It is a slideshow and doesn't load up from the original link, but hopefully that works. It emphasizes that the duration of barbiturates in the environment is UNKNOWN but there are some data showing that effective composting would reduce the presence of residuals within a few months.
Nothing and nobody is going to dig up a properly-buried carcass that's six feet down, in a populated pasture that's kept up with and supervised by humans. Not on my "worry" list.
Just in the last week, here in GA, there were two dogs who died after ingesting euthanasia drugs in a carcass. I don't know if the carcass was being composted or if it had been left out in the open but I'll see if I can find a link to the story. I don't support composting of a horse/animal euthanized with drugs for this reason. I think it is far safer to cremate or do as we do, and have the county bury them six feet deep to prevent them from possibly being dug up.
Don't get confused between my personality & my attitude. My personality is who I am, my attitude depends on who you are.
I also have an article from Penn State on equine composting but can not link it here. If you would like to have it please send me a PM with your email address & I'll try to get it to you as soon as I can.