A sire is the male parent of a horse. The term appears in pedigree documentation, where the sire’s name is listed in the upper half of a breeding certificate and the female parent listed alongside the sire (female parent) in the lower half. A reproductive status that enables the sire role can become a sire by producing one or more live offspring; the distinction between “stallion” and “sire” is that stallion describes the horse’s sex and reproductive status, while sire describes its relationship to a specific offspring.
In selective breeding, the sire’s genetic contribution is evaluated through its progeny — the performance, conformation, and soundness of its foal crops over several seasons. A stallion with a strong, consistent sire record commands a higher breeding fee. Pedigree research traces back through successive sires to identify the foundational progenitors of a breed line: in Thoroughbreds, the male tail-line traces to one of three founding sires.
In breed registration, both sire and dam must typically be registered members of the same breed for the offspring to qualify as a purebred. DNA parentage verification using microsatellite or SNP panels is now required by most major registries to confirm that the recorded sire is the actual biological father, a safeguard against misidentification when multiple stallions are on a property or AI records are incomplete.
Further Reading: The Wikipedia entry on sire in horse breeding covers pedigree notation and the sire’s role. The three founding sires of the Thoroughbred breed are the canonical example of how a sire’s line defines a modern breed.