I have a one year old, male GP. He is an excellent dog except for the fact that he has been chasing semi trucks that drive by our house. I realize I could pen him up to stop it, but that would make it hard for him to protect my free ranging flock. I have caught him doing it twice, but I know he does it during the day when I'm not home. Each time I caught him doing it, I really scolded him and penned him up for a day or two. That has not worked. Any suggestions?