Probiotics are among the most widely fed horse supplements in the UK, and among the most misunderstood. This guide explains what an equine probiotic is, how it is thought to work in the hindgut, which organisms are used and what the research does and does not support. It is written to help you decide whether a probiotic has a sensible place in your horse's routine, and how to judge a product if it does.
What is an equine probiotic?
The International Scientific Association for Probiotics and Prebiotics (ISAPP) defines probiotics as live microorganisms that, when given in adequate amounts, confer a health benefit on the host. Two parts of that definition matter for horse owners. The organisms must be alive when the horse eats them, and they must be given in a quantity that has been shown to do something. A product containing dead organisms, or too few live ones, does not meet the definition however it is described on the tub.
In UK feed law there is no legal category called a supplement. As the British Equestrian Trade Association (BETA) explains in its guide to labels, most products sold as supplements are complementary feeds, and technical ingredients such as live yeast appear under the "Additives" heading with an inclusion rate. Live yeast authorised for horses is classed as a zootechnical additive in the functional group of digestibility enhancers. That is why you will often see a strain code and a count in colony forming units (CFU) per kilogram rather than a simple ingredient name.
How probiotics are thought to work in the hindgut
The horse is a hindgut fermenter. Grass, hay and haylage are broken down largely by microbes in the caecum and colon, which turn fibre into volatile fatty acids that the horse absorbs as energy. A 2022 review by Chaucheyras-Durand and colleagues notes that horses rely on this microbial fermentation for more than half of their maintenance energy. When fibre-digesting groups dominate, lactate stays low and the hindgut environment stays stable.
That balance can shift. The same review describes how high-starch diets lower hindgut pH, reduce fibre-degrading activity and encourage lactate-producing bacteria, and how even a two-hour road journey has been linked to measurable changes in faecal bacteria. Our guide to hindgut health covers these risk factors in more detail.
Proposed mechanisms
Researchers have suggested several ways a probiotic might help. Live yeast may scavenge oxygen in the gut, creating conditions that suit the strictly anaerobic bacteria that digest fibre. Some yeasts and bacteria may help steady pH when starch reaches the hindgut, and others may compete with unwanted organisms for space and nutrients. These ideas are plausible and some have laboratory support, but the review authors are clear that more work in live horses is still needed.
Probiotics do not usually stay
One point surprises many owners. A 2014 review in the Journal of Veterinary Internal Medicine by Schoster, Weese and Guardabassi found that probiotic strains were generally shed for only days after dosing. Most probiotics therefore pass through rather than settling permanently, which is why products are fed daily for as long as support is wanted.
Which organisms are used?
Live yeast (Saccharomyces cerevisiae)
Live brewer's yeast, Saccharomyces cerevisiae, is the most common probiotic organism in UK horse products. A 2022 review in the journal Animals by Perricone and colleagues describes it as the yeast most used in horse nutrition, usually with the aim of helping horses on poor-quality forage or high-starch diets get more from their feed. Strains are authorised individually and the conditions are strain-specific. When one strain's authorisation for horses came up for renewal, for example, the European Food Safety Authority (EFSA) found it safe but could not conclude that a proposed lower minimum dose was effective.
Lactic acid bacteria and others
Some products contain lactic acid bacteria such as Lactobacillus or Enterococcus species, sometimes alongside Bifidobacterium or Bacillus strains. These are familiar from human probiotics, but the Chaucheyras-Durand review cautions that their role in the horse may be different, and that any benefit may be more relevant to foals or to the upper gut than to the hindgut. Many products also combine a probiotic with prebiotic fibres, which our guide to prebiotics and postbiotics explains.
What the evidence shows, and where it is limited
The fair summary is that the evidence is mixed and depends heavily on the organism, the dose and the horse. Independent reviews reach broadly similar conclusions.
| Question | What reviews report |
|---|---|
| Does live yeast improve fibre digestion? | Some trials show better digestibility of fibre fractions; others show no effect. |
| Does live yeast help on high-starch diets? | Limited evidence of higher caecal pH and lower lactate in some high-concentrate trials, not seen on high-forage diets. |
| Do probiotics help adult horses with colitis? | Evidence is weak and results are inconsistent. |
| Are probiotics suitable for foals? | Results are mixed; one product was linked to more diarrhoea, so caution is advised. |
| Do products contain what the label says? | Independent testing has repeatedly found gaps between label claims and contents. |
Digestion and the hindgut
The Perricone review found that some trials report better digestibility of dry matter and fibre with live yeast, while others show no effect. In horses on high-concentrate diets, one study reported higher caecal pH and lower caecal and colonic lactate, but similar benefits were not seen on high-forage diets. The authors also observed that the individual horse often has more influence on faecal microbes than the yeast does, and that study designs, doses and diets vary so widely that results are hard to combine.
Clinical conditions
For disease the evidence is weaker. Schoster and colleagues concluded that the overall evidence for probiotics in controlling disease in horses is weak, with inconsistent results in enterocolitis and no reduction in Salmonella shedding in the trials they reviewed. An ISAPP article by Professor Kelly Swanson, updated in 2025, also describes the evidence for horses with enterocolitis as weak and advises caution in foals, after one Lactobacillus product was associated with diarrhoea and more veterinary treatment.
What is actually in the tub
Quality is a separate concern. In a 2021 study from Washington State University, reported by EquiManagement, none of 11 commercial equine probiotics fully met their label claims for the number or viability of organisms, and only three were consistent between batches. The Schoster review found similar problems in North American products. These studies did not test UK products specifically, but they show why a named strain, a stated count and careful storage matter.
When owners consider a probiotic
Most owners use probiotics around situations known to unsettle the gut: a change of forage, the move to spring grass or a new yard, travel and competition, or a spell of box rest. Some feed them to horses on higher-starch diets for hard work, or to older horses that struggle to hold condition. These uses fit the proposed mechanisms, although trial evidence for each specific situation is limited. A probiotic is never a substitute for gradual feed changes, plenty of forage and a steady routine.
How to choose an equine probiotic
A few questions separate a carefully made product from a vague one. They apply equally to powders, pellets and pastes.
- Named organism and strain. The label should identify the organism and, for authorised additives, the strain or identification number.
- Stated count and daily dose. A CFU figure per kilogram, together with a clear feeding rate, lets you work out the daily intake.
- Format. Powders, pellets and pastes suit different routines; our comparison of probiotic powder, pellets and paste sets out the trade-offs.
- Evidence for that strain. Ask whether the manufacturer can point to published work on the strain and dose it uses.
- Competition status. If you compete, check whether the company is accredited under the BETA NOPS scheme and check your governing body's rules.
- Cost per day. Divide the price by the number of days a pack lasts at the full feeding rate.
Our full checklist is in how to choose a horse supplement, and our desk reviews apply the same questions to individual products. For a side-by-side view, see the best horse probiotics in the UK.
Storage and shelf life
Live organisms are fragile. Heat, moisture and time all reduce viability, and the Washington State researchers suggested that manufacturing, transport and storage may all contribute to the gap between label and contents. Keep tubs sealed, cool and dry, away from direct sunlight and out of hot lorries and tack rooms in summer. Check the best before date and batch number, which BETA says should appear on all packaging, and follow any guidance on use after opening. Our guide on how to feed a probiotic covers introduction, storage and record keeping in practice.