I like to feed my neighbours chickens & ducks the weeds from my veg patch but I'm rather unsure about which ones are safe & nutritious and which are toxic & should be avoided. I've scanned the info available on the web & have come up with some conflicting reports. One long list of weeds to be avoided even included Chenopodium sp which is also known as Fat Hen; supposedly it may be too high in nitrates. Nettles too may give them dermatitus etc. This long list of so-called toxic weeds seems to have been copied & posted on a range of sites.
Unfortunately I don't particularly trust these chooks to know what's good for them as the first thing I saw one pecking at was a piece of Nightshade which had got into my weed bucket by mistake & which I hurriedly had to remove.
I don't believe they've had much access to a variety of greenery at their first residence & now they are situated on a cleared piece of land mainly under trees, so they are probably pecking at whatever they can.
We live in the south of the UK and I would welcome some comments from permies with experience in these matters!