Saw something in a scrap yard that might work. They were feeding cars into
a huge grinder. Came out the bottom in small pieces that were dumped onto a
conveyer belt. These pieces included plastic, cloth, metal, etc. The belt
was moving fairly quickly at an upward angle. When the pieces got to the
end of the belt, they would fly off in an arc. The heavy metal pieces would
fly farther then the lighter plastics and cloth. Boards were set to deflect
the pieces into different piles. If your manure is dry it should be lighter
then the rocks and this might work.