For a determined lab with time on his/her hands and a hankering to either get out or join his/her neighbors, that wire strung along the bottom will last about 20 minutes...tops! Especially if the dogs have never been separated before, one comes in heat, or if any have separation anxiety towards not being with you. Or, simple boredom. You may also find that separating the dogs leads to "fence fighting". This can sound vicious, the majority of the time it is done either in play or during stressful times such as a female in heat close by (this can also occur among females) or during meals, or when you have one dog out and not the rest.
Our dogs do it when we have one in the backyard and one or two in the airing yard as a game (all females) as they race each other up and down the fence line, stopping on each end to snarl, bark, and growl at each other. They all get along just fine together, by the way. Our males do it when a female is close to coming in (early warning system) and as a dominance type of "strut" past each others kennels when being aired separately.
You will also be surprised at who the instigator will be among your dogs, it may not be the most dominant dog at all!