No, a horse practically cannot vomit. Its anatomy makes the reflux of stomach contents towards the esophagus extremely difficult, if not impossible in the vast majority of cases, unlike in dogs or humans.
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An almost impassable anatomical barrier
In the horse, the esophagus joins the stomach at a very closed angle, almost at a right angle, which creates a sort of natural valve. At this location is a particularly powerful and muscular cardiac sphincter, capable of withstanding considerable internal pressure. When the stomach distends under the effect of food overload or excessive fermentation, this sphincter contracts even more instead of giving way, preventing any return of gastric contents to the mouth.
Added to this is an almost absent gag reflex: the nerve centers which, in other species, coordinate abdominal contractions and the opening of the cardia to expel the contents of the stomach are poorly functional in the horse. The wall of the equine stomach is itself relatively fragile and not very extensible: its volume, of the order of only 8 to 15 liters for a 500 kg animal, is very small compared to the size of the digestive tract as a whole.
A vital emergency rather than a protective mechanism
This inability transforms a simple excess of food into a potentially fatal situation: if the stomach continues to distend without being able to empty itself forward or evacuate by natural means, the pressure can go so far as to cause gastric rupture, almost always fatal. This is why a suspicion of overload, colic or abdominal distention in a horse must always be considered as a veterinary emergency to be treated immediately, without waiting for possible vomiting which will not come.
