My five year old lab used to play with all dogs - then she was bitten on the nose by a collie. Now she is more wary and will sometimes become aggressive with other dogs - maybe 1 in 20 off lead - but I never know what triggers it. She is fine with people and other labs. I exercise her a lot - we go to training class - and I try to socialise her with as many dogs as possible . But sometimes it backfires and she might take against one . Hackles rise and she tries to use her strength to overpower and grab neck . Any tips ?
Can you see anything different that the 1 in 20 other dog is doing? Or looks like? Gender? Age? What does your girl do, more explicitly? Is it possible she is filling the "higher ranking role" in this article on Puppy License to Misbehave? http://www.dogstardaily.com/training/dog-communication
To be able to help your girl you do need to identify, as @Snowshoe says, what the triggers are. It could be related to the collie that bit her nose and she has generalized it to a certain size/colour/sex of a dog. If she is trying to attack dogs as frequently 1 in 20, and trying to overpower them and grab their necks it is a behaviour that needs to be resolved quickly before any serious damage is done. I think a session with a good behaviourist who only uses positive reinforcement methods would help identify and produce a plan of action to resolve the problem.
Our first Lab, Ginger, was bitten by a rather mean GSD and she was actively aggressive towards GSD from that point on. She never actually got in a fight with one, just postured. She seemed to be fine with all other dogs. Our current two Labs are not aggressive towards any other dogs, but they definitely favor certain dogs, generally Gun Dog breeds.
