So far it has only been his own vomit. BUT after carefully examining his vomit the other day since he had quite a few bouts of it, it was full of chunks of grass (the chunky grass that collects on the mower blade), corn stalks, straw, small rocks and what looked like the remnants of a small mouse.
It was his first day in his dog kennel and he got brave enough to explore the creek after we let him out.