"To know the breed" means to have knowledge or understanding about a specific type or group of something, typically referring to animals. It can also imply having experience or expertise in dealing with a particular situation or category.
Breed rescues exist because the people involved know the breed best — they often consist of past and current breeders, owners of the breed, and trainers. (scsheltierescue.com)
But those devoted few who knew the breed knew it in intimate detail. (nyx.net)