Bold Ruler was also the dominant American sire of sires in the 1970s. His daughters have also made him a noteworthy broodmare sire. He was the Leading broodmare sire of 1962 and 1964. Easy Goer has also been an influential broodmare sire. In 1992, Secretariat was the leading broodmare sire in North America. Others sire fillies of great abilities but not colts. Bigger males tend to sire bigger offspring. The combination of sire and dam also affects the offspring phenotype. Both males engage in mating, but only the dominant males can sire offspring. This bull became the foundation sire for the breed. Sir Gaylord became an important sire, whose offspring included Epsom Derby winner Sir Ivor. Males sire few offspring in periods when they are not in musth. He is now considered the foundation sire of the breed. Stray dogs can produce litters in which every puppy has a different sire. In a lifetime, a successful bull could easily sire over 500 pups.