I can't tell you about the haylage, though if it's tiny amounts the chances of a sheep eating a dangerous dose I would have thought was small.
But I and my Dad have both had problems with rats carrying poison blocks and then other animals getting them, so we've both switched to loose grain, which has to be eaten in situ so can't be moved about. Of course the animal can still go elsewhere and die, but that won't affect herbivores at least.