Shuttling Thoroughbred stallions between Northern and Southern hemisphere farms for breeding began in earnest around 1992. Stallions had been shuttling from Great Britain and Ireland prior to that time. Between 1996 and 2002, 117 stallions from the United States were shuttled to the Southern Hemisphere in the late part of the year to complete a second breeding season. It was recently determined that the effect of this type of breeding schedule on live foal percentages depends on the individual stallion and his book size.

Pete Sheerin, DVM, Dipl. ACT, of Rood and Riddle Equine Hospital in Lexington, Ky., presented the results from his retrospective study comparing U.S., Australia, and New Zealand breeding and foaling records at the clinic’s annual client education seminar on Jan. 5. He explained that the reproductive cycle of the stallion can be altered by manipulating day length, and although this type of change is much more pronounced in the mare, both mares and stallions have a hormone increase in the spring and fall, followed by dormancy in the late fall/winter. While stallion cycles have not been extensively studied, in prior research examining the effects of season and photoperiod on stallions, “their cycle could be shifted by altering day length so that they peaked or rose sooner,” said Sheerin