Hi
Apparently in small amounts, it's OK. Hens do like the taste of Comfrey but it does contain Pyrrolizidine alkaloid! Which are liver toxins.
It is said that the damage from the toxin is built up over time and some of the toxins can be transferred eggs if eaten in large quantities over a prolonged period. In small quantities it's probably safe, but due to this I would advise against feeding comfrey.
If they are free ranging for short periods and eat small amounts here and there I wouldn't worry to much.
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