A new study asks whether racehorses have hit their genetic peak

For decades there was an apparent paradox in horse-racing. The sport is lucrative (Mage, the winner of this year’s Kentucky Derby, earned his owner $1.9m) and simple—the fastest horse wins. Horses with good results and a good pedigree are used as breeding stock for the next generation. Horse-breeders were armed with plenty of data, a single trait to optimise, and strong incentives to do so. Yet several studies suggested that, despite their efforts, race times were not improving.The most common explanation was that, physiologically speaking, it was increasingly difficult to breed a horse that ran faster than existing horses already do. The modern thoroughbred racing horse dates back at least three centuries. Perhaps the years of selective breeding had already discovered and exploited almost all of the breed’s genetic potential.