A term widely used in various rural community clubs and industries (such as 4H , Dairy Farmer's, etc.): when one or more individuals participate in anal sex while in a barn and shit finds it's way onto the hay...you have a "Haystack Brownie".
(figure of speech) referring to something small that is difficult to find in a large space, it quite literally would be like searching for a needle in a haystack, obviously nigh impossible.