Arsenic? Geeze. Old Tommy Woodcock's been gone for years so they can't blame him.
Seriously though, that's so old school. Makes you wonder where it comes from. Maybe some people are feeding Roxarsone or something like that? It's an arsenic compound that's fed...or at least it used to be fed....to young shed grown meat chickens in order to maximise their feed conversion. Either that...or the horse has been having a regular whack at some chook feed Strange.