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Bill's Columns
THE EFFECT OF SHUTTLING ON STALLION FERTILITY, MORTALITY, AND PERFORMANCE
Date: February 28th, 2018
By Dr. Emily Plant
In 1990, Coolmore and Robert Sangster partnered with Arrowfield and John Messara to purchase Danehill to stand dual-hemisphere duty in Ireland and Australia. Danehill, by Danzig, was an American-bred turf sprinter who was a sensational sire in both the Northern and Southern Hemispheres. His success sparked the explosion of standing stallions in dual locations, and the resulting mixing of genes from around the world is no doubt revolutionizing the breed.
Controversy surrounding the practice centers around the assumption that stallions are harmed by the absence of an "off season", that using them on double duty is detrimental in some shape, either by reducing the length of the horses's life, decreasing his fertility, or making him less likely to sire good horses. A lot of questions have been asked, and a lot of speculation has taken place regarding the topic, but reliable research on the subject matter is scarce. Emily's research, presented below, sets out to provide evidence-based answers to some of the most important questions surrounding shuttling.
This study is excerpted from our private research publication, known as The Stallion Spectator Ratings. This is the first time we have ever published for public consumption anything from this book, as it is available only by private subscription to a limited number of entities. We've made an exception to our publishing rules for this one topic, as the news is simply too big to keep to ourselves. Emily finds that shuttle stallions do not have lower fertility, do not die younger, and shuttling does not affect their success rate as as sires of good horses. As far as we know, this is the first study of its kind, although it is worth noting that researchers in Veterinary Science have recently published research which also finds no difference in fertility between those horses who shuttle and those who do not.
We trust you will find this research of interest and we will be interested to hear what people think!

The horse business is positively full of interesting questions which are difficult to answer. Without true scientific evidence to back up suppositions, we’re left to make assumptions about the truth. One of these such ‘difficult questions’ is how shuttling stallions to stand dual-hemisphere duty affects their health and performance. In this modern world where news knows no bounds of time or space, we receive the published information and make assumptions. We can receive realtime updates of every moment in a stallion’s life- look for example to California Chrome and American Pharoah, who received extensive media attention of their travels to the Southern Hemisphere for the 2017 covering season. With this instant access to information, we hear all of the news- both the good and the bad. We know when the stallions are happy and enjoying their life of perpetual springtime grazing virtually year-round on green pastures; we also know when things go wrong.
It's true, there have been high-profile cases of stallions dying while standing on Southern Hemisphere time or while in transit to or from their destination. Harlan’s Holiday comes to mind, who was euthanized while standing in Argentina at age 14 in 2013. War Pass died at age 5 after collapsing in his paddock the day after returning from Australia. The dearly departed Scat Daddy was a dual-hemisphere star who suffered cardiac collapse at age 11.
There’s also a belief that shuttling will impact a stallion in terms of his fertility, or otherwise deplete his ability to sire top horses. Criticisms are leveled that it is simply unnatural for a stallion to breed year-round, and that the winter off season is necessary for his health and wellbeing.
Discontent to accept this suppositions as fact, we set out here to statistically test these questions.
  • Do stallions experience a detriment in fertility when standing dual-hemisphere duty?
    • Fertility is defined here as the percentage of live foals reported from mares bred. This data is available only for the Northern Hemisphere, and is dependent on accurate reporting both in terms of number of mares bred and the number of live foals. This figure is not necessarily the same as a medical definition of fertility as measured by veterinarians.
  • Are shuttle stallions more likely to die an early death?
    • We studied the age and cause of death for a subset of horses who died at age 19 or younger. We wanted to focus on early causes of death, and attempt to exclude horses who died due to old age to isolate a specific population for in-depth study. The result is 140 stallions, 102 who stood in the Northern Hemisphere only, 38 who shuttled.
  • Does shuttling impact a stallion’s ability to sire top horses?
    • Percentage of A Runners and G1G2 winners are used to measure performance and compare the subset of shuttle stallions to those who did not shuttle.)
This study was sourced using data from the US Jockey Club and is intended to analyze the population of shuttle stallions versus those stallions who do not shuttle. All efforts were taken to ensure data is an accurate representation, however errors or omissions may occur due to incomplete or inaccurate records.
Our research finds that shuttle stallions do not suffer adverse effects in terms of fertility, mortality, and performance.
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