Dr Jazmin Markey – PhD, PAS
When horse owners think about digestive health, attention usually turns to the stomach. Gastric ulcers and stomach acid are common topics of discussion, but the stomach is only one part of a much larger digestive system.
Horses are hindgut fermenters, meaning they rely on the caecum and large colon to digest the structural carbohydrates, or fibre, found in forage. Rather than breaking down this fibre themselves, horses depend on billions of naturally occurring microorganisms living within the hindgut to do it for them.
As these microorganisms ferment fibre, they produce volatile fatty acids (VFAs), which provide a major source of energy for the horse while helping to support normal digestive function. Without this microbial community, horses would be unable to efficiently utilise one of the most important components of their natural diet: forage.
Because this microbial community plays such an important role in digestion, maintaining a healthy hindgut is about much more than preventing digestive upset. When the balance of this ecosystem begins to change, fibre fermentation becomes less efficient, normal digestive function may be disrupted, and the overall health of the hindgut can be affected.
Under normal circumstances, most starch, a non-structural carbohydrate, is digested enzymatically and absorbed within the small intestine before reaching the hindgut. However, horses have a limited capacity to digest a high concentration of starch at any one time. When large concentrate (grain) meals are fed or dietary changes are made too quickly, total starch intake can exceed digestive capacity.
A portion of that starch may then escape small-intestinal digestion and enter the caecum and large colon. Once it reaches the hindgut, this undigested starch becomes available for microbial fermentation.
Because starch is more rapidly fermentable than the structural carbohydrates found in forage, its arrival provides an additional substrate for starch-utilising microorganisms. Fermentation patterns begin to shift, production of volatile fatty acids and lactic acid may increase, and hindgut pH begins to decline.
As the environment becomes more acidic, the microbial community also changes. Populations and activity of fibre-digesting microorganisms may decline, while microorganisms better able to utilise starch or tolerate the more acidic environment become more prominent.
The horse may therefore become less efficient at fermenting fibre, which is the primary function the hindgut is evolved to perform. This disruption of the normal hindgut environment is commonly referred to ashindgut acidosis.
Rather than describing one specific disease, the term refers to a reduction in hindgut pH accompanied by changes in fermentation and microbial balance that may interfere with normal fibre digestion and gastrointestinal function.
The microorganisms living within the hindgut do far more than simply break down fibre. When this microbial balance is disrupted, the horse becomes less efficient at utilising one of its most abundant and important nutrient sources: forage.
As fibre digestion becomes less efficient, the effects may extend beyond the hindgut itself. Research suggests that disturbances in hindgut fermentation have been associated with changes in manure consistency, increased gas production and general digestive discomfort.
In some horses, prolonged disruption of the hindgut environment may also contribute to changes in body condition, performance and overall gastrointestinal health.
Importantly, these signs are not unique to hindgut acidosis and may occur with a variety of digestive disorders. Rather than serving as a diagnosis itself, hindgut acidosis should be viewed as one potential contributor to digestive disturbances in susceptible horses.
For this reason, persistent gastrointestinal or performance concerns should always be evaluated alongside other possible causes in consultation with a veterinarian.
One of the biggest misconceptions surrounding hindgut health is that starch should always be avoided in the diet. In reality, starch can be an appropriate source of energy for many horses when incorporated into a balanced feeding programme.
The key is not the complete removal of starch, but rather ensuring it is fed in amounts and in a manner that allows the small intestine to digest as much as possible before it reaches the hindgut.
Supporting hindgut health is more about maintaining a stable environment where beneficial microorganisms can continue to thrive. Appropriate forage intake, sensible concentrate (grain) meal sizes, gradual dietary transitions and consistent feeding practices can all help support this balance.
Ultimately, a healthy hindgut is built through consistent nutritional management rather than quick fixes. By supporting the microorganisms responsible for fermenting fibre, owners are also supporting one of the most remarkable and important digestive partnerships within the horse.
Supporting hindgut health involves more than simply choosing the right feed. Practical considerations may include:
Building the diet around forage, the horse's primary source of structural carbohydrates (fibre).
When concentrate (grain) feeding is required, divide the daily ration into multiple smaller meals rather than large single meals whenever practical.
Introduce dietary changes gradually to allow the hindgut microbiome time to adapt.
Increase calories strategically rather than simply increasing starch intake.
Match concentrate type and feeding rate to the horse's workload and individual nutritional requirements.
Remember that supporting hindgut health means feeding the microorganisms as well as the horse.
Ultimately, supporting hindgut health is about creating an environment where the horse, and the microorganisms it depends upon, can thrive.
Supporting hindgut health begins with maximising nutrient digestibility. To help achieve this, Keyflow® utilises steam extrusion and micronisation, advanced feed-processing technologies that improve starch digestibility, among other nutrients, making starch more accessible to digestive and absorptive processes in the small intestine.
By improving pre-caecal starch digestion, less undigested starch is available to reach the hindgut, where it may alter normal microbial fermentation.
Combined with a range of low-starch, high-fibre feeding options, Keyflow® feeding programmes can be tailored to horses with different workloads, body condition goals and digestive needs.
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