Yes some dogs can be desensitised in this way, but in my experince, even though they may be calm at this time, in this place, with these sheep (essentially a preconcocted training environment) that is no guarnantee that they will remain like this if they come across one sheep alone or indeed a runnign sheep or flock, or worse still a flock that i already being chased by another dog!

Remember even trained sheepdogs can go "wild" when they are used in an unfamiliar place or on unfamiliar breeds of sheep!

The risk is so great to the sheep and thei unborn lambs of pregnant, that in my view the only really safe way of keeping dogs from harming sheep and unborn lambs is to keep them on lead, and if it is a big dog, using a halti or something similar.