A breed registry that accepts horses of multiple breeds and backgrounds is called?

An open stud book is a registry that allows for the acceptance of horses from various breeds and backgrounds. This type of registry promotes inclusivity by enabling horses of different lineage, whether registered or unregistered, to be recorded within the same breed association. This approach can encourage genetic diversity and the introduction of new traits and characteristics into the breed.

In contrast, a closed stud book would only accept horses of a specific breed and lineage, maintaining strict standards regarding ancestry and quality. These differing principles are what set open stud books apart and contribute to their flexibility in accommodating a wider range of horses, ultimately enriching the breed by allowing contributions from various sources.

The other terms mentioned, such as formal breed registry and hybrid registry, do not accurately capture the inclusive nature of an open stud book. A formal breed registry typically refers to a recognized body overseeing the registration and pedigree of horses but does not specify whether it accepts multiple breeds. A hybrid registry might imply a mix of breeding practices rather than focusing specifically on acceptance criteria for various breeds.