Old-fashioned tonic or modern-day myth?
Apple cider vinegar (ACV) has been poured into horse feed buckets for decades thought to be a splash of something sharp to ‘clean the system,’ ‘acidify the gut,’ or ‘ward off flies.’ But how much of this is folklore, and how much is backed by science?
A brief history of vinegar in animal care
The use of vinegar as a health tonic dates back thousands of years. Ancient Greek and Roman texts describe its use to ‘purify’ and ‘sharpen’ the body, while early horsemen added it to feed or water to ‘brighten the coat’ and ‘sweeten the stomach.’ Its modern revival in equine management owes more to anecdote than research, but there are kernels of truth behind some traditional claims.
What it actually is
Apple cider vinegar is a fermented liquid produced when crushed apples are exposed to yeast, converting their natural sugars first into alcohol and then, through acetic acid bacteria, into acetic acid, which is the compound responsible for its characteristic tang and antimicrobial activity.
Commercial ACV typically contains 4–6% acetic acid, as well as trace minerals, pectin, and small amounts of polyphenols (plant antioxidants).
What the research says (and doesn’t)
Unlike probiotics, omega-3s, or vitamin E, there is limited controlled research on the use of ACV in horses. Most studies on vinegar’s effects have been conducted in humans or livestock.
Still, here’s what science – and careful inference – suggest:
- Urinary pH: The popular claim that ACV ‘acidifies the urine’ has some merit in other species, but equine kidneys maintain a strong buffering capacity. In horses, it’s unlikely to have a significant impact unless given in large (and potentially unpalatable) doses.
- Digestive health: A small 2017 study showed that acetic acid may slow gastric emptying in humans, improving glycaemic control. Some owners extrapolate this to horses, believing ACV may ‘balance’ stomach acidity or deter ulcers, but no equine studies confirm this, and many suggest the opposite may in fact be true.
- Antimicrobial properties: Acetic acid does inhibit certain bacteria and fungi in vitro, but that doesn’t automatically translate to benefits inside a living gut. Still, it may help maintain hygiene in feed buckets, automatic waterers, or when used as a mild stable disinfectant.
- Insulin sensitivity: A handful of studies in humans and rodents suggest vinegar may modestly improve insulin sensitivity. For easy keepers and those with Equine Metabolic Syndrome, some owners see it as a gentle dietary support, though again, evidence in horses is lacking.
The myths and the margins
The ‘fly deterrent’ claim remains largely anecdotal. Horses fed ACV don’t appear to attract fewer insects in controlled conditions, though vinegar-based sprays may offer mild topical effects when mixed with essential oils.
Similarly, the idea that ACV can ‘dissolve calcium deposits’ or ‘flush toxins’ has no scientific basis and risks oversimplifying the complex chemistry of the equine body.
When it might be useful
ACV may genuinely help in these cases:
- To encourage picky eaters, a light drizzle adds tang and moisture, improving palatability.
- As a natural rinse for buckets or brushes, it offers mild antimicrobial cleaning without harsh residues.
- Mixed with water as a post-work coat rinse, it can help remove sweat residue and restore coat shine.
Risks
Despite its popularity, ACV is not recommended for horses with gastric or hindgut ulcers. Its acidic nature (pH ≈ 2.5–3.5) can increase stomach acidity, particularly in the sensitive non-glandular region of the equine stomach where most ulcers occur. Rather than neutralising acid, vinegar can heighten irritation or discomfort, especially in horses already prone to gastric pain or girthiness.
Veterinary nutritionists caution that while small, diluted amounts of ACV are unlikely to harm healthy horses, there is no peer-reviewed evidence showing that it prevents or heals ulcers. In fact, early reports and expert commentary suggest it may worsen gastric inflammation in susceptible individuals.
If you suspect your horse may have ulcers – or if you’re already managing a diagnosed case – avoid adding ACV until advised by your vet. Focus instead on proven management strategies such as ad-lib forage, reduced starch intake, turnout, and veterinary-guided treatment.
Ultimately, for ulcer-prone horses, ACV could do more harm than good.
Did you know?
In competitive sporthorse populations, 6 to 9 out of every 10 horses will develop some form of gastric ulcer during their careers.
Feeding according to facts
Like many old stable remedies, apple cider vinegar occupies a grey area between tradition and science. It’s unlikely to transform health or performance, but equally, it’s not without its small merits when used wisely. However, it should be added with great caution in horses with suspected ulcers, and as the rate of ulcers in sporthorses is purported to be much higher than we recognise, this may rule out its use in many of our equines.
Did you know?
Ulcers are extremely common in performance horses, so much so that equine gastric ulcer syndrome is often described as a ‘disease of domestication and athleticism.’
In summary
It’s not a miracle, but not a myth either. It’s an ingredient that keeps us asking better questions, and that’s exactly what makes it intriguing.