The horse’s digestive system is rather unique in comparison with many animals that have also evolved to survive on a forage or grass-based diet. The horse can function on high volumes of forages that contain lower nutritional values, compared to those grasses often more suitable for livestock, such as dairy pastures. The horse can achieve this due to the structure of its digestive system.

The average horse’s digestive system is around 100ft in length and runs from their mouth to their anus and comprises of sections referred to as the foregut and the hindgut.

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The Mouth & Oesophagus
Saliva is produced as the horse chews, but unlike many species the saliva produced contains little to no enzymes. Meaning that insignificant chemical digestion occurs before the food travels down the 1.5m long oesophagus and into the stomach.

The Foregut
The foregut comprises of the stomach and small intestines. The stomach produces hydrochloric acid and two main enzymes called pepsin and lipase. Only the lower portion (glandular region) of the stomach contains a mucosal lining as protection against the impact of the acid and enzymes. As such the upper area (Squamous region) can be at an increased risk of developing ulcerations due to the presence of acid, but lack of a protective layer. The stomach then leads onto the small intestines and these can be divided into three regions, the duodenum, jejunum and the ileum.

The combination of bile and enzymes within the small intestines break down fats, proteins and some carbohydrates and the end products of these should absorb before the food passes onto the hindgut.

The Hindgut
The hindgut (sometimes referred to as the large intestines) starts after the end of the small intestines. The Hindgut comprises of the cecum, the large colon and onto the small colon, and is approximately 28ft long in the average 500kg horse. Up to this point digestion of food has been mainly driven by enzymes, but in the hindgut digestion is driven microbes. These are populations of bacteria and protozoa, collectively sometimes referred to as the gut biome.

In a healthy gut these microbes work symbiotically (with the horse) to digest roughage/fibre to produce energy for the horse in the form of Volitale Fatty Acids (VFA’s) or short-chain fatty acids (SCFA’s). It should be possible to provide the total energy requirements of many horses with these from a forage only based diet. Any additional feed or supplement should be provided to supply extra energy or essential nutrients missing from the fibre provision for that horses requirements.

Over recent years it appears as though the foregut, and in particular the stomach, has received greater focus than the hindgut where it comes to managing and caring for our horses. This increased focus on the stomach has been driven by the greater understanding of and treatment options for Equine Gastric Ulcer syndrome (EGUS).

But as a horse’s diets should be fibre/forage based and the hindgut is this main site for its digestion focusing more on the stomach could be short-sighted.

Hindgut dysfunction
The hindgut biome can be susceptible to upset or dysfunction from a number of causes, including certain medications, stress, sudden changes in diet, high grain- based diets, insufficient fibre, or infections, to name but a few. Hindgut dysfunction occurs when an unhealthy balance of the different microbial populations develops.

This is a very different process to EGUS (which occurs in the stomach), as hindgut dysfunction occurs in the caecum and colon and as a result of increased levels of lactate producing bacteria.

This dysfunction can result in a condition called hindgut acidosis. Increases in the lactate producing bacteria cause the environment within the gut to drop from a neutral pH to one far more acidic. The consequences of the hindgut becoming acidic is a cascade of events. The presence of acid causes the death of other healthy bacteria, which then release toxins within the hindgut. These then result in inflammation of the gut wall and increased permeability of the gut lining, which allows for the passage of these toxins into the blood stream and onto the rest of the body.

Impact of Hindgut Acidosis
It is not surprising that with the combination of acid, inflammation, damage to the integrity of the gut wall and toxins entering the blood stream, the consequences of hindgut acidosis can impact on the whole horse. But the array of issues that can be attributed to increased levels of lactate producing bacteria can be alarming.

Probably the best known consequence of hindgut acidosis is sadly laminitis, where the toxins that have ‘leaked’ into the blood stream negatively impact on the sensitive structures of the lamini. Impeding their function and ability to hold the pedal boned and the hoof wall together. There are a number of other causes to laminitis, but the processes involved in hindgut acidosis are at play when the naughty pony that’s broken into the feed room and consumed huge amounts of hard feed goes onto develop laminitis. Other frequently associated symptoms can be poor body condition or appetite. As fibre should be the largest part of a horse’s diet and as the hindgut is the main site of fibre digestion it is understandably easier to grasp how issues in this area could result in poor weights, appetite and even performance. 

However, some of the other consequences linked with hindgut acidosis and behaviour are not necessarily so easy to initially understand. Several studies in horses as well as other animal species have shown hindgut acidosis to cause changes in behaviours linked with flight responses and even aggression. A series of published studies identified that horses with higher levels of these bacteria where not only more likely to spook, but that when they did spook, they did so quicker, faster and
for a longer distance. Other studies have identified that horses with raised lactate producing bacteria in the hindgut were also more likely to display stereotypical behaviours such as windsucking or box walking, and have raised cortisol levels, both potential measures of stress levels. Further studies in rats found that the rate of fighting and even resultant deaths increased as did the lactate levels in the hindgut.

Further studies across a variety of species have linked high lactate levels in the hindgut with poor immune response, slow healing rates and even an increased frequency for the requirements of antibiotics.

While it is not possible to directly relate the findings in a study in one species to another, it can highlight towards the disease processes and issues caused by a poor hindgut biome.

The importance of a healthy hindgut biome and the full impact on a horses is still not fully understood. But it is clear that a disturbance in the micro-organisms within the hindgut can negatively affect the whole horse, it’s health, it’s performance and even its behaviour.