Culling usually means to put down, kill or euthanize an animal. Generally this is done if the animal has some sort of defect that could impair it's ability to thrive. For breeders, it is usually to spare the animal having to suffer.
Culling doesn't always mean that the animal is euthanized. It can mean it is removed from any breeding projects. But usually when we refer to 'culling', it means the animal has to be destroyed to preent the possibility that genetic defects do not get passed on.
When I you culling it is my way of just selecting the best that I have produce for that season and getting rid of the rest that I am not going to be using in my next season, but it is never for me killing them.