Bugle in Herd bulls
Many people know that a huge herd bull with lots of cows will not come in to a bugle very often. Most of the time he will head the other direction. However herd bulls will respond often to a locator bugle and will tell you where they are going to be bedded.
The key to calling in a herd bull is to get into his bedding grounds and threaten his herd. Start off sounding like a smaller bull, then as he gets more aggressive start cutting off his bugle with your own. I have called in 400 class bulls just by ticking them off.
Remember they are like any other man and if you can make them mad, they will come after you. This experience comes from 15 years of calling bulls in Arizona and New Mexico. I have called in huge bulls in the middle of the early hunts while other hunters are trying to locate them and sneak up on them or swing around and cut them off at the pass.
I have also noticed that most huge herd bulls will not leave their cows for one more, sometimes the only way to get them is with the bugle.