I do think that hormones do play a part in problems between girls and boys too for that matter so perhaps spaying would help. I belong to a dog breeders site and this problem has come up more than once and although I have not had issues myself (knocking on wood right now!) it is a common problem. I have seen other breeders say that 2 dogs (male) will fight and you'll wind up with vet bills but two girls will fight to kill each other. Anyways, I do think you need to take this problem very seriously. I belong to a wonderful trainging site. You might want to check it out and see what they say....
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Good luck!