Milkweed is indeed toxic, but it is also very bitter. I've never had a problem with my free ranging chickens eating it... There are so many other, more delicious, plants for them to eat. I once saw a cottontail rabbit eat three quarters of a large milkweed leaf. I assume it was self-medicating for something. No idea if it harmed it or not, of course, but if this was common behaviour you would think it would impact on the local population and there has been no indication of this. I believe some members of the milkweed family are more toxic than others.