A horse’s digestive system is highly sensitive to change due to its unique structure and function. Horses have evolved as slow grazing feeders, which means their digestive system is not designed to handle large amounts of food at once. Instead, it requires a continuous, small amount of food to pass through it to remain healthy. The equine digestive tract digests portions of feed enzymatically in the foregut and ferments in the hindgut. Sudden changes in diet can disrupt the delicate balance of gut flora, leading to issues such as diarrhoea, colic, and laminitis.

Lisa Elliott Mc – Equine Nutrition Solutions discusses the intricacies of the horse’s digestive system, exploring the reasons why it’s so susceptible to change and how we can help minimise the impact through feeding and nutrition.

Understanding The Equine Digestive System

The equine digestive system consists of several interconnected organs, each playing a unique role in the process of digestion.

• The Mouth is where digestion begins with the horse’s teeth and saliva. Unlike humans, horses have hypsodont teeth, which continuously erupt throughout their lives, aiding in the grinding of fibrous plant material.
• The Oesophagus is a muscular tube that transports food from the mouth to the stomach. It has a one-way passage, preventing the horse from vomiting.
• The Stomach is relatively small with a limited capacity of 9-15 litres, reflective of natural trickle feeding, and represents just 10% of the digestive tract with a similar size to a rugby ball. Unlike other animals, horses secrete stomach acid continuously, regardless of whether they are eating or not. Feed in the stomach is mixed with acid and the enzyme pepsin which helps to start the breakdown of feed particles and protein.
• The Small Intestine is where most of the nutrient absorption occurs, thanks to its extensive surface area, and resembles a folded pipe. This is a very busy organ and continues the enzymatic digestion of protein along with the digestion of fats/oils, starch, and sugar.
• The Large Intestine consists of the caecum and large colon, followed by the small colon and then rectum, where the droppings are formed. The Caecum is 1.2 m and has a volume of 28-36 litres and the Colon is 3-3.5 metres and has a volume of 86 litres which together is big in comparison to the rest of the digestive tract. The large colon also contains a small number of folds or ‘flexures’ which connect its four main parts (right and left ventral and right and left dorsal). The Large intestine makes up 60% of the digestive system and because of its sheer size is often referred to as the ‘hindgut’. The hindgut is responsible for the digestion of fibre and its large size in proportion to the rest of the digestive tract is testament to just how important fibre is in the diet. However, horses cannot digest fibre by themselves, they don’t possess the enzymes to do that, so they need to rely on billions of microbes inside the hindgut who digest fibre through fermentation and live in a delicately balanced community within an optimum pH of 6.5 – 7. The large intestine ends with the small colon and rectum which are responsible for water and electrolyte absorption before waste elimination.  

Sensitivity to Changes in Diet

The horse’s digestive system is adapted to continuous grazing of fibrous forage. Whilst this unique design facilitates the processing and digestion of continuous fibre, modern management can lead to abrupt changes in feeding routines and diet which aren’t in sympathy with their digestive system and microbial population.

Limited Stomach Capacity. The horse’s stomach has a relatively small capacity, and unlike ours, it’s unable to expand to accommodate the size of the meal consumed. This small capacity means the stomach empties regularly into the small intestine. If a large meal is fed, the stomach may empty quicker. This has a knock-on effect to the small intestine because a relatively large feed bolus will travel through quickly and overwhelm it, leading to poor digestion and the potential for undigested starch to reach the hindgut which can lead to laminitis or colic.

Constant Acid Production A horse’s stomach produces acid constantly whether they are eating or not. Horses should be chewing fibre regularly which produces saliva to naturally buffer stomach acid.  If fibre intake is changed or reduced suddenly, the acid build up can result in damage to the stomach lining and ultimately lead to the formation of gastric ulcers which compromise digestive health.

Limited Digestion of Starch. While small amounts of starch and sugar are generally well digested, in the small intestine, horses have a limited ability to digest starch. This means that if cereal-based meals with a high starch content are fed there’s potential for undigested starch to reach the hindgut for fermentation by hindgut microbes.  High levels of starch will be rapidly fermented by lactic acid producing bacteria, who proliferate, resulting in microbial dysbiosis, a significant drop in the hindgut pH, and a state of acidosis. 

Hindgut acidosis will cause the death of beneficial fibre-digesting microbes, resulting in compromised fibre digestion, along with inflammation and deceased permeability of the gut wall, allowing the passage of endotoxins and other harmful compounds into the bloodstream. The result of this chain of events can be digestive upset and loose droppings but more seriously can end up in colic or laminitis.

Sensitive Microbial Balance. The hindgut is home to a diverse bacterial population, coexisting with other gut microbes in a finely balanced community that benefits all members, thriving at an optimal pH of 6.5 to 7.

This delicate balance means that any sudden changes in diet between different feeds or more significantly between forages, for example, changing from grass to hay can have significant consequences for the digestive system.  It can result in rapid fermentation, proliferation of less desirable microbes, dysbiosis and a more acidic environment in the gut, as well as compromised fibre digestion.

The consequences of sudden dietary change highlights just how sensitive our horse’s digestive systems are and is a major risk factor for colic, with research indicating that the risk increases further with changes to a new forage.

Hydration and Water Intake. Horses need a steady supply of fresh, clean water for digestive health and to ensure smooth passage of gut contents. Alterations in diet or surroundings can reduce their water consumption, increasing the risk of impaction colic, especially in the tighter bends of the large colon, when the contents of the hindgut dry out.

Tips to help Minimise Digestive Sensitivity

To ensure optimum digestive health, it’s essential to consider the design and sensitivities of a horse’s digestive system and feed them in a way which reflects and supports their unique dietary needs.

1. Feed Plenty of Fibre, this should be ab-lib whether it’s grazing or preserved forages like hay or haylage. This mimics the horse’s natural grazing behaviour, keeps the hindgut microbiome happy and balanced, and helps maximise digestive health.
2. Keep Starch to a Minimum – If your horse needs extra feed in addition to forage, ensure this is provided in a high fibre and low starch form. Feeding highly digestive fibre as opposed to cereal based feeds which are often high in starch, is ideal to provide extra energy without compromising digestive health. All products in the Coligone Range are formulated to provide high levels of fibre whilst being very low in starch.
3. Feed Little and Often, this is essential to maintain digestive heath and prevent upset. Ideally, feeds should be no more than 2kg per feed and fed by weight not volume because different feeds weigh different amounts.  
4. Make Diet Changes Gradually, ideally over 2-3 weeks when changing forage or feed. This will allow the microbial population in the hindgut to adjust, reducing the risk of digestive disturbances. Feeding Coligone Powder to support beneficial microbes when transitioning to a new feed or forage can further help minimise sensitivity, mitigate digestive upset and support a healthier digestive system throughout the process.
5. Support Healthy Acid Levels is essential to help minimise sensitivity and prevent digestive upset. Coligone Liquid and Powder supply generous levels of natural acid buffers. When fed daily it can help to neutralise acid within the stomach and support healthy acid levels throughout the entire digestive tract.
6. Maximise Hydration by providing daily salt in the feed or in the form of a lick can help increase water intake to support optimum gut motility and digestion. Coligone Healthy Lick can help meet your horse’s daily salt needs, which is essential for optimum hydration. Additionally, Coligone Healthy Lick will promote the production of saliva to help buffer acid, and reduce stress, all of which helps maintain optimum digestive health.
7. Regular Exercise will help promote healthy gut motility and further reduce the risk of digestive issues.