An analysis of inbreeding over 45 years shows the biggest increase during 1996-2006.
A 2011 study showing an increase in inbreeding in the Thoroughbred during a 45-year period from 1961-2006 also concluded the majority of the increase occurred during the last 10 years of the study period—a time coinciding with a sharp rise in the number of stallions being bred to books of 100 mares or more. Dr. Matthew Binns was the lead author of the study “Inbreeding in the Thoroughbred horse” that appeared in a June 2011 edition of Animal Genetics. The genotyping of 467 Thoroughbreds born between 1961-2006 showed an increase in the average inbreeding coefficient. More significantly, the study notes, the majority of the increase occurred during 1996-06, when the number of North American stallions breeding 100 or more mares in a given season rose from 14 to 128. In 1996, 14 North American stallions covered 100 mares or more. Only five years earlier only one stallion—Alydar—had bred a book of mares exceeding 100.
“My conclusion was that the data was showing the start of a trend that could become worrisome and needed monitoring,” Binns told BloodHorse. “It was starting to show this increase as a result of the big books.”