"Mr. Bob," they call him: a nice combination of deference and affection. Both have been amply earned, over the six decades since Robert Hall Jr. established a feed mill that has attained global reach while remaining an unmistakably local, Bluegrass institution. But as befits a man who sleeps in the room where he was born, 94 years ago, the story has roots extending to another era altogether; to a boyhood when Hall--presumably then plain "Master Bob"--saw for himself the difference that good diet could make to livestock. "I was nine...