behaviour issues towards other female dogs

Question
Dear Dr. Jill Connor,

I have a two and a half year old Newfie named Ginny. She has a gentle and friendly character, and is well behaved. We have had her since she was a puppy. She proved to be very social towards humans (especially children) and other animals (dogs, cats)
From early on, we consulted a certified master trainer who assisted us in training Ginny to be obedient. Ginny responded well to this training.
Our Newfie often attends an off-leash park were she displays good social behavior, sometimes slightly submissive.
However, she has had about 6 sudden outbursts (in the last 16 months) of dominant behavior displaying growling and charging at another dog.
She gives little to no warning, then suddenly charges in an almost aggressive way. In some cases, she knew the other dog and played well before, other times the dog were unknown to her. Initially; I thought this must have been related to the fact that she was coming into heat; her personality does change before her period. After her third heat, we went to the park and kept her on-leash; introducing her to every dog with her butt first.
I also considered this might have been possessive behavior as this often happened when a ball or stick was around which Ginny claimed as hers.
Today; she had a sudden onset of "attacking" and growling again (she was carrying a float)and this created a very unpleasant siuation.... She has not actually bitten the other dog, but it sure looks like she trying(?) to do this. This behavior in combination with her size does not go down well. The other dog owner pulled her away; Ginny let her and was calm when I got to her. I instantly pushed her down onto her back to dominate her an told her "no".
It has never occurred with human interaction, she is wonderful with (my) kids and visits elderly people frequently; then she is very well behaved and displays none of this behavior.
I would very much appreciate your expert advice on how to handle or prevent this from occurring again.
Tomorrow she is scheduled for surgery; she is getting spayed. Might that influence her "outbursts" in a positive manner?

Kind Regards, Ginny's owner.  

Answer
DO NOT SPAY THIS BITCH without an estrogen titer.  If she has recently been in estrus, you must WAIT; if she's close to coming into estrus, the behavior that is influenced by estrogen will be set for life.  Do the blood work and re-schedule the appointment based upon the results.

Your dog requires evaluation by a certified applied animal behaviorist.  Her sudden outbursts most likely have real cause (perhaps a conditioned response to an incident with another dog at the park that you didn't witness or misperceived).  If she is resource guarding (as you describe, ".....she had a sudden onset of "attacking" and growling again (she was carrying a float)...." this behavior can begin toward humans.  Any possibility of a dog this size to demonstrate aggression needs in person professional assistance.  DO NOT force her onto her back;  you are NOT a dog, "dominance" routines DO NOT work, they provoke fear, anxiety and distrust toward the human conspecific(s) in the dog's environment.  Further, should you intervene in this manner when the dog's behavior is locked on to whatever motivation produces aggression, YOU WILL BE BITTEN and perhaps quite severely.

Stay OUT of the dog park until you are able to find a professional.  Do NOT bring in a dog trainer, "master" or otherwise.  Contact the following person by email and request referral to another behaviorist in your area of Canada if she is too far away:
Pamela Reid
Ontario, Canada
preid@psych.torontu.edu

Meanwhile: if there are young children in your home, instruct them never to attempt to remove treats, food or toys from this dog.  The behavior you describe may have a reasonable explanation (as I suggested above), something that can be rehabilitated without a great deal of trouble, but for this breed the behavior is extremely aberrant and I would take precautions to insure that children and elderly are not in a position of vulnerability.  IF the "master trainer" you spoke of used coercion, pain, domination (which is possible given the "alpha rollover" you described), START OVER.  Ask the behaviorist for referral to a positive reinforcement training venue where dogs and humans are working for reward/praise and start with a novice obedience class.  Your dog will learn to voluntarily offer "obedience" behaviors with the added benefit of being with other dogs who are also working.  This will give you and the trainer the opportunity to observe any possible triggers to her "sudden" aggression (among other dogs) and address it properly.