I have a 5year old Dally. He was castrated at 18 months because he was very humpy with other dogs when playing. Now he has no real interest in humping anything other than labs/golden retrievers. These however, need immediate and urgent humping without even a "Hi there and hello sniff".

Can anyone shed any light on why he would have such a penchant for a particular breed?

I try to keep an eye out for any and put him back on the lead, but sometime we will come across one unexpectedly. Even worse, they can be elderly, and not able to do much more but just stand there looking pained.

This may be because he is not sure of himself, I have seen it many times when a dog feels insecure then humping seems to be the answer. It may well be something else but without watching his behaviours it is difficult to say, one thing it is not is sexual and probably never was although I see he was castrated very young and humping is a juvenile activity that most dogs grow out of once adult and it is possible he is stuck, so to speak, in his juvenile stage and is permanently a teenager..

Try going back to basic training and put emphasis on the leave command.

Perfectly normal for an 18mnth old dog to want to hump a lot.  During adolescence they have 8 times more testosterone than they will have as adults.  Had you not neutered him, it would have settled down again.  Why oh why do people neuter without a second thought.... vets don't help either as they don't give a full account of the cons.

I don't think he really has time to be insecure though. A lab is spied, and he is straight there trying to mount any available part of it. Also, it is the the older ones he really likes. The ones with the bad back and rickety legs. We can recall him away from younger ones but if he sees an old one hobbling a long there is nothing we can do to stop it.

Rabid I did ask quite a few people at the time about getting him neutered. I asked on here, I asked the trainer at dog class, I asked at a Dalmatian show, and yes, I did ask the vet. His humping was out of control and lead walking alone did not give him enough exercise, so we had to take steps which meant we could let him off lead. We battled with this from him being 8 months old and eventually had enough. I hate that we did it as it was always our plan to leave him entire, but we could see no other option.
You post didn't actually answer my question at all, nor give any useful advice other than to berate me for something which was 3 and a half years ago and cannot be undone.

Probably retained juvenile behaviour, are the dogs he picks male or female? Mind you the only way is to teach a leave and return.

They can be either. He isn't fussy!

I can understand if it is retained behaviour of some sort, I just find it odd that it is only one breed and wondered if there could be a reason that that oddity.

Well they are pretty dogs!!
Sorry, I've been trying to stop myself saying that all through this thread.

Actually, I did wonder if he has had a best friend in the past from either of the breeds and somehow sees them in a more interesting manner now. Dogs are odd creatures at times. One of my dogs seemed to take no interest whatsoever in the tv and I assumed she couldn't see it somehow. One day a friend popped in with a video she had taken of my dog playing with her dog, her best friend. Suddenly Cally was there, glued to the telly. Obviously it had been a case of 'nothing good on the telly' until then.

Still think it may be because he is worried by these dogs for some reason, once owned a GSD bitch who was very worried by other dogs including the bitch she lived with and her answer was to hump first and hope the other enjoyed the experience.

My other thought is that you castrated him in his peak humping period and that he has remained at that stage, the fact it is only docile and ageing dogs who get his attention may be because he has been told off by fitter dogs and so now avoids them.

To repeat myself; teach a good recall and a strong leave and that should stop the problem or keep him on a lead when there are other dogs around, you are not likely to stop the humping at this stage but you can try, it is usual for it to stop when the dog reaches adulthood but it is obvious that in your dogs case this is not going to happen.