Regarding horse bedding. Walnut is definitely toxic. Owners with a few horses would certainly pick it up if its nearby. Bigger operations would probably get a trailer load periodically. I know the place we keep our horse has the bags trucked in and stored. A new trend is to use a pelletized product. It's more expensive, but is more efficient as the "used" material clumps together making it easier to clean a stall and you end up using less material over time. Don't know about price. Check local feed stores or TSC.
I know this is an older post but for the others. I believe biomass is a solution to field and woodland products. Find a biomass facility in your area and sell the product.