I'm not sure I understand why you want to cull the younger ones to start over,especially if they are laying. Also,If you intend on starting over, teaching the dogs to kill,would make them want to try getting into your pen even more. I had a dog, that got out of its kennel. Actually squeezed through the door, and get into my family members flock in the back yard. Killed 5,would have killed them all if the rest werent in the shed on top other cages. I rehomed him. This dog was never by chickens,only in the kennel. Training them to kill? Doesn't sound smart to me.