Most of what a horse eats is fibre, and the horse cannot break most of that fibre down with its own digestive enzymes. The work falls to the caecum and colon, together known as the hindgut, where a dense population of bacteria, fungi and other microbes ferments grass, hay and haylage. This page explains how that system works, why it is sensitive to change and what owners can sensibly do to keep it steady. It is editorial guidance from Horse Probiotics rather than veterinary advice, so involve your vet in any concern about your own horse.
What the hindgut does
The hindgut is a large fermentation chamber. Microbes there break down the cellulose and hemicellulose in plant cell walls and produce volatile fatty acids, mainly acetate, propionate and butyrate, which the horse absorbs and uses as fuel. A 2019 review of equine microbiome research in Animal Microbiome describes these microbes as providing a substantial proportion of the horse's daily energy needs. In practical terms, a horse living mainly on forage is powered to a large degree by its gut microbes.
That arrangement works well when the diet is steady and fibrous. The microbial population adapts to what it is given, and different feeds favour different groups of organisms. A study by Dougal and colleagues found that adding a starch-rich or oil-rich feed to a hay diet changed the faecal bacterial community, and that the hay-only diet had the largest shared core of bacteria across the horses studied. The hindgut is always responding to the ration in front of it, which is why consistency matters so much.
What upsets the hindgut
Starch reaching the caecum
Cereal starch is meant to be digested in the small intestine. When large meals are fed, some starch can pass through undigested and reach the hindgut, where it is fermented quickly. In a study published in the Journal of Animal Science, horses moved abruptly onto a higher-starch concentrate ration had lower caecal pH over the following week than horses on a lower-starch ration, along with shifts in which bacterial families were most abundant. Two horses on the higher-starch ration showed signs consistent with lactic acidosis. It was a small study, but it shows why the size of each hard feed matters.
Sudden changes
A systematic review by researchers at the University of Nottingham, published in PLOS ONE, found that changes in feed and recent changes in housing had the strongest evidence among management-related risk factors for colic. The authors concluded that avoiding changes, or introducing them gradually, should be a key part of preventative management. A new batch of hay, a move onto spring grass and a switch from living out to being stabled all count as changes from the gut's point of view.
Long gaps, stress and medication
Horses evolved to eat little and often for much of the day, and long periods without forage work against that design. Travel, competition, fasting and some medicines, particularly antibiotics, are also recognised influences on the gut microbiota. The Animal Microbiome review notes that transport, fasting and exercise each produce measurable shifts in faecal bacteria, although what those shifts mean for an individual horse is not always clear.
Management comes first
No supplement can make up for a ration that works against the hindgut. These are the foundations most vets and nutritionists start with, and they cost far less than any tub on the shelf.
- Forage as the base of the diet. The European College of Equine Internal Medicine consensus statement on gastric ulcers suggests that horses fed hay receive at least 1.5 kg of dry matter per 100 kg of bodyweight each day. For a good-doer, the amount and type of forage may need adjusting for weight control with advice from your vet or a nutritionist. Our article on how much forage a horse needs walks through the calculation.
- Small hard feeds. Split concentrate rations into several meals rather than one or two large ones, and use fibre and oil to supply extra energy where your horse needs it.
- Gradual change. Blending old and new forage over around two weeks is a widely used approach, and spring grazing is best built up a little at a time.
- Constant clean water, with extra checks in freezing weather.
- Movement. Turnout and regular exercise support normal gut motility.
- Teeth and worms. Redwings advises dental checks at least once a year and a programme of faecal worm egg counts with targeted worming.
Where supplements may or may not help
Gut supplements for horses usually contain some combination of live yeast, prebiotic fibres, yeast cell wall fractions and buffers. Our guide to gut balancers explains each ingredient group in detail. The honest summary is that the evidence is mixed, and it varies by ingredient.
A 2022 review of live yeast (Saccharomyces cerevisiae) in Animals found that results on digestibility and gut microbes were contradictory and highly variable, depending on the strain, dose, diet and horses studied. Effects on lactate-producing and lactate-using bacteria were more consistent in horses on high-concentrate diets than in horses on forage-based diets. That suggests yeast is most likely to be relevant where a horse needs a substantial hard feed, and less so for a good-doer living on hay.
For probiotic bacteria the picture is weaker still. A review in the Journal of Veterinary Internal Medicine concluded that evidence for using probiotics to control disease in horses is weak. It also reported quality problems, with only 2 of 13 veterinary and human probiotic products tested containing the labelled organism at the stated concentration. Probiotics appear to be safe in healthy adult horses, but safety is not the same as proven benefit.
In practice, a gut supplement is easiest to justify as support during a predictable period of pressure, such as a change of forage, used alongside the management steps above. It is hardest to justify as a substitute for them. If you are weighing up products, our supplement buying guide and the best horse probiotics comparison explain what to look for on a label, and the reviews hub covers individual products using the manufacturers' published information.
Hindgut or stomach?
Owners sometimes use "gut supplement" to mean anything digestive, but the stomach and the hindgut are very different environments. Gastric ulcers can only be confirmed by gastroscopy, and the products marketed for the stomach work in a different way from hindgut products. Our comparison of a gut balancer and a gastric supplement sets out the differences.
Signs to watch
Hindgut disruption tends to show gradually rather than all at once. If you notice any of the following, keep a simple daily diary so that you and your vet can see a pattern rather than a single bad day.
- Droppings looser than is normal for your horse, or liquid passed before or after droppings, often called free faecal water
- Fewer droppings, or drier droppings than usual
- Picking at feed or forage, or leaving a usual meal
- Gradual loss of condition despite adequate feeding
- Dullness, reduced energy or a change in attitude to work