Closed stud book

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A closed stud book is a stud book or breed registry that will no longer accept any outside blood for improvement of a particular breed of animal, and the registered animals are the foundation for the breed, with all subsequent offspring tracing back to the foundation stock. For example, the Trakehner horse has a closed stud book, and will not allow any other stallions to be registered, even if they are extremely successful. Therefore, all Trakehners that are registered are descended from the same stallions and mares that were registered at the time the stud book was closed.

A closed stud book allows the breed to stay very pure to its type, but limits its ability to be improved. This may put a breed at a disadvantage, especially in horse breeding, where an animal is worth more if it is successful in competition even if it is not pure. It also limits the gene pool, which may make certain undesirable characteristics become accentuated in the breed, such as a poor conformational fault or a disease.

The American Kennel Club is an example of a kennel club with primarily closed books for dogs; it allows new breeds to develop under its Foundation Stock Service, but such dogs are not eligible for competition in AKC conformation shows. For the breed to move to the Miscellaneous class and then to fully recognized status, the breed's stud books must be closed.