Since this came back for review, I'll add my 2 cents back to the mix in reference to sisal twine not breaking down. For the longest time, I thought that was the case as well, until talking to a friend years ago, whom has a brother that works in a large kill plant.
He said, sisal in small amounts, breaks down. But large amounts of sisal can form into, what the brother describes as, 'a black (tar)ry-type solid mess'. It has a completely different look to it from plastic twine balls. Take it for what it's worth.