stylistic:tone, structural:summarization
Breeds

In most horse races, entry is restricted to certain breeds; that is, the horse must have a sire (father) and a dam (mother) who are studbook-approved individuals of whatever breed is racing. For example, in a normal harness race, the horse's sire and dam must both be pure Standardbreds. The exception to this is in Quarter Horse racing, where an Appendix Quarter Horse may be considered eligible to race against (standard) Quarter Horses. The designation of "Appendix" refers to the addendum section, or Appendix, of the Official Quarter Horse registry. An Appendix Quarter Horse is a horse that has either one Quarter Horse parent and one parent of any other eligible breed (such as Thoroughbred, the most common Appendix cross), two parents that are registered Appendix Quarter Horses, or one parent that is a Quarter Horse and one parent that is an Appendix Quarter Horse. AQHA also issues a "Racing Register of Merit", which allows a horse to race on Quarter Horse tracks, but not be considered a Quarter Horse for breeding purposes (unless other requirements are met).

A stallion who has won many races may be put up to stud when he is retired. Artificial insemination and embryo transfer technology (allowed only in some breeds) have brought changes to the traditions and ease of breeding.

Pedigrees of stallions are recorded in various books and websites, such as Weatherbys Stallion Book, the Australian Stud Book and Thoroughbred Heritage.