I had walnuts growing in the sheep pastures at the ex's place, and we heard that the leaves were poisonous to them, but the sheep ate the leaves and never had any problems with them (wet or dry). Nor did the goats. We had Icelandic, Soay and Shetland cross sheep. I have never been able to find a reliable source that said that walnut leaves were poisonous to sheep. They used to eat the cambium off the walnut trees as well, and I had to wrap them in woven fence wire.
I have cut and burned a lot of black and white walnut. Good wood. Never had bad issues. For me sycamore has the most ashes of any wood I have burned, and it is on my no collect list. Walnut is not the hottest of woods, especially for its weight. However, I would fetch, cut and burn it. I will take all y'all have out there. 'Specially if its free.
My no collect list of wood species: Tree of
Heaven Hell, Black Cottonwood, Willow (any type), Sycamore, Ginko (bark is poisonous),
White Pine, and I may be allergic to Black Locust.