Dual Hemisphere Breeding and Live Foal Percentages

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. "If you can alter their cycle, what does that do to their internal light or time clock?"

Sheerin had hypothesized that dual-hemisphere breeding does not affect the live foal percentage. For the study, he collected a group of shuttling stallions' ages, number of mares bred and live foal percentages in the United States and Southern hemisphere, and the total number of mares bred in a year. The controls included Kentucky stallions grouped by age, book size, and age and book size combined. "Since most of the stallions that shuttle are from Kentucky, the management (of the control groups) was similar to those who shuttled," he said.

There were no significant differences in the live foal rate in the United States versus that in New Zealand, with exception of three stallions in 1999 with better live foal rates in New Zealand. When live foal rates were different between the United States and Australia, they were almost always lower in the United States. "Ten stallions had significantly lower live foal rates in the U.S. for all years shuttled," said Sheerin. He said seven of 14 shuttled stallions had significantly lower live foal rates in the United States in 1996.

The majority of stallions which shuttled to New Zealand covered significantly fewer mares in the United States. Stallions that shuttled to Australia tended to breed more mares in the US, but it was not a statistically significant difference.

Because of mare reproductive loss syndrome that occurred in Kentucky in 2001 and 2002, Sheerin expected to see reduced foal rates in the United States for those breeding years, and he was correct—13 of 14 stallions had reduced foal rates for those covering seasons. Australia had a significant drop in foal rates from 2000 to 2001 (reason unknown at this time). There was not enough data to gauge changes in the foal rates from year-to-year in New Zealand.

There was a positive correlation between book size and live foal percentages in the U.S. breeding seasons (a larger book size equaled more live foals), but not in Australia. Again, there wasn't enough data to make that analysis for New Zealand breeding seasons. The age of the stallion didn't seem to be a predictor of live foal percentage, but Sheerin said this could have been because of the low number of older stallions which are shuttled. There seemed to be a positive correlation between stud fee and number of mares bred in the United States for all years except 1996 (higher stud fee equaled a better live foal rate), and also a positive correlation between stud fee and live foal rate (except in 1999 and 1996). In 1996 this could be due to the low number of stallions that shuttled.

Mares in the Southern Hemisphere typically are managed differently for breeding than U.S. mares, and this might be one possible explanation for lower live foal rates in the United States. "For the most part, the mares go to the (Southern Hemisphere) farms and are on one management scheme (rather than being trailered in just for the breeding), and their breeding times can be selected better. Here, that's not always what happens. We try for the most ideal time, do the best we can, but it doesn't always work out that way," he said.

The number of mares bred appeared to be country-dependent, and management probably was dependent on the stallions' book sizes as well, Sheerin suggested. Previous research has shown that in the United States, there is a positive correlation between book size and live foal rate. The thought is that when a large number of mares is being bred, the level of management must increase to compensate for the additional mares.

The strongest conclusion that Sheerin had about the effect of shuttling was that, "it depends on the stallion. Some stallions will tolerate it, others won't. It's a hit or miss. We're not sure why, but those that can't (tolerate shuttling) pull down the average (live foal rates)."


About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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