Well, ideally it's when they're actually calves, so in that case we don't need to knock them out entirely. Just get them immobilized and give them something local for the pain. But there's been a case or two where a farmer's left it too long, or originally planned to use an animal for breeding but changed their mind. Adult bulls, it's safer to knock them out entirely, and certainly my preference. But owners sometimes get nervous because of their investment and ask that you use just enough to calm the animal down to reduce the risk of an overdose.

Granted, they're generally not thinking of the risk to the vet, but so it goes.