There must be something you feel IS bad about treated hay, or you wouldn’t be of the mindset you won’t buy it if treated, even though you have no idea what it’s treated with.
Like I said, the only treatment I’ve ever heard of is spraying with proprionic acid, and that’s GOOD thing, to make hay that might not end up good, BE good. Almost always it’s alfalfa, but depending on the baling conditions can be grass hays as well.