I have two bitches where the aggression between them is getting worse.  The oldest is a 10 year old JRT X Collie who is fixed and the youngest is a Collie cross, we don't know what with, but some sort of larger breed most likely.  She is 3 and has not been fixed yet.  It is always the younger bitch who starts it seemingly over nothing and the older bitch does not fight back.  We are very concerned that one or both of them are going to get seriously hurt and they are separated at all times now unless some can closely supervise.   This is causing a bad atmosphere and tension and we are struggling.  I spoke to a trainer who said when two bitch dogs start to fight then you have to rehome one because you can't trust them.  Is this my only option?

IMHO yes, it is your only option.  Remember the saying "Dogs fight to breed, bitches fight to breathe."  A bitch is far more likely to fight to the death.

The situation is really not fair to your oldie, she needs peace in her later years.  The youngster needs to go to an experienced home where she is an only dog.

Do it before you have a crisis.  Like now.

I also believe it is your only option too. Once a bitch decided to fight separation is the only fail safe way of avoiding a disaster. Either you manage it 150% all the timer and never ever let your guard down or you rehome one.

I wouldn't say it is always the case that they will never come back from it, it all depends on triggers, intensity of aggression, any medical stuff that might be involved etc.  But it can be very difficult to manage and work through, especially with the types of dog you have as they have that extra intensity and tendency to react and finish a fight, even if they don't start it.  Far less likely to back down and calm down without considering that extra intensity of bitches as well, so rehoming is likely the best choice in these circumstances.

I have managed a mali with major bullying and aggression towards my other dogs, primarily the girls, for 3 years now.  She is just about fine now (with ongoing management, she will never be 100%) and I can honestly say, if rehoming had ever been an option for her, she would not still be here.  It is utterly draining and not just for the humans involved, but for the victims as well.  I used to be all for helping people through it if there was a chance but having lived with her, I have very much changed my opinion.

Don't let your guard down for a minute. my two giant breeds fought and the older one had to be PTS with her injuries. Same age difference as yours. This was in the 70's and i vowed never to have 2 bitches again.

I'd go ahead and get the other one spayed, for starters.   Unfortunately however, once bitches take against each other, all too often there is no way back - I too go with the fight to breed (males) but fight to survive (breath?) - bitches.   If this is escalating now, and I'd suggest the younger one is sensing the ageing in the older one, and you think there could be serious injury, you can only keep them apart when you are not there, feed separated, watch with toys or any other trigger, OR sadly think about rehoming the younger one, in a home where she's either the only dog, or with a male who will back off.

I'd not want to live with a potentially explosive situation like this.    And when I had two males who started all this - well it was the younger one - challenging his uncle for the position of top dog, having tried all I knew to get him to settle down again, eventually I had to find him a home - a lovely lady with 2 bitches of the same breed.   He went off happily and when we saw him some months later, he greeted us happily but turned back to what he had been doing!   Result!   Peace back in our small pack.

Whilst I would still say to rehome the younger bitch, it would be worth getting both dogs checked out as medical issues and/or pain can trigger aggression in both directions (picking on a weak dog, pain causing reactivity etc.  That mali I mentioned had major muscular pain problems which hugely exacerbated her aggression, which were going on before I got her at 17 months old (just to show that age is irrelevant).  It's still an issue to a degree (i.e. still under treatment, it's not mended yet) but her pain is reduced through chiropractic and what's left is managed so it doesn't affect her behaviour now.

Even if you do rehome, it would be good to identify any possible problems first and with collies being quite wired, tense dogs a lot of the time, it is possible that something is sore somewhere and the aggression towards the older bitch could be aggravating it (as it did here).

i bred bullmastiffs.out of a litter i kept 3 bitches..5 years was harmony until one day for no apparent reason one turned on the other.ripped her ear to shreds and took the other girls eye out..i had them all spayed but nothing worked..i ended up having to use dog gates for separation.the bitch that attacked carried on in perfect harmony with the other girl until they both passed away.it was just the one bitch she set her sight on strangely.i only have one girl left now with one eye..never ever would i have 2 bitches again.its so hard but i would rehome for the sake of your senior girl.once a bitch gets the better of another bitch theres no going back sadly.hope everything works out well.