Animals have pretty good instincts about what they can and can't eat. Our sheep happily leave things like lords-and-ladies alone, as well the odd stray daffs that pop up in the field.
The exception is anything 'wilted', and therefor also anything baled into hay/silage. I think this is where the ragwort problem comes in. No doubt it's the same with yew too.
On the other hand, the wilting technique can be useful for getting things to eat nettles, for example. Which is why my next investment is a scythe.... much nicer than a load of roundup or mcpa.