Complex sentence: People who have not considered these factors often stint their breeding stock in terms of veterinary care and adequate maintenance, the puppies they produce may have genetic disorders or undisclosed illnesses; it is these people who are disparaged as'backyard breeders'.
Split into simpler sentences: People who have not considered these factors often stint their breeding stock in terms of veterinary care and adequate maintenance. It is these people who are disparaged as'backyard breeders'.