Open stud book

An open stud book is a type of breed registry for animals, particularly horses, wherein animals may be registered even if their parents or earlier ancestors were not previously registered with that particular entity. Usually an open stud book has strict studbook selection criteria that require an animal to meet a certain standard of conformation, performance or both. In contrast, a closed stud book only allows registration of animals whose parents are also registered with that particular breed.

An open stud book allows breeders to strengthen breeds by including individuals who conform to the breed standard but are of outside origin. Some horse breeds allow crossbreds who meet specific criteria to be registered. One example is American Quarter Horse, which still accepts horses of Thoroughbred breeding. Among dogs, an example of an open stud book would be the registries maintained by the American Kennel Club as its Foundation Stock Service.

In some cases, an open stud book may eventually become closed once the breed type is deemed to be fully set.

Another form of open registry is a registry based on performance or conformation, called in some societies "Registry on Merit." In such registries, an eligible animal that meets certain criteria is eligible to be registered "on merit," regardless of ancestry. In some cases, even unknown or undocumented ancestry may be permitted.

The Registry on Merit or ROM may be tied to percentage of bloodline, conformation, or classification or may be based solely on performance.

More controversial open stud books are those where there are few, if any qualifications for animals other than a single trait, such as color, particularly when the color is not a true-breeding characteristic. However, some breeds have a standard color or color preference that is one criterion among others used to register animals.