We only dock one third off of our working cocker pups tails and ensure that they go to working homes.
We dock a third because ideally we want the dog to keep as much of its tail as possible, but we need to remove enough to ensure that the tail is not long enough to become split if it is whacked against branches, thick brambles etc... the longer its length the more fragile and vulnerable the tail will be in a working environment.
Tails if damaged tend to split up the length of the tail rather than across creating a 'split' paintbrush type of injury that can easily get infected and is difficult to heal. Often dogs who suffer a split tail end up having most if not all of their tail surgically removed because the nature of the injury tends to lead to tissue infection.
So we have our dogs docked because we know that they will be used for work and want to give them the best chance of not sustaining such an injury during their lives, but also want them to have a tail to wag
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I do think that cosmetic docking is unnecessary.
The RSPCA is slightly misguided when it lumps all 'breeds of working dog' into one category and so states that docking isn't necessary because you don't get docked Labradors for instance. Spaniels are used to go deep into thick cover and flush out game therefore their risk to getting tail damage is significantly higher than for say Labradors and retrievers which are generally used for retrieving game from land and water and don't work in close cover.
Anyway, I will be very interested to see what happens in Scotland regarding working spaniels and tail injuries.
Mary xxx