A 3 year old girl and an 18 month old girl had a very loud growl out upon returning home together from obedience training last night(they both went to training). They have gotten along great until this point. The 3yo growled loudly this am when the 18 mo got too close to her crate. The 18 month old growled when the 3 year old was walked near the play yard where she was exercising. Before this, no issue with these two at all that we noticed. Neither is currently in season, but we are watching closely. 18 month old had first season 2 months ago without temperament change then.

They nosed each other friendly like at our training spot yesterday - I'm bewildered about what and why this suddenly happened and how to handle this. Ideas?

Personally I would place the trouble maker of the pair. Once they get like this they never get over it. It is a lot of work keeping a dog separated from the pack and you have to worry about her being accidentally being let out with the other dog. Bitch fights can be really bad. Don't let it get to that point. I had one that growls at everyone but they all just ignore her and she is wagging while she does it. Not a serious thing, but your's are getting ready to get into it big time.

I placed a "trouble maker" once. It turned out I placed the wrong one. It can be hard to figure out who the perpetrator is. If you do decide to place one of the dogs, place the one you like least. Chances are neither girl will have a problem with other dogs. It is unpredictable.

As a bitch owner, I've dealt with this in the past. You have to be very careful to really get the one that is causing the problem. Usually it is the one that looks more "submissive", but it really is insecure. Giving special time one to one to this insecure bitch might amend things. Emptying the anal glands of all involved can set thing a level down as it causes them lose their identity for a few hours.
Bitches go through a "bitchy" stage when they are about 1-2 month from coming in season. This same bitch that is giving you problem can be very well no problem at all once she comes in season and stay like that for 3-5 months.

I've also observed that this problem bitches are a problem between 1 1/2 years and 3 years old. After that they just quit being in a constant fight for dominance.

If they keep at it, you can go around it by not letting them interact through the fences or crates. This will trigger the worst of their behavior. Let them together when you are present AND CAN STOP A FIGHT IF IT WOULD ACTUALLY HAPPEN. Watch the signs of dominance before they escalate to a growl or fight level. Keep them completely separated when you are not there.

Get in there! If they start, get in there, grab each by the cheek while standing over both of them and give them a NO! Dominate the situation and let them know it is unacceptable behavior !! You need to lead the pack or it will go bad quickly.

I would spay and place both of them as I feel this is not normal lab behavior!!

All my dogs live together and at times I have had up to 8 breeding males running together. Not a growl to be heard. I have even bred a male and turned him out with the rest right after the breeding. Everyone (males and females) give him a good smell and then off they go.

When I have a litter, all my dogs and my cat help with the puppies. We are a family and all live together as such.
My labs are known for their great behavior and are used as working dogs in all venues.

I have also spayed a CH BOB bitch that produced one litter with champions in it and gave her to a single dog pet home. She growled and then went after another bitch. That was enough for me.

I was not going to live in fear and I was not going to set up kennels to keep dogs apart. I have labs just for that reason.

How hard is it to place a dog like this?

I would think it would be difficult. Won't the new owners want to know why your placing the dog? Question is, when placed how do you know the dog won't go after another dog.

I've had Labs for almost 40 years and multiple girls in my kennel over 30 years, and I've never had two that I had to keep separated when I was not with them- ever! Likewise for the boys, even when girls are in season. I do have two girls that play so roughly when I first let them out for their evening walk that I keep one on leash for the first minute or two until the excitement ebbs. I can turn them out into the airing yard (not as exciting as the daily walk) and they are fine together -best friends - no wrestling.

How would you deal with it?

Had a Ch bitch with all clearances. She was a definite improvement over her mother in many ways except temperament.

She did not like other dogs! At all. Same sort of thing - started with the occasional growl and moved up to full out attack mode in a matter of weeks. She was good with our pack, it was dogs met while out walking, at shows etc. Her's is an insecurity issue - tried working through it, put obedience titles on her, etc. She was well socialized from a young age with people and other animals.

I did place her with full disclosure to anyone who inquired about her "intolerance" of other dogs. Loves people and cats, is a dream to work with in obedience and at the Vet etc. She lives with a wonderful couple who have two cats, live at the end of a dead end road and have no plans to add another dog ever. They love her to death and she boards with us up to 12 weeks each year as they love to travel. She grumbles at the dogs that might be new to our pack between our visits, but a reminder from me that it's not an appropriate manner and she's fine. But, for me it wasn't the Labrador temperament that is my goal for breeding.

So in all my recommendation for your question on how to approach her behavior when talking to prospective new owners is full disclosure.