alpha dog

Question
We own a shih-tzu mix male, who is about 11 or 12 now (we rescued him) and is very docile and submissive. We just acquired another rescue, a lab/German shepherd/Australian shepherd mix, who is female. She came to us two weeks ago, very meek and shy with men, and appeared to have no interest in our other dog, Shep. But now, Molly, is displaying some agressive behavior towards Shep, such as not letting him come upstairs, herding him around the house, etc. They yelped and barked at each other twice last night. Should I be worried that Molly will actually hurt Shep? My vet told me to let Molly be the Alpha, and we just want to make sure both dogs remain healthy. I do feed Molly first (after pretending to eat some myself), pet her first, etc. My vet also said to stay out of the room if Molly exhibits this behavior and let them work it out themselves. True? Any advice?

Answer
Your veterinarian has given you very poor advice without any apparent evaluation of temperament, observing interaction between the two dogs, etc.  Veterinarians don't automatically qualify as behavior experts.

Molly needs to be put on long, strong house leash (training leash) and her accessibility to your entire house needs to be stopped immediately.  As the product of an apparent herding/guarding mix, she is most likely attempting to control your older dog BUT HE WAS THERE FIRST, this is HIS territory, and he is far too elderly to attempt to protect it.  Molly has apparently not been socialized to other dogs and may not have lived with another dog in her former "home"; further, her ignoring the older dog is not a good sign, as this is a display of dominance and/or lack of socialization (perhaps even fear); the new dog member of the household normally "closets" him/herself for the first few days and observes the resident dogs and humans, attempting to find a place for itself in the hierarchy of the household.  From your description, Molly does not seem to have behaved in a normal manner in this regard.  You have no idea of her past life experience and this may very well have involved dog to dog aggression.  You need to contain Molly on house leash, institute an immediate regimen of positive reinforcement training in order to teach her ONE solid behavior ("SIT" but use a different word) and, after she demonstrates compliance to this command 100% of the time (about two weeks or so, indoors only), put her on a Nothing In Life Is Free regimen: she earns EVERYTHING (being fed, being taken in/out, being petted, etc.)  for at least six months.  This will put you clearly in charge and demote her psychologically.  DO NOT LET THESE DOGS "WORK IT OUT" BETWEEN THEM; your elderly Shih Tsu mix is no match for this bitch and she can hurt him seriously.  Contain her in a small room (kitchen) away from Shep when you are not at home; do not allow her run of the house for any reason at any time in the forseeable future; prevent her from bullying Shep by restraining her on house tab (and removing her if she demonstrates dominance toward him, simply pick up the tab and walk away, ignore her for ten seconds, release her and keep observing).  Protect your older dog; feed them separately so she cannot see who gets 'fed first'; do NOT do any so called dominance exercises with this dog, she is not properly socialized to humans (from your description); instead, teach her that "work" is reward (go to ClickerTraining.com)  Read about multiple dog households (Patricia McConnell Ph.D. has a  book on this topic) and dog behavior (John Fisher's "think Dog").  If you feel this is not something you can do without guidance, find a certified applied animal behaviorist (referral from veterinary college in your geographical area.)