if any will this have on the stomach, the small intestine or the hindgut. If you find something that is not ideal, look for ways you can make it suit the horse a little better. For example if your horse is not able to graze and you are only able to feed your horse twice a day, put its hay into a feeder like a small hole hay net so it takes much more time to eat the hay than if it was just put into an open feeder or on the ground.

Feeding to suit the structure of the gut and keeping it healthy means you will always have a horse that is healthy, on the inside and out.

 

This newsletter by Dr. Nerida Richards was originally posted to the FeedXL user forums for FeedXL subscribers in December, 2012. If you would like be among the first to receive our newsletters then please consider subscribing to FeedXL.

Keeping the hindgut healthy

Like humans, horses are classified as monogastrics, however unlike humans, horses have a highly specialised and enlarged caecum and colon, collectively known as the ‘hindgut’.

In this newsletter we investigate the role the hindgut plays in maintaining overall health, what the implications are for an unhealthy hindgut and how you can keep your horse’s hindgut healthy.

What role does the hindgut play in maintaining overall health?  

The hindgut plays some important roles in horse health including:

  1. Fibre Digestion: horses don’t have the enzymes necessary to digest fibrous feeds like pasture and hay. However, in their hindgut they house many millions of bacteria that exist in a symbiotic relationship with the horse. The horse provides them with somewhere warm and moist to live that has a constant supply of fibrous ‘food’ which the bacteria digest via a process of fermentation. In return, the bacteria give the horse a majority of the energy contained in the fibrous feeds in the form of volatile fatty acids (VFAs), which the horse absorbs and burns as fuel for muscle and organs.
  2. Hydration: the hindgut and the fibrous material within it provide a reservoir of water for the horse which may be absorbed when needed to keep the horse hydrated.
  3. Electrolyte Balance: the hindgut and the fibrous feeds within it also serve as a resource of electrolytes that can be absorbed when needed.
  4. Vitamin Supply: the bacteria that ferment fibrous feeds also produce vitamins that may be absorbed and used by the horse, including B-group vitamins like thiamine (vitamin B1) and biotin as well as vitamin K.

What are the implications of an unhealthy hindgut?

If the hindgut is compromised, many health problems can arise including:

  1. Colic: too little fibre, dehydration, the accumulation of dirt and sand or the fermentation of grain in the hindgut are just a few situations that may lead to serious cases of colic.
  2. Weight loss: an unhealthy hindgut that does not have access to ample forage or has unbalanced bacterial populations due to the fermentation of grain can lead to a reduced feed use efficiency and weight loss.
  3. Diarrhoea: a hindgut that is irritated by the accumulation of dirt and sand or parasites, rapid changing of diet ingredients, too little forage or the excessive fermentation of grains can all lead to diarrhoea.
  4. Vitamin deficiency: the fermentation of grains or prolonged use of oral antibiotics can disturb bacterial populations in the hindgut which in turn leads to a reduced production of vitamins like the B‐group vitamins and vitamin K.
  5. Loss of appetite: allowing grains to be fermented in the hindgut can lead to the production of thiaminase, an enzyme that destroys the vitamin thiamine (B1). A thiamine deficiency can then lead to a loss of appetite.
  6. Altered behaviour: studies have shown that horses with acidic hindgut contents resulting from the fermentation of grain are more likely to exhibit abnormal behaviour.

How can you keep your horse’s hindgut healthy?

Follow these 7 tips for keeping your horse’s hindgut functioning normally:

  1. Feed plenty of forage: a healthy hindgut is almost completely dependent on feeding enough forage. Bulky forage is needed to keep the hindgut full to prevent it from physically collapsing on itself or twisting up in a severe case of colic. Healthy bacterial populations are also dependent on having lots of fibre available for fermentation.
    As a rule feed a minimum of 1 kg per every 100 kg of bodyweight in forage per day (1% of bodyweight). The more forage you can feed the better, so unless you have a good reason for limiting your horse’s forage intake, feed a completely forage diet and only use grains when absolutely necessary.
    Note: If you are concerned about a weight disadvantage in racing and other performance horses due to large amounts of forage being carried in the gut, reduce the amount of forage being fed slightly in the days leading up to a race or event.
  2. Feed cooked grains to prevent grain being fermented in the hindgut: if you feed grains, always use a cooked grain (ie steam flaked, micronised, extruded or boiled) as uncooked grains, with the exception of oats, are poorly digested in the horse’s small intestine and will almost certainly end up being fermented in the hindgut. Oats may be fed uncooked.
  3. Feed in small meals: when feeding grains, never exceed 0.5 kg per 100 kg of bodyweight (0.5% bodyweight) in any one meal. Feeding grains in larger meals will make the feed travel quickly through the small intestine and will push undigested grains into the hindgut where they will be fermented.
  4. Make sure your horse is drinking enough water: water intake is crucial for maintaining a healthy hindgut. Dehydration will result in the hindgut contents drying out too much and can lead to problems like impaction colic.
    If you live in a cold climate offering your horses warmed water may help to increase their water intake (for more detail on this read here: http://www.thehorse.com/ViewArticle.aspx?ID=66). If you suspect your horse is not drinking enough water, try adding molasses or other flavourings your horse might like to the water. Make sure horses feel safe around their watering point and that they always have access to water and a salt lick.
  5. Remove sand and dirt from the hindgut regularly: if your horse is grazing in dry/drought conditions or is housed in a dry lot or sand yard you should feed psyllium husk on a regular basis to remove sand and dirt from the hindgut to prevent it accumulating to the point where it will cause problems.
    Feeding 50 grams of psyllium husk per 100 kg bodyweight for 5 days in every one month will help to remove any sand or dirt that may have accumulated in the hindgut.
  6. Make all dietary changes slowly: 4horsessudden changes in feed can upset the balance of bacteria in the hindgut or can lead to diarrhoea. Making dietary changes slowly over a period of 7 to 14 days (taking longer for the more dramatic changes – i.e. if you were changing from Timothy hay to a blend of timothy and alfalfa/lucerne hay you could do this over 7 days. If however you were changing to a diet that included grains you should introduce the grains slowly over 14 days or longer depending on how well the horse adapts to the new diet) will help to reduce or eliminate any negative impact of a new diet.
  7. Avoid the prolonged use of oral antibiotics: where possible try to use injectable antibiotics. If your horse does have to be on oral antibiotics, feed as much forage as possible during and following the treatment to maintain an environment in the hindgut that favours the more beneficial fibre fermenting bacteria.

    Note:
    Most ‘probiotics’ have limited usefulness when it comes to repopulating a horse’s hindgut following antibiotic treatment. Many probiotics contain Lactobacillus spp bacteria which are useful for killing off pathogenic bacteria, but are not really what you want your horse’s hindgut populated with.

When you are feeding, always think about the hindgut!

When you are making feed choices for your horse you should always be thinking about how you will impact the hindgut. Using the 7 tips listed above will ensure your horse’s hindgut will function well to provide your horse with a good supply of energy and beneficial vitamins and problems like colic, loss of appetite, diarrhoea and abnormal behaviour will be avoided.

 

This newsletter by Dr. Nerida Richards was originally posted to the FeedXL user forums for FeedXL subscribers in January, 2010. If you would like be among the first to receive our newsletters then please consider subscribing to FeedXL.

Avoiding gastric ulcers

Equine Gastric Ulcers: Using feeding management to reduce their incidence and severity

Equine gastric ulcer syndrome (EGUS) is a major equine health problem worldwide. Some studies have reported an incidence of ulcers in performance horses in excess of 90% of horses training. A more recent study conducted in Western Australia found 53% of horses had ulcers.

Ulcers negatively and sometimes severely affect a horse’s ability to perform. They cause pain and discomfort, reduce a horse’s appetite which in turn limits its capacity to maintain bodyweight and lead to the development of vices including windsucking and crib biting.

While gastric ulcers have long been recognised as a major health concern there seemingly wasn’t much progress made in preventing them in performance horses. New research conducted in Australia and the USA is however starting to shine some light on how ulcers can be avoided.

What are Gastric Ulcers?

Gastric ulcers are lesions that are found in the stomach of horses. The horse’s stomach is made up of 2 major regions, the upper ‘squamous’ area and the lower ‘glandular’ area. The majority of ulcers in adult performance horses occur either in the squamous area or at the junction of the squamaous and glandular regions.

It is thought that the lack of buffering and protection from gastric acids in the upper squamous area of the stomach is what makes it more prone to ulceration when compared to the lower glandular area which secretes mucous to protect itself from the gastric acids that are continuously secreted into the stomach.

Why do gastric ulcers occur?

Gastric ulcers are a ‘mulit-factorial’ disease, meaning they are caused by many things. The following factors have been identified as possible causes of gastric ulcers:

  1. Training—horses in training are known to have a higher incidence and also more severe gastric ulceration than horses not in work. In a recent study it was reported that the risk of developing moderate to severe gastric ulceration increased 1.7 times for every week that a horse was in training (Lester et al. 2008).
  2. Training location—in thoroughbreds, horses that were exercised on a track on the property where they lived had 3.3 times less chance of having gastric ulcers (Lester et al.2008).
  3. Turnout/paddock time—horses that were given access to some turnout time were less likely to develop ulcers (Lester et al. 2008).
  4. Turnout time with paddock mates—horses turned out with other horses are even less likely to develop ulcers than horses turned out alone (Lester et al. 2008).
  5. Stress/nervousness—talkback radio playing in stables was found to increase the likelihood of thoroughbred horses developing ulcers, suggesting stress is a risk factor for ulcers (Lester et al. 2008).
  6. Exercise on an empty stomach—as a horse exercises the pressure inside the stomach increases which forces the highly acidic gastric contents from the glandular area up into the unprotected squamous area (Lorenzo-Figueras et al. 2002). Exercising horses on a close to empty stomach (as would be the case in horses exercised after an overnight fast) makes it is easy for the acidic contents of the stomach to be pushed up into the squamous upper region of the stomach where it can cause ulceration.
  7. Forage type—lucerne/alfalfa hay appears to have a protective effect on the equine stomach and appears to reduce the incidence of gastric ulcers in horses (Nadeau et al.2000; Lybbert 2007).
  8. Feeding Frequency—feed deprivation such as might occur during transport and long periods between meals lowers the pH in the equine stomach and increases the risk of gastric ulceration (Murray 1994).
  9. Non-Steroidal Anti-inflammatory Drugs (NSAID)—drugs like phenylbutazone have been shown to increase the risk of ulcers, particularly in the glandular region of the stomach.

How can feeding management reducing the risk?

Reducing the risk of gastric ulcers is not just a matter of changing one or two things and hoping it works. You need to assess your horse management systems and make changes wherever your horses are exposed to one of the above risk factors. Some feeding management practices that may help reduce the incidence and severity of gastric ulcers are:

  • Don’t exercise horses on an empty stomach—providing a small meal of lucerne/alfalfa hay prior to exercise will:
    1. Help to stop the acidic contents from the glandular region of the stomach splashing up into the squamous region where it can cause ulcers;
    2. Provide a buffering effect by causing the horse to produce saliva while it is chewing the hay and through the buffering effect of lucerne/alfalfa hay.
  • Provide a small meal of lucerne/alfalfa hay immediately following exercise—the Western Australian study which found horses trained off site had a higher incidence of gastric ulcers suggests that the time taken to return home following training and thus time between the completion of training and breakfast and perhaps the stress associated with travelling is increasing the incidence of ulcers. Providing a small meal of lucerne/alfalfa following training will again help buffer the horse’s stomach and protect it from gastric ulceration.
  • Provide turnout time (with paddock mates where possible) as often as possible—paddock turnout will help to reduce a horse’s stress level and if pasture is available will provide the horse with an opportunity to graze, and thus continuously produce saliva to help buffer the stomach.
  • Provide regular small meals and constant access to hay—allowing the horse to feed continuously during the day and night will help to reduce the likelihood of gastric ulcers developing. Divide the horse’s daily concentrate ration into as many meals as you can to be fed during the day and evening and provide hay (preferably not all as lucerne/alfalfa hay, some grass hay will provide variety in the diet and keep the horses protein intake in check).
    If you are travelling long distances with your horse take regular breaks to provide small meals during the trip. Providing hay in a hay net will also provide the horse with an opportunity to continue eating during transport (if the hay is dusty dampen it down). If you are concerned about the horse’s gutfill leading into a competition, reduce the amount of hay you are feeding for 2 days leading up to an event. However be careful not to reduce total forage intake to less than 1% bodyweight per day.
    Using these feeding management strategies in combination with strategies to reduce stress and the impact of NSAID drugs will help to reduce a horse’s risk of developing gastric ulcers.

If your horse already has ulcers you must treat them

While one study has shown that feeding lucerne/alfalfa hay has been shown to reduce the severity of ulcers already present in horses and long periods of pasture turnout will sometimes allow a horse to resolve gastric ulcer issues, if your horse already has ulcers you must treat them with a registered ulcer treatment. Talk to your vet about the best treatment regime for your horses.

Finally…

Gastric ulcers are a serious and common health problem in horses that will affect their overall wellbeing and performance. While researchers still don’t have a full understanding of how and why they occur with such a high incidence, using the feeding and management strategies outlined above will help to reduce your horse’s risk of developing gastric ulceration.

 

This newsletter by Dr. Nerida Richards was originally posted to the FeedXL user forums for FeedXL subscribers in June, 2009. If you would like be among the first to receive our newsletters then please consider subscribing to FeedXL.