In the world of equine nutrition there seems to be many buzz words bounding around and often without a full understanding of their meaning or implications. The terms Pre and Probiotics are a perfect example of this scenario. Their terms are frequently heard in the field of equine nutrition, and can be easily found printed on many horse feed and supplement packages. As an independent equine nutritionist I hear first-hand from owners that have chosen to use supplements with a pre or probiotic inclusion. But where these have been chosen with a misconception of how they work, or even what conditions they can help to manage or support. Having a correct understanding of an inclusion in a feed or supplement, be that a pre or probiotic or otherwise, can help to ensure ideal products are chosen, but that misguided spend is also avoided.


Probiotics were first defined in 1965 (Lilley & Stillwell,1965) and have since been redefined as “a source of live, naturally occurring micro-organisms” (Yoon & Stern 1995). These micro-organisms include yeast and certain types of bacteria, which when fed have a beneficial impact on the bacterial populations within the hindgut. Feeding of yeast has been shown to have benefits to both digestion and health in horses in clinical and research settings. Their inclusion on packaging can be found listed under ‘zootechnical additives’.


Whereas Prebiotics are not living microorganisms, rather they are a form of indigestible carbohydrates. Prebiotics don’t supply a food source for the horse itself, instead they provide a food source for the beneficial bacteria within the hindgut. Supporting these microorganisms with available prebiotics helps to ensure they have sufficient numbers and stability to maintain hindgut health. The inclusion of certain prebiotics in a horses ration has been shown to limit the negative impact that excessive starch can have on hindgut health and reduce the risk of colic (Geor et al 2013).

Providing a probiotic is supplying the actual beneficial microorganisms to support their population within the hindgut. Providing prebiotics is supplying a nutritional support package for the microorganisms already within the hindgut, helping to maintain a healthy population within the hindgut.

The role of the the gut biome

The bacterial population within the gut is made up of different types of bacteria and protozoa, which collectively are often referred to as the gut biome. It is this biome that is responsible for breaking down the fibre from the diet and converting it into an energy source, in the form of Volitle Fatty Acids (VFA’s), available for the horse. This occurs within the hindgut, primarily the caecum but also the large and small colons. The gut biome has also been shown to synthesise B vitamins, including thiomine and biotin. These are important for the functioning of the nervous system and metabolic pathways and growth especially that of the hoof, respectively. The requirement of the horse for these B vitamins should be achievable with a diet of sufficient fibre and a healthy hindgut biome. Although for horses with poor hoof quality an additional supply of biotin maybe prove beneficial.

Challenges to the gut biome

The efficient conversion of fibre to energy requires a healthy and functioning gut biome. There are many factors that can jeopardize this, including;
Sudden changes to a horses diet, including sources of grass or dried forage.
Bucket feeds exceeding 2kg in weight.
Bucket feeds exceeding 1g/kg body weight/meal in sugar and starch (500g for a 500kg horse).
Certain infections or medications.

These listed are just a few of the factors that can cause, or increase the risk of a disturbance, to the gut biome from occurring. A disturbance in the gut biome can result in the rapid proliferation of a group of bacteria called Amylolytic bacteria, these not only produce the required VFA’s but they also produce Lactic Acid. The production of lactic acid within the hindgut then causes a drop in the pH. Causing the area to become acidic, which is damaging to the preferable bacteria. As these preferable bacteria begin to die off due to the acidic environment, they release endo and exotoxins. These toxins, combined with the lactic acid are irritating and damaging to the gut wall itself. Resulting in an inflammatory response by the gut wall, where the tissue of the gut begins to swell and stretch. This reduces its integrity as a barrier between the contents or digestive tract and the rest of the horses body. At this point a condition called ‘hindgut acidosis’ can occur, increasing the risk of colic, laminitis, a sour disposition and poor performance. Disturbances in the hindgut biome have also been linked with poor appetite, poor coat and hoof quality, higher infection rates, increased flight or spook responses, slower healing rates and even impact on the horses hind limb movement.

Limitations of Pre and Pro biotics

While the benefits of pre and probiotic inclusions have been well documented for supporting horses health and digestion, they do have their limitations. Their inclusion will not compensate for a poorly balanced diet, a diet deficient in its fibre provision, a diet with meals exceeding the capacity of the digestive system, a diet of excessive sugar and starch levels or a diet that is rapidly changed. Also as their mode of action is in supporting a healthy bacterial population within the hindgut the inclusion of pre and probiotics in a diet to support a horse with Equine Gastric Ulcer Syndrome (EGUS) is potentially misguided. Although there is a school of thought that EGUS can occur as a secondary condition due to pain or issues elsewhere in the body. In the situation where there is the potential that EGUS may have developed secondary due to hindgut acidosis, the inclusion of pre and pro biotics in combination with dietary and management changes may be beneficial.

Best Practice

Ensuring correct management and appropriate diets, and that any changes take place over a period of 1-2 weeks are the most efficient management tools to prevent disturbances to the gut biome. However, providing additional support in the form of pre and prebiotics can be an appropriate measure to support the gut biome and increase its ability to cope with changes or challenges. In situations where previous factors have potentially increased the risk of such a disturbance or even hindgut acidosis from occurring the inclusion of pre and probiotics maybe required to re-establish a healthy population of these essential microorganisms.

Geor, R.J., Harris, P.A., & Coenen, M., 2013 Equine Applied and clinical nutrition. Health welfare and performance. Saunders Elsevier, 575-576
Lilley, D.M., & Stillwell, R.H., 1965 Probiotics: Growth promoting factors produced by microorganisms. Science 147, 747-748
Yoon, I.K., Stern, M.D., 1995. Influence of direct fed micobials on ruminal microbial fermentation and performance of ruminants: A review. Asian-Australasian Journal of Animal Sciene 8, 533-555

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