Jeffrey Fawcett (Senior Research Scientist, iTHEMS) and his research was featured in an article in The Horse.

If you want to breed an older mare, you might consider how old she was when she was first bred.
According to a new study in Japanese racehorses, broodmare fertility appears to decrease slowly over the years, on a downhill slope that follows her number of breeding years—and not just her age.

“Our results suggest that the fertility of a mare is the highest at (her) first year breeding and gradually declines every year,” said Jeffrey Fawcett, PhD, a biologist and senior research scientist at RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS), in Saitama, Japan.

To read more, please visit the related link.


  1. Jeffrey A Fawcett, Hideki Innan, Takeru Tsuchiya, Fumio Sato, Effect of advancing age on the reproductive performance of Japanese Thoroughbred broodmares, Journal of Equine Science, 32, 2 (2021), doi: 10.1294/jes.32.31