Honestly, if you have great variety, even a potentially dangerous plant is OK in small quantity. I have a "pasture" which is, politely, a biodiverse polyculture. (Basically, I've deliberately seeded it with everything I could get cheaply which might self-propagate in my climate, then let things go). Even when the local wildlife carries bracken fern or soda apple into the area, and it takes root, the chickens and goats either avoid it completely or taste such a tiny amount that it isn't harmful to them.
Of course, my pasture looks like an acre and a half of weeds - so if that's going to be a problem with the neighbors, it may not work for you - but yes, a lot of variety will help provide through multiple seasons, reduce (somewhat) seasonal gorging on treats, and help to ensure that even mildly dangerous flora aren't eaten to the point of concerning symptoms presenting in your flock.