well to be honest, im not so clean on the ybs, they only get cleaned out once a week, till yb racing starts, i just dont have time, spending the time i do have on the o/bs. And touch wood, i dont think ive ever had it, had the odd bird go back, but never the whole lot.
a few of the top guys round here say they get it every year, And top the fed a fair few times, so they must come round :-)