Studies have shown it is still viable even after 2 plus years.
This is one I found:
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.