I couldn't access that article, but the PFAF entry says it's killed people who've drunk milk of cows that have eaten it. So it looks like your chickens are choosing wiselyI guess it's snake root. Ageratina altissima
I also need to get rid of it according to this article.
https://www.wqad.com/article/news/l...ther/526-ae911032-d04b-4360-983e-5783d22b3228
Apparently it's poisonous to mammals but birds can eat the seed and spread it.
ETA it's has a white flower that is one of the last to bloom
Bees like it