Complex sentence: Retired to stud in 1974, Secretariat ultimately became notable as a broodmare sire, as though a number of his progeny had success at the track, he was most influential though through his daughters, becoming the leading broodmare sire in 1992.
Split into simpler sentences: Retired to stud in 1974, Secretariat became noted as a broodmare sire, though a number of his progeny had success at the track. He was most influential though through his daughters, becoming the leading broodmare sire in 1992.