The horse’s digestive system and how gut health can impact behaviour

The Digestive system and how gut health can impact equine behaviour

The horse’s digestive system and how gut health can impact behaviour

Written by Loni Loftus, Clinical Animal Behaviourist

In this article Clinical Animal Behaviourist, Loni Loftus uncovers the mechanics of the horse’s digestive system and looks at how the health of the equine gut can have a direct effect on behaviour, including mental and physical health.

We have all heard the old adage of ‘no foot, no horse’ well ‘no gut, no horse’ is just as worthy a pronouncement!

The horse is a non-ruminant herbivore – this means that they are specialised to eat mainly plants. The equine gut or gastrointestinal tract (GIT) is the largest organ in the horse’s body and its health status has significant implications on both the physical and mental wellbeing of the horse.

The horse’s digestive system

From a biological viewpoint the digestive tract is made up of the mouth (including teeth, tongue and salivary glands) and the oesophagus, travelling down to the stomach, followed by the small intestine and large intestine (which includes the all-important caecum) before reaching the rectum and anus where faecal matter is expelled (Figure 1).

Figure 1 – Pie chart to show the average percentage capacity of the digestive tract contributed to by each section.
Figure 1 – Pie chart to show the average percentage capacity of the digestive tract contributed to by each section.

A closer look at the overall process of digestion

The stomach is the size of a rugby ball when empty and holds 9-18 litres on average. Food arrives in the stomach from the oesophagus through a non-return valve called the cardiac sphincter (this valve is the reason why horses cannot be sick) and stimulates the release of digestive juices. Food, called ingesta, passes through the stomach taking between 20 minutes-3 hours and larger meals cause faster passage to the small intestine. In the stomach food is hydrolysed by acid and protein digestion begins alongside a small amount of soluble sugar and starch fermentation occurring.

Ingesta then passes into the small intestine (made of up three sections, the duodenum, jejunum and ileum) through the pyloric sphincter where all the major starches, lipids (fats) and proteins are broken down. This is the main place where nutrients are absorbed from the horses’ diet into the body for energy, growth and repair. The food is now called chyme and passes through the small intestine via muscular contractions of the intestinal wall called peristalsis. The small intestine is around 21-25 metres long, with a capacity of 50 litres, and the transit time of food through it is on average 45 minutes.

The large intestine is composed of the small colon, large colon and caecum. The small colon is around 3-4 metres long but only holds up to 14 litres of fluid whereas the similar length large colon holds up to 82 litres of fluid. Water and nutrients are extracted from these parts of the digestive tract. The caecum holds around 35 litres of fluid and food arrives here around three hours post meal. The caecum is responsible for the fermentation of fibre such as hay and the cellulose is broken down into volatile fatty acids which are used as an energy source.


Waste is formed into faeces and excreted via the rectum and anus.

The smooth running of this process to provide the horse with sufficient, but not excess, energy requires careful thought and attention to the design of the horse’s digestive tract, the mental (emotional) and physical health of the horse and the workload or exercise level of the horse.

the horse eating grass
A horse: The stomach is the size of a rugby ball when empty and holds 9-18 litres on average

Feeding for mental (emotional) and physical health

Due to the design of the digestive tract the equine stomach is quite small in relation to the size of the horse and the intestines are large to allow for a significant volume of roughage such as hay or grass. In order to function optimally the horse should be provided with access to ad libitum forage at all times as this mimics the eating patterns of the horse in a natural environment and provides a steady stream of low energy value roughage through the tract to maintain movement (which reduces likelihood of colic incidences) and maintain healthy gut pH (which reduces likelihood of gastric ulcers).

Providing forage ad lib also allows the horse to behaviourally mimic its natural environment through regular chewing and ingestion of long stalk forage – this helps to maintain a positive emotional state with chewing shown to reduce stress and promote release of endorphins or ‘feel-good’ hormones. The provision of ad lib forage combined with access to turnout where the horse can roam, browse and graze is an ideal scenario for the keeping of our domestic horses, allowing them to life a lifestyle that is as close to their natural ethological state as possible. Free-ranging horses spend a significant proportion of time (around 46%) grazing with a wide range of other behavioural actions performed, many of these being social behaviours with other horses.

Unfortunately, many horses are fed large quantities of high starch concentrate feed split over one, two or three meals a day with reduced forage ration and restricted turnout. This can affect not only digestive health but also the horse’s daily time budget which can lead to the development of unwanted behaviours such as over-exuberance, aggression and repetitive behaviours such as box-walking, crib-biting, weaving, wind-sucking and so on.

a horse crib biting
Crib-biting is a stable vice associated with stress and can cause stomach and digestive issues

Research has shown that being stabled for more than 13 hours a day is associated with abnormal oral and ingestive behaviour in horses.

These unwanted behaviours generally arise due to one of the following mechanisms, or a combination of these:

  • A medical condition causing physiological changes and/or pain; such as Equine Gastric Ulceration Syndrome
  • Excess energy production due to high feed values and limited exercise
  • Lack of turnout
  • Lack of social interaction with other horses
  • Boredom leading to displacement behaviours which can become repetitive and addictive
  • Fear/Anxiety due to being stabled alone or punished for unwanted behaviours resulting from the above causes.
a horse in a stable looking out = - Research has shown that being stabled for more than 13 hours a day is associated with abnormal oral and ingestive behaviour in horses.
Research has shown that being stabled for more than 13 hours a day is associated with abnormal oral and ingestive behaviour in horses.

More often nowadays we are seeing the need to restrict the grazing and turnout of our horses and ponies in order to manage a variety of health issues such as obesity, laminitis, PPID and other metabolic conditions. This can prove challenging in terms of ensuring we provide for the horse’s natural needs whilst ensuring their physiological health remains optimal. Similarly trying to provide for the needs of a horse on box-rest whilst recovering from injury or surgery can be just as challenging and equally important.

How can I ensure I feed my horse for both physical and emotional health?

If your horse does not require any restrictions to grazing or turnout due to health conditions then follow the traffic light system in figure 2.

Figure 2 – Traffic light horse turnout management system
Figure 2 – Traffic light horse turnout management system

If your horse does require restriction or non-standard management of the diet in order to facilitate management of weight or medical conditions it is vital that any veterinary advice is followed.

If grazing is to be restricted it is important that the opportunities to socialise, move and browse remain available for the individual. Opportunities to socialise can be facilitated through providing a known affiliative companion to spend time with when both grazing and when more restricted. This can be facilitated through the use of larger non-grass paddocks for turnout of more than one horse, or at least the ability to graze next to each other within reach for mutual grooming to be enabled.

A picture of a horse with turnout
Horses on box rest may be easily deprived of both social companionship and browsing/foraging access.

Horses can be encouraged to move around during turnout (whether restricted or not) through the use of a track system to provide them with defined grazing, browsing and investigative areas, often using a loop or figure 8 type set-up. The systems can be designed in any way to suit the horses using them and can include non-grass areas such as woodchip and gravel areas, hills, rolling areas and treasure hunt type activities to forage for and seek food items for added enrichment. It is always important to consider the design and use of these systems carefully to promote health and relaxation for all horses using them.

For horses and ponies who may need to be stabled or restricted for longer periods walking in hand (if not precluded through a medical condition) to take them for browsing and grazing walks, often called ‘picnic walks’, can be a good way of providing for their movement time-budget as well as allowing them the positive experiences of satisfying their foraging behaviours and at the same time can contribute to developing a positive relationship between horse and owner.

Horses on box rest may be easily deprived of both social companionship and browsing/foraging access. Wherever possible it is an advantage to provide a horse on box rest with a companion. Some situations may allow for larger shared stabling/barn arrangements for social companionship whilst others may provide for an outside stable where the resting horse can be in contact with and/or line of sight of other horses. These horses should be provided with the opportunity to allogroom with others (some of which can be provided by human ‘grooming’ of the horse).

In order to reduce boredom and potential negative emotional states including depression and frustration it can be valuable to provide a range of enrichment resources for the restricted horse. These may include horseballs, foraging games such as hiding carrots or similar in the straw bed for the horse to find, hiding or dividing food around a non-grass turnout area, introducing scentwork for horses, doing in-hand training little and often to mentally stimulate the horse and providing the horse with puzzles to solve in order to earn low calorie food rewards.

Feeding excess energy can be a common cause of significant behavioural issues related to exuberant or over-enthusiastic behaviour which can be seen as bucking (sometimes accompanied by some squealing), leaping, bolting, appearing highly strung and hypervigilant (such as shying at lots of stimuli). Whilst there are many other reasons why we may see these behaviours in the horse (the most common alternative reason being pain) clinical animal behaviourists are regularly referred equine cases where an imbalance in energy consumption versus energy expenditure is causing unwanted behaviours and breaking down the human-animal relationship. In these cases, it is vital that the diet of the horse is carefully calculated, monitored and slowly altered to ensure the provision of the optimal level of energy and balance of macro and micronutrients for good health and calm behaviour. Following correction of the diet there is often a requirement to undertake some training or re-training work to undo learnt behaviours and conditioned responses developed during the period of suboptimal feed management.

It is important that this training is carefully planned by an appropriately qualified and accredited clinical animal behaviourist in order to ensure that the unwanted behaviours are replaced with new, wanted behaviours without any techniques being used that are aversive to the horse (either through causing pain, fear or frustration) and therefore potentially dangerous to both horse and handler. Suitably qualified, accredited and experienced clinical animal behaviourists specialising in equines can be found via the Animal Behaviour and Training Council’s list of accredited practitioners at

For specific and bespoke advice regarding feeding your horse you should consult an appropriately qualified equine nutritionist (see  or RCVS Specialist in Equine nutrition .

The field of equine nutrition is not well regulated and as such anyone can call themselves a nutritionist. Look for someone who holds a Masters degree or PhD (Doctorate) in Animal/Equine nutrition or approach one of the well-known feed companies for advice as their representatives are often overseen by well qualified nutritionists. Don’t be afraid to ask for evidence of a practitioner’s credentials to ensure that you are receiving the correct advice for you and your horse.

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Information provided in this article is based on research preformed by the author.


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