It is the conditions that some of these crops may be growing in, NYREDS.
Over-fertilizing with nitrogen, flooding, drought and other things may cause almost any plant to become toxic. For grazing animals, eating a heavy load of these "altered" forage plants can be a serious problem.
This is why
Cornell says, "IMPORTANT:
Just because something is on the poisonous plants list doesn't mean it can't be a good food or feed, and just because it is absent from the list doesn't mean it is safe . . . "
Most chickens in this country never see a plant. Even those that are allowed to free-range in our backyards don't get a whole lot - their diet is still, probably, 70% or 80% commercial feed. They aren't quite so vulnerable to "plants that have gone bad" nor to being forced to eat contaminated hay or silage or go hungry - those kinds of things.
Of course, their commercial feed can and does sometime become contaminated with things like toxic weed seeds. Usually, that results in meat or egg production problems rather than outright illness or death. I guess, breeder farms even have problems with egg fertility that they blame on weed seed contamination.
Mostly, a chicken isn't likely to get itself into too much trouble - but, that's just my opinion. I still wouldn't even think about tossing potato vines into their pen.
Steve