I have a two year old gsd who has chased his tail from being a pup.He now sits with it in his mouth and is striping some of the hair from the end of the tail.Does anyone have any ideas on how to stop this behaviour.

Hi
My bitch also does this and it is when she is aggitated a bit like a child seeking attention but its more a nervous thing she bit her tail till she cried and made it bleed. I thought I would ignore her at first but this didnt work. So I distracted her and made her sit beside me and stroked her but she still offered to do it not so much though. So in the end I resorted to threats I'm afaraid with a rolled up news paper and this has worked I just say to her now No dont chase your tail.
and she has forgotten about it now and hardly does it at all. So I reckon it is habit forming and you have to break the habit with enforcement of a form of punishment as they would much rather move away from punishment. Its like a kid that suddenly starts to have tics and twitches  they grow out of it but she must be the highly strung type like my girl and you have to stimulate these types keep them busy buy her a new toy and only give it her for half an hour a day.

Sorry him bless him

our year old CKCS does it. She makes herself so dizzy that she someytimes falls over. We distract her and that usually works.