Leylandi is an evergreen, and all evergreens are generally bad for livestock. However, the previous posts are correct- whilst Leylandi isn't very good, it's not really, really poisionous, like yew of rhodenren (sp?). I have seen goats eat some leylandi and they are fine with it- however I wouldn't like to see them eat any more than a few mouthfuls.
Not sure about chickens, but I can't see them being that interested in it in any case.
Beth