Any info on comfrey for chickens?

[twandx]

Comfrey grows all over my yard and my year old Black Stars love to eat the leaves. I know it has been an herbal plant that is used by many people, but that awhile ago there were claims made concerning its PA toxicity. Does anyone know any study that might indicate it is harmful for chickens?

 

[Amyable]

I have comfrey growing here too. It's used for healing properties, topically. What is "PA toxicity"?

 

[Sunny the Hippie Chick]

Ive heard you are not supposed to eat it and only use it on your skin..