My 4-month-old puppy barks during training. Any suggestions?

My four and half-month-old lab puppy has started barking while training. I don't think that I have accidentally cued this, and it may be frustration or excitement. But I want to make sure it doesn't become a habit, as it will not be acceptable in an obedience ring. I start each session with engagement. I have not noticed whining, and I have not trained a bark on cue. When he barks I tell him no and do not mark until he is quiet and engaged. Any suggestions?

Leerburg's Answer:

I would teach a bark on cue, that way you can teach a quiet on cue as well.

For dogs or puppies that bark when they are excited or trying to get my attention, I use my NO marker (as you do) then stop what I am doing and break engagement from them until they are quiet.