The ones I have assisted with went quick and easy. That is the goal which ever way it is done. My vet gives an agent that puts the horse to sleep like the stuff they give you in surgery, then he gives an agent that stops the heart. There is no poison to contaminate anything.
So I wanted to research to be sure, but:
Veterinary use Benzodiazepines
are used in veterinary practice in the treatment of various disorders and conditions..... for induction of anesthesia and as adjuncts to anesthesia
Funny thing about Benzodiazepines (and the thing I wanted to check for sure: "On June 25, 2009 American recording artist and entertainer Michael Jackson died of acute propofol and benzodiazepine intoxication after suffering a cardiac arrest at his home "
Both courtesy of Wiki by the way, but we researched Michael Jackson as part of Death Investigations in College. He was taking Lorazepam, which I've been prescribed in the past. If you've read the warning statement that comes with that drug, you'd probably be a little more hesitant to let it sit in the field for 'Nature" and the wild animals to dispose of. I'm pretty sure it IS poisonous considering it can cause liver or kidney failure, or:
" Other adverse effects include confusion, ataxia, anterograde amnesia and hangover effects. With long-term use of benzodiazepines, it is unclear whether cognitive impairments fully return to normal after cessation of therapy; cognitive deficits persist for at least six months after withdrawal, but longer than six months may be required for recovery of cognitive function. Lorazepam appears to have more profound adverse effects on memory than other benzodiazepines; it impairs both explicit and implicit memory"
Due to it's original intended purpose, it also causes: "sedative/hypnotic, muscle relaxant, anxiolytic, amnesic, and anticonvulsant"...
It should not be taken by the pregnant or elderly, and seeing as how no one would be monitoring who ate the dead horse.....