General Feeding – Understanding the Basics

By Julia Gray, MSc. Eq. Science

Developing an understanding of digestive anatomy and digestive physiology can help minimise the risk of digestive disturbances in the horse due to diet and feeding practices.

The health of the digestive system is one of the factors essential for allowing the horse to perform to its full potential, while a sound understanding of the digestive system can also aid to increase efficiency from both a cost and a nutritional point of view.

The role of the horse in society has changed significantly since its first domestication, which is believed to have taken place around 4000-3000 BC.  The purpose of the horse has changed from being a food source, to a means of transport, to being instrumental in wars, to being used for leisure and sports purposes.

The ‘natural’ diet of fresh forage was not sufficient any more to meet increased nutritional demands that arose as a result of the changing role of the horse in society. Cereal grains and by-products, as well as oils, were therefore introduced to the horse’s diet.

Apart from changing feedstuff, changes in management such as restricted feeding times and restricted turnout occurred with domestication of the horse.  While there are numerous benefits in the use of feedstuff other than forage, some inappropriate feeding practices or dietary imbalances linked to this might be disadvantageous to the health and performance of the horse.

Thus, digestive anatomy and physiology and potential problem areas will be briefly outlined in the following article, to help develop a basic understanding of equine digestive anatomy and physiology.

A unique digestive system

Horses are non-ruminant herbivores and, as such, their digestive system has adjusted to eating high-fibre diets including feedstuff like grass, leaves and herbs.  In nature, a horse will spend most of the day, i.e. 16-18 hours, grazing.  That means the digestive system has adapted to a diet based on the principle of ‘little, but often’.

Hence, the horse is a so-called trickle feeder.  On average, horses consume 2 kg dry matter per 100kg bodyweight (BW) or 2-3% of their BW per day.

The digestive system of a horse can be thought of in two sections.  The first section (stomach, small intestine) has similarities to the monogastric digestive system of, for example, a dog or man - while the second system is a bit like the rumen of a cow, in that fermentation of structural fibre by bacteria takes place in the horse’s hindgut (large intestine).

So, does that mean the digestive system of a horse is equal to that of a dog plus a cow?

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The answer would have to be ‘no’.

The horse is unique and therefore needs to be considered as such in order to avoid potential problems - and to maintain health and optimal performance.

From top to bottom…

The entire digestive tract can be divided up into the mouth and oesophagus, the stomach, the small intestine and the large intestine - each plays a different, but important role in digesting the feed and making nutrients available for the body to use as well as excreting the ‘unwanted’ material.

Total passage time of the feed through the digestive system depends on type and composition of the feed ingested.  It can vary between 35 – 50 hours in total, with about 61.3% of the total volume being made up by the large intestine.

In general, total passage time is faster for hay than for concentrates (non-roughage meals), while being quicker for large feed quantities than for small ones.

The digestive process

   • Ingestion: Taking up food into the body using mouth and lips
   • Digestion: Breakdown of ingested food into its simplest forms
   • Absorption: Nutrients are absorbed into the circulatory system and carried to where they are needed or stored
   • Assimilation: The use of these nutrients by the body for different functions
   • Egestion: Removal of undigested material and waste products from the body
 

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The mouth and oesophagus

The food is taken into the mouth by a combination of the use of lips, tongue and teeth. The lips are very sensitive and mobile and the horse is as such able to selectively choose and reject feed.  This is useful for preventing the ingestion of an unwanted foreign body, but unhelpful when the horse, for example, rejects vitamin and mineral pellets.

The food will be chewed or masticated by complex lateral and vertical jaw movements. Duration of feed intake depends on the type of feed, with roughages being ingested much slower than concentrates.  Furthermore, chewing rate per kilogram (kg) of feed ingested also depends on the type of the feed.  Figures A and B display average duration of feed intake and chewing rate for hay and oats/per kg.

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Figure A: Average Chewing Rate       Figure B: Average Duration of Feed Intake

Mixing chaff with concentrates lengthens feed intake and increases chewing rate, whereas moistening of feed will shorten the duration of feed intake.  It is also interesting to note that ponies generally take longer to ingest feed than horses.

The risk of developing dental hooks (figure C) increases with reduced chewing activity and, therefore, with a low fibre/high concentrate diet.  A low fibre/high concentrate diet may further have implications for equine behaviour as the horse will spend less time eating and has potentially more time to develop behavioural abnormalities such as box-walking or wood chewing.

The mechanical breakdown of feed is also important for optimal digestion of the food in the small intestine and for undisturbed passage of digesta into the large intestine.

Figure C:

Dental Hook

Unlike in the dog, saliva in the horse is produced as a response to chewing - and not in anticipation of the feed.  About 3-5 kg saliva can be produced per kg of roughage and only 1-2 kg per kg concentrate.  This means the more the horse chews, the more saliva is produced.  Saliva is not only important for lubricating the food to allow for smooth swallowing, but also for neutralising stomach acid.

The stomach

The stomach of the horse is relatively small as it has adapted to small meal sizes.  It makes up about 8.5% of the total digestive tract volume, and has a capacity of about 9-15 L.  Passage time of the feed through the stomach can vary between 1 and 5 hours - depending on feed type.

Gastric emptying rate is quicker after larger meals, which would then cause the feed to also pass more quickly through the small intestine, allowing for less time for digestion and absorption of nutrients.

As quick ingestion and little saliva production may lead to gastric disturbances, care should be taken to feed little and often. A maximum of 0.4 kg concentrates per 100 kg bodyweight should not be exceeded per meal.

As can be seen in figure D, the stomach is divided into two parts, a non-glandular and a glandular section.  Bacterial fermentation of soluble carbohydrates begins in the squamous, non-glandular section.  For example, soluble carbohydrates such as starch are converted into lactic acid.

This process is stopped as the stomach contents move into the glandular region, where stomach contents will be mixed with gastric acid.  With larger concentrate meals, the swallowed feed (bolus) will have higher dry matter content and the stomach contents will be mixed with the stomach acid at a reduced rate, which increases the risk of dysfermentation.

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Figure D:

The equine stomach and its regions

Equine Gastric Ulcer Syndrome

Gastric ulcers are a rather common occurrence in performance horse, with prevalence rates of 80-90 % for racehorses in training, 60 % for performance horses, but also around 30 % for leisure horses.

Ulcers mostly occur in the non-glandular section, particularly around the cuticular ridge, which is a raised area between the more acidic glandular and the less acidic non-glandular region.  While ulcers can be asymptomatic, they can also be caused by poor body condition, poor performance and colic.

Diagnosis is possible through endoscopic examination of the stomach by a qualified veterinarian.  Causes are multi-factorial, but often relate back to low fibre, high starch diets combined with large meal sizes, periods of fasting, stress and the mechanical effects of exercise.

The small intestine

The small intestine is approximately 2 m long and is the site of digestion and absorption of simple sugars (up to 90%), protein (up to 70%) as well as oils and fats (up to 90%).  The small intestine is divided into three parts - the duodenum, jejunum and ileum.

Mean transit time of feed through the small intestine is 1.5 hours.  All nutrients from digestion in the small intestine - except fatty acids from breakdown of lipids - are taken to the liver via the blood for storage.  Fatty acids are absorbed via the lymph system.

Amylytic, proteolytic and lipolytic enzymes are secreted with the pancreatic juice.  These enzymes assist the breakdown of carbohydrates, protein and oils/fats.  Furthermore, the pancreatic juice contains alkalines helping to neutralise acidic stomach contents as they pass into the small intestine.

Bile, which is continuously secreted into the small intestine by the liver as the food passes through, also has buffering capacities and aids to emulsify lipids (fats & oils) to allow for breakdown of these.

Mucosal enzymes secreted into the small intestine are also important for carbohydrate and protein digestion and absorption.  Adult horses (~500kg BW) secrete around 100L fluids per day into the preceacal section of the gastrointestinal tract.

Starch digestion

The capacity of the small intestine to digest starch, which is the main energy source from cereal grains, is limited and is thought to decrease as the amount of starch fed per meal increases.  This means feeding more per meal does not necessarily result in more energy available for the horse.

On the contrary, feeding larger amounts of concentrates containing high amounts of starch can lead to a decrease of pH in the small intestine as a result of increased lactic acid production.  This could damage the mucosal membrane and cause disruptions of peristalsis (rhythmic involuntary muscular contractions, which move ingesta along the digestive tract).

Also, undigested starch or other soluble carbohydrates may then pass into the large intestine, where normal microbial activity may be disrupted potentially leading to hindgut acidosis, colic and laminits.  Hindgut acidosis can damage the mucous membrane in the large intestine making it permeable for endotoxins produced as a result of dying ‘good’ bacteria.

These toxins can then pass through the blood into the vascular system of the hooves, potentially causing laminitis.  Processing of cereal grain can have a profound impact on starch digestion in the small intestine, see table A.  Preceacal starch digestibility can significantly increase depending on processing technique.

Feedstuffs Type of processing Preceacal starch digestibility (%)
Oats Whole or Rolled 80 – 90
Barley Rolled 22 – 75
Maize Whole 29
  Ground 47
  Thermo-Mechanical 90

Table A: Effect of cereal grain processing on starch digestibility

The large intestine

The large intestine is also divided into three sections - the caecum, colon and rectum (figure E).  It is only approximately 8 m long, but comprises about 61.3 % of total digestive tract volume.  Average passage time of digesta through the large intestine is approximately 40 hours.

Digestion in the large intestine is very different to precaecal digestion, as suggested by the term ‘post-gastric hindgut fermentation’ describing the digestive system of the horse.

The large intestine is the site of microbial fermentation of structural carbohydrates (cellulose, hemi-cellulose, pectins) or the fibrous content of the diet. Lignin, another complex carbohydrate, cannot be digested by the bacteria in the large intestine.

Feeding large proportions of highly lignified fibre, such as straw should therefore be avoided.  Straw intake should be kept at a maximum of 0.8 kg/100 kg BW per day.  If the microbial flora in the hindgut is healthy, as encouraged by a continuous supply of fermentable fibre to the hindgut, the water soluble B-vitamins will be synthesised by the microbes.

Consequently, horses rarely suffer from B-vitamin deficiency.  Most of the microbial fermentation takes place in the caecum.  The volatile fatty acids (VFAs) produced, are mainly absorbed in the colon.  Most water present in the digestive tract is absorbed in the colon of the large intestine.

Different feedstuffs have different water-holding and –releasing properties, so that hay, for example, can act as a water reservoir in the gastrointestinal tract, which can be important for animals such as endurance horses.

VFAs from microbial fermentation of hay, for example, are a source of energy to the horse.  Due to the long duration of fibre digestion, energy from fibre breakdown is also referred to as slow-release energy.

Protein digestion in the large intestine is not very efficient, so it is important to provide the horse with high-quality protein through the diet, which can be broken down and absorbed in the small intestine.  The rectum is a short tube to expel the faeces, giving them their distinctive shape.

Figure E: The large intestine

Practical feeding management to limit digestive disturbances

By mimicking a diet the horse would eat in the wild, a beneficial effect on the digestive system of the horse can be achieved and more natural feeding behaviour is promoted.  Some guidelines for practical feeding management of the horse to limit digestive disturbances and promote health and performance are summed up in the following.

Adequate forage intake

Ideally, forages such as grass, hay or haylage should constitute at least about 50 % of the horse’s diet.  At least 1kg chewable forage should be fed per 100 kg BW.  This would mean that a 500 kg adult horse receives at least 5 kg forage per day.

Forage fed to the horse should always be of high quality and as dust-free as possible - and free from mould.  However, many forages do not contain appropriate levels of vitamins, minerals and maybe amino acids to meet the requirements of a horse, and should therefore be balanced by a suitable feed or balancer.

Little and often

Feeding frequent small meals to ensure that concentrate meal size does not exceed 0.4 kg / 100 kg BW per meal will help to maintain digestive health and increase digestive efficiency.  Remember that feeding more at one time, for example, to increase the horse’s energy intake, sometimes has the opposite effect!

Feed starch sources with high precaecal digestibility

To limit the delivery of rapidly fermentable substrate to the hindgut and the development of associated problems, feed starch sources with high preceacal digestibility.  Cereal grain processing maximises digestibility in the small intestine, thus helping to limit overflow of starch into the hindgut.

Use alternative energy sources

Add alternative energy sources such as oils and highly digestible super fibres e.g. soya bean hulls and beet pulp, to the horse’s diet to meet its energy demands without overfeeding on starch.  Oil should not be fed to a horse at more than 100 ml / 100 kg BW.

Gradual dietary changes

Always introduce any dietary changes gradually to allow the horse’s system to adapt to the new feed or feedstuff slowly.  Remember that dietary changes in nature don’t happen overnight, but slowly with changing seasons.  New feeds (concentrates or forage) should ideally be introduced over a period of 7 – 14 days.

Delay exercise after feeding

Avoid (especially all strenuous) exercise for a period of 2 to 3 hours post-feeding of a concentrate meal.

Store all feed appropriately

To prevent loss of nutrient value and development of fungal, bacterial or pest contamination, all feed should be stored in a cool and well-aired place away from direct sunlight, preferably in pest-proof bins.

And of course, don’t forget to always provide access to plenty of fresh water!

References

• Pferdefütterung, 4th edition. Helmut Meyer and Manfred Coenen.
• Equine Nutrition & Feeding, 3rd edition. David Frape

For further information on feeding your horse, contact the RED MILLS nutrition team.