Studbook selection is a process used in certain breeds of horses to select breeding stock. It allows a breed registry to direct the evolution of the breed towards the ideal by eliminating unhealthy or undesirable animals from the population. The removal of individuals from a population is called culling, and does not suggest killing the animal in question. Typically, culls are castrated or they and their offspring are unable to be registered.
Registries which implement studbook selection differ from registries which require only two parents of the proper pedigree or registration status. For example, a horse foal with two Thoroughbred parents is almost certainly a Thoroughbred, but a foal with two Oldenburg parents may not be accepted through studbook selection to be an Oldenburg.
The pattern of studbook selection varies from nation to nation and registry to registry, but among horse registries, particularly warmblood registries, the general outline includes an inspection of foals before formal registration, evaluation of conformation and movement of broodmares, and evaluation of the conformation, movement, health, and performance ability of stallions. Decisions about which mares and stallions are suitable breeding stock are made by elected breed judges, who strive for objectivity and transparency.