I've had good luck using a pronghorn decoy once they really get into the rut. If you're able to sneak to within their bubble, like less than 100 yards, and pop up with the decoy unexpectedly, they have a tendency to come in fast and check it out. If you try to walk up to them from across the prairie holding it in front of you, not so much.. Or if you can figure out where they're headed and set the decoy up in front of them, I've seen them come into that also but it takes a lot of luck for them to come the right way. I haven't tried the moo cow yet, if only it came in black..
And like Bydand said, I probably wouldn't use it during rifle season.