Stump break is the same as stump bust except it is commonly done on the farm and to farm animals when they're still young. It's the act of tying a horse, cow, or other farm animal to a tree or stump so they cannot get away and mounting them sexually.
Dad: Boy, we've got a long day of work today. We've gotta stump break 5 colts and that mule today.
Son: But dad, I just fucked 7 calfs yesterday.