A foal is born with a digestive system that has to make one of the biggest dietary journeys in the animal world, from mare's milk to a diet based on fibre, in a matter of months. Along the way it builds the microbial population that will ferment its forage for the rest of its life. This page looks at how that population develops, what the research says about supplements in foals, and how owners and breeders can support youngstock through weaning and growth. It is editorial guidance from Horse Probiotics, and for foals more than any other group your vet should make the decisions.
How a foal's gut develops
Research suggests the foal's gut microbiota develops quickly. A study published in Scientific Reports followed ten foals from birth to after weaning and found that the composition of their gut bacteria shifted most between days 20 and 50, when fibre-fermenting species became more dominant. After about day 50 the authors found no significant further changes in species abundance, and they concluded that relative stability was reached within 50 days of birth.
The same researchers note that foals eat their mothers' droppings, a behaviour called coprophagy, almost from birth and most noticeably in the first two months. They suggest this may help to stabilise the developing microbiota by supplying a steady source of microbes, although that remains a hypothesis rather than a tested finding. Owners are sometimes alarmed by it, but it is a normal part of foal behaviour.
Diarrhoea in young foals
Loose droppings are common in young foals. In a randomised trial on breeding farms, published in the Journal of Veterinary Internal Medicine, diarrhoea occurred in 59% of the 72 foals studied, and foals aged 8 to 15 days were at the highest risk. Many cases are mild, but foals have small fluid reserves and can deteriorate quickly. Diarrhoea in a foal that is dull, not suckling or has a temperature is an emergency.
What research says about probiotics in foals
Because diarrhoea is so common, probiotics have been tested as a way to prevent it. The results are a reason for caution. In the trial above, foals were given either a multi-strain probiotic or a placebo for three weeks. There was no difference in the overall rate of diarrhoea, but foals in the probiotic group were more likely to develop diarrhoea needing veterinary treatment: 12 of the 18 foals that needed treatment had received the probiotic, compared with 6 in the placebo group. The authors concluded there was no benefit from the three-week course and that potential adverse effects were noted. Quality testing also found far fewer viable bacteria in the sachets than the label claimed.
An earlier review by the same research group in the Journal of Veterinary Internal Medicine described another foal trial in which a Lactobacillus pentosus product was associated with more diarrhoea and more need for veterinary intervention. The review's authors called for larger controlled studies of different strains and products before conclusions could be drawn, and the authors of the later trial advised that commercial products without published safety and efficacy data should be approached with caution.
None of this proves that every probiotic is unsuitable for every foal. It does mean that giving a probiotic to a young foal is a veterinary decision, not something to try from the feed room shelf. Our equine probiotics guide explains why strain, dose and quality control matter so much.
Supporting youngstock through weaning
A study in Frontiers in Physiology describes weaning as one of the most stressful events in a horse's life. The researchers found that separation from the mare reshaped the gut microbiota in every foal they studied, and that certain microbial profiles after weaning were associated with lower salivary cortisol and better growth. The authors are clear that this shows an association rather than cause and effect. By contrast, the smaller Scientific Reports study found weaning did not have a major impact on microbial composition, which shows how much there is still to learn.
Practical weaning plans vary between studs, and your vet and breeder will know what suits your youngstock. Common principles include making sure the foal is already eating forage and any hard feed confidently before weaning, keeping familiar companions together, avoiding stacking other changes such as a yard move on top of weaning, and keeping forage and clean water easily available.
Growing youngstock
Once weaned, youngstock need a diet that supports steady rather than rapid growth. Good forage and grazing remain the foundation, with the ration balanced for vitamins, minerals and protein. Our vitamins and minerals guide and the comparison of a feed balancer and a vitamin and mineral supplement explain the options. Worm control for youngstock differs from adult horses, so ask your vet to plan a programme that includes faecal worm egg counts.
Youngsters face several firsts in their first two years: first handling, first farrier visits, first vaccinations and often a first move to a new yard or field group. Each of these is manageable on its own, and spacing them out where possible gives the gut and the horse time to settle between changes. Keep forage and routine as consistent as you can through each one, and record weight and condition regularly so that a slowdown in growth or a drop in condition is noticed early. A weigh tape is not perfectly accurate for youngstock, but used consistently it shows the trend.
Where supplements may or may not help
For weanlings and yearlings, the evidence on gut supplements is no stronger than in adult horses, and in most studies it is weaker. If your vet agrees a gut supplement is worth trying around weaning or a change of yard, choose a product that names its strains and inclusion levels, and use our supplement buying guide to compare labels. The reviews hub notes where a manufacturer states that a product is suitable for youngstock.
Signs to watch
- Diarrhoea, especially if the foal is dull or not suckling
- Reduced suckling or a full, uncomfortable udder on the mare
- Dullness, lethargy or a raised temperature
- Signs of abdominal pain such as rolling, lying on the back or straining
- Poor growth or loss of condition after weaning