Hi Lins,
I'm a firm believer in that you can train a dog to do anything it is physically capable of doing. Like wise you can de-train a dog to do something if you have the time and the knowledge combined with the resources. A lurcher by its breeding is a `chaser/killer', therefore you have to de-train it from its instincts. This can only be done with the co-operation of a sympathetic sheep farmer and should not be attempted without permission!
If a dog has drawn blood, however, you `may' succeed in de-training it but I would never trust the dog around sheep again no matter how good a trainer you are.
There are no `niceties' in this de-training, no treats, no `positive' angles you can take, the brutal truth of the matter is you have to make the dog `fear' the sheep, and you have to turn the whole ethos of predator and prey on its head. This is different in training `prevention' where you are aiming more towards ignoring sheep as potential prey and channeling the dogs interest elsewhere. ( Prevention is always better than the cure, and easier!)
You should note that I emphasized the dog should fear the sheep and not the handler, or the consequences laid out by the handler subsequent to the dog `worrying'. ( A dog following through with its insticts will always remain stronger in the dogs mind than any subsequent punishment, and the damage to the sheep will already have been done!) This ironically, is where the `incorrect' use of an electric collar can and does work, and why it is often advocated for use in such situations. But then IMV you can't then really call it a training tool can you? as its really being used as aversion therapy ( subject to successful operator interpretation of the dogs thinking!)
There are various methods used by individual sheep farmers to make a dog `wary', all follow a general theme. Firstly you make the dog vunerable and take away the stimulus to chase, namely by having the sheep and the dog in a confined space and allowing the sheep to intimidate the dog, whilst preventing the dog from retaliating ( if it was so inclined) Most dogs will very quickly show signs of anxiety when alone and surrounded by sheep in close quarters, ears will drop and head carriage will lower. Normally you as a handler would act to reassure the dog, however in this scenario even when the dog looks to you for help you must give preference to the sheep and only protect the dog from `serious' harm, remember this is about de-training a strong instinct, its no job for the fluffy bunny brigade, the dog needs to come away from this experience with a few bruises, be it physical but more importantly psychological! The next time it sees a sheep out in the open it has to remember this experience as unpleasant, and that woolly jumper as a monster. One session properly handled is usually enough for most dogs, a few hard cases may need a second, I've not heard of a dog that needed a third..but perhaps they ended up getting shot anyway!
As an aside, my first Munster hated sheep, why? Because as a poor hard up farm worker I couldn't see the point of having a working sheep dog as well as a gundog that I could send in any direction and stop where I wanted her, ( and therefore move stock where I wanted them). This did not go down to well with the Munster who treated the whole `sheepdog' thing with contempt! However She never chased a sheep in her life, wink!
Peter