Why is your horse vomiting?

Horses have a band of muscle around the oesophagus as it enters the stomach. This band works as a one-way valve. Food passes down the oesophagus into the stomach as the valve relaxes, but when it squeezes down on the opening,

it cuts off the passage for food going back up. In horses, this muscle closes so powerfully they simply can't vomit.

Also, the oesophagus meets the stomach at an angle, which enhances the cut-off function when the horse's stomach is bloated with food or gas. Then, the stomach wall pushes against the valve, closing the oesophagus off even more completely.

It's very bad for the horse if material does come up from the stomach. Grass sickness can sometimes cause vomiting - damage to the nervous system means the valve mechanism fails to close. Stomach contents flow into the oesophagus and horses 'vomit' through their nostrils, or risk inhaling stomach contents, causing aspiration pneumonia. Because horses can't vomit, materials have to be evacuated through the gut - this is why they're prone to colic.