Originally Posted by loosie
But three different vets asserted to me that horses *could* vomit - as explained, it's just very difficult & therefore rare for them.
I think what is causing debate here is technical terms. Vomiting actually has a very specific definition which is different from regurgitating. Vomiting implies a distinct set of steps and involves contents of the stomach and sometimes small intestine refluxing back up into the esophagus and out the mouth. If the contents have not yet passed into the stomach, they can be regurgitated out of the esophagus and hence not "technically" vomited but most vets will refer to it as such. So, you're both right!
To the OP though, feed aversion can be a sign of something wrong with the grain (as another poster said). It can be mixing error or a mycotoxin, any number of things. To be safe, what I would do is ditch the grain and get a new bag. Yes, it's throwing money away but could save you a ton down the road, even the lives of your horses!!