Border Collies do not bark while working.
They will bark when they uncertain or are playing (flushing livestock off a fence). You can tell by their tails if they are serious or not. The tails on Border Collies that are serious are down to tucked up under their belly; when their tails are up high they are uncertain or playing.
Try to break the bad habit of what we call "fence rage"; barking and racing along a fence line to get a reaction (or to flush off the fence) animals on the other side of the fence. We use "that'll do" or "leave it" when they are paying inappropriate attention to something followed by a redirection of their attention to something appropriate.
