Read this for another chapter of "Mad Stuff":
Customer spots poison stems in salad
Actually, unless I'm totally mistaken, these weeds are pulled out in many countries because they're so dangerous. Cows and other cattle can die if they ingest them as they will distroy their livers. I've been known to pull them out of my back garden, so my dogs wouldn't be tempted to eat them.
Thing is, most animals won't touch them, but then again, some young and inexperienced ones might and die a painful death :(