I would cow call if I was ready to and if the bull was unaware of me and was just moving out because it was nervous or spooked by other game.

But really, within 100 yards I'd normally just put a bullet through his lungs while he was on the run. Have done than many times. Within 100 yards it should not be difficult for a skilled shot. But I realize very few hunters practise moving shots any more, and if you are not confident hitting moving game, calling or even whistling can sometimes make them pause long enough to settle your crosshairs.