Feeding sport horses for a more stable microbiome

As athletes, sport horses have additional energy and nutritional requirements, with racehorses and 3-day eventers requiring up to two times more energy compared to leisure horses. Feeding forage like hay and haylage alone cannot meet these needs, so it is necessary to feed concentrates or hard feed to make up for shortfalls. Traditionally, feedstuffs usually contain high levels of starch due to the grains used within the formulation to provide the additional energy that high performing equines needed, so it’s important to be aware of the impact this can have on the microbiome.

More recently, many low starch alternatives have become available within the market that can equally provide enough additional energy through the utilisation of digestible fibre sources and precision formulation. However, despite our best efforts to carefully manage starch intakes via these lower starch alternatives, due to the additional energy requirements and more than one feed being fed, an accumulation of starch from many sources can present a significant challenge to digestion if not managed carefully. 

Understanding and measuring starch

It is widely accepted that equine management and diet remain at odds with the natural physiology of the equine gastrointestinal tract and so it is difficult to ensure the microbiome remains in a harmonious state. Even when starch is fed within the suggested limits, it can still induce adverse microbial changes within the hindgut (caecum and colon)1. Regardless of the type of feed you are providing, it’s essential to stay within the manufacturer’s recommended feeding rates.

To truly know how much starch is in your horse’s daily diet, any grain or concentrates fed should be weighed and starch content calculated, especially if feeding more than one type of feed. You can calculate this with the starch content amount that is printed on the feed label(s) and applying that to the daily weight of feed being given. For example, if a horse receives 2kg (recommended feeding rate) of Feed A which has 8% starch and 2kg (recommended feeding rate) of hard feed B which has 20% starch every day, then that horse is consuming 560g of starch daily (280g in both morning and evening feed).

The absolute maximum that should be fed daily is 1.5g of starch per kilogram of body weight2, but ideally horses should consume much lower levels than this. For example, a 500kg horse that is fed twice a day should receive no more than 500g per meal (1kg per day).

What happens when we exceed the recommended levels of starch?

Starch should be digested within the foregut (stomach and small intestine) of the horse, however when too much is fed, the starch digesting enzyme amylase cannot cope letting undigested starch bypass the foregut. Leftover, undigested starch makes its way to the hindgut, where it is quickly digested by lactic-acid-producing microbes, who proliferate rapidly and create more lactic acid to make the hindgut more acidic, killing off beneficial fibrolytic microbes. This in turn alters the fermentation patterns within the hindgut resulting in key beneficial metabolites no longer being produced.

As the conditions within the hindgut continue to deteriorate, a significant drop in hindgut pH occurs, resulting in hindgut acidosis. In addition to decreasing how much feed is digested, this triggers an array of metabolic and behavioural problems, in addition to an unsurprising drop in performance as the body cannot support optimum metabolic function.  

Additionally, it’s important to remember that all starch sources are not equal when it comes to equine nutrition.

Focus on fibre first

It is no surprise the industry as a whole is re-thinking how best to feed high-performing equine athletes.

Each feed stuff, grain or fibre, can supply different levels of energy due to different rates of fermentability. Some grains ferment in the hindgut more quickly than others, therefore the more rapidly fermentable starch sources such as flaked maize are more likely to cause greater degrees of microbial dysbiosis (an imbalance of microbial species that can cause a wide ranges of issues in horses). Equally, compared to hay the materials like haylage, sugar beet pulp and soya hulls ferment more slowly, meaning they supply a greater amount of energy with little to no negative impact on the hindgut microbiome.

Recent interesting research3 reviewed work compared traditional high starch diets (e.g. oats, barley and maize) to those high in fibre (e.g. haylage, sugar beet pulp and soya hulls) when fed to elite performing racehorses.

Compelling results showed that horses fed high-fibre diets were better able to handle the mental and physical demands of high performance. The all-fibre diets resulted in a higher number of OTUs, making up nearly 15% of the overall microbiome, instead of 5% for a diet that contained starch. Put more simply, the high-fibre diet resulted in a more stable, optimally balanced microbiome with enhanced metabolite production. This also resulted in a healthy environment for beneficial microbes to flourish (reducing lactic acid build-up) and showed faster recovery rates in the horses.

Overall, research is suggesting that feeding more high energy yielding, fibrous food sources would substantially improve the welfare of the performance horse by supporting a more stable microbiome.

Microbial sequencing work proves that Actisaf® supports fibre digesting microbes and in turn enhances the fibre digesting capacity of the hindgut, further enhancing fibre digestion and beneficial metabolite production for optimum metabolic function and energy supply.



1 Bulmer, Louise S. (2020) High-starch diets increase behavioural reactivity, alter hindgut microbiota and brain neurochemistry in horses. PhD thesis, University of Glasgow.

2 Luthersson et al. (2010) Risk factors associated with equine gastric ulceration syndrome (EGUS) in 201 horses in Denmark. Equine Veterinary Journal. 41(7), pp. 625-630.

3 Richardson, K., and Murray, J.A.M.D. (2016) Fiber for performance horses: a review. Journal of Equine Veterinary Science, 46, pp. 31-39.