Fresh horse poo isn't really recommended for digging into soil. It needs to be well rotted first, so it is dark, crumbly and just about odour free. I understand fresh horse poo can be put around roses as long as it doesn't come into contact with the stems. I have heard of sheep droppings being used dunked in a water butt as you describe, I'm sure someone will be able to advise on use as a feed.