How terribly sad. I heard that botulism doesn't really kill things, that it's not the main cause, it's respiratory failure that does them in. When they did an autopsy did that somehow check for that?
I'd rule out EPM because of the reasons Mango explained.
Although, This scares me because I feed the same grain to one of my geldings now. He's been on it for weeks and hasn't had any issues. Did anything at all come back in the grain sample?
"The greatest pleasure of a dog is that you may make a fool of yourself with
him and not only will he not scold you, but he will make a fool of himself too."