I have a GSD also, and there is no way I would let the yorkies loose to run but in my backyard its fenced..When I walk them they are on harness, have you seen how fast these little ones are...Even in the backyard, when I call them in, one or two will come, but I have to always chase Lexie down, its like a game, and she can run circles around me, so I bring in the big guns, and get big Max (GSD) to get her...thank goodness she can't out run him to much, she runs under his legs....