Social media is a frenzy of opinions, from politics to ponies, and everything in between. But one abnormal topic recently caught fire in the frenzy not once, but twice: stallion shuttling.

With the recent announcements that American Pharoah will ship to Coolmore Australia at the conclusion of the North American breeding season, while California Chrome will head to Chile, the keyboard warriors took aim, fired…and missed.

There were accusations of animal neglect, abuse, and questions as to why the industry doesn’t allow artificial insemination. A good friend of mine reached out and asked for a scientific explanation as to why it’s good for stallions to shuttle between hemispheres, so that’s what I’ll attempt to do. (Most stallions do not shuttle, though it’s becoming more common as travel becomes easier.)

Let’s start in the Northern Hemisphere, with thoroughbred breeding hubs like the United States, Ireland and Japan, and where the days are longest between May and September—or, summer. Conversely, for breeding hubs in the Southern Hemisphere, like Australia and South America, summer is the opposite—November to March.

Why does this matter? Well, quite simply, because the horse’s entire reproductive tract is controlled by those precious hours of daylight. Sunlight tells the brain to actively secrete the hormones that make our mares tease-hot and ovulate, thereby enabling them to get pregnant. They even control (to a lesser extent) the amount of super swimmers a stallion can produce.

Through artificial strategies we have manipulated this to enable our mares to cycle earlier in the year—in both hemispheres—mostly by placing them under artificial lights or turning the light on in their stalls. This convinces their brains to do this earlier, thus allowing us to begin breeding season on February 15th, ensuring a Northern Hemisphere birth date as close to January 1st as possible.

The Southern Hemisphere does this as well, only because their summers are the inverse of ours they strategize for a different time. In Australia and New Zealand, for example, the universal Thoroughbred birthday is August 1st, but in Chile, the recognized birth date is July 1st. But one fact remains true across the globe: All thoroughbred stallions must breed their book of mares by live cover, thus eliminating the option for artificial insemination, embryo transfer or cloning. This is why some stallions are shuttled.

The process of shuttling a thoroughbred stallion means they are transported from one hemisphere to the next, which keeps the amount of daylight at a long, constant state, enabling them to produce testosterone—and therefore sperm—at a steady state.

Northern Hemisphere breeding season: foals in February.

Stallions travel via air and it’s a rather simple and straight forward process. They are assisted by their grooms and a veterinarian for every flight and the majority ship in a box stall without the bumpy roads to annoy their fragile legs.

More importantly is they then breed a second book of mares in that season. With some stallions breeding books of greater than 200 mares, this increase often raises the concerns of the public…

It has to decrease fertility, right?

They have to eventually burn out, right?

They couldn’t possibly breed that many mares, right?


The average stallion produces enough sperm to breed seven mares a day in the wild. So by that math a stallion produces enough sperm to cover 2,500 mares a year. So why don’t they?


The limiting factor to book size is not the amount of sperm that can be produced, it is the number of mares you can excite the stallion with. In most live cover scenarios this is approximately 1-2 a day. I have witnessed, at most, a stallion breeding 5 mares in one day—below the natural limit.

So does shuttling have an effect on their fertility? It has to, right? They have to eventually burn out and stop getting mares pregnant, no?


An awesome study at Texas A&M was recently published in Theriogenology, a journal which studies animal reproduction. Headed by Dr. Stephanie Walbornn, the study examined the effect of dual hemisphere breeding on a variety of outcomes that are used to evaluate stallion performance: seasonal pregnancy rate, first cycle conception rate, and pregnancy rate per cycle.

From Walbornn et al. (2017): Table 5

Each of these are indicative of a stallion’s intrinsic fertility, and each taken into consideration when choosing a stallion’s book size. Stallion owners understand a healthy stallion is a productive stallion. Body condition, increased temperatures due to viral or bacterial infections and joint health all impact sperm production and libido. A sick stallion cannot breed at a high capacity.

A stallion that is not getting mares pregnant is not profitable. In the thoroughbred industry, the majority of these lavish six-figure stud fees are only paid when the mare produces a foal that is able to stand up and nurse. If the stallion has passed his book size capacity and is therefore either shooting blanks or simply refusing to breed, the farms are making zero.

Increasing blood in our pedigrees is essential. We would have never known Zenyatta or Winx, two of the greatest race mares ever, if Darley had not shuttled Street Cry between Kentucky and Australia. We would not have Animal Kingdom if Leroixdesanimeux never arrived from Argentina. And, we would not have beautiful Galileo foals in my backyard if globetrotting stallions was not encouraged. Limiting ourselves to North America, or scarier, just Kentucky, would be devastating.

The outcome of this necessary study by TAMU was simple: there were no negative effects of breeding a stallion in dual hemispheres. In fact, one of the only significant outcomes of the variables evaluated was that the stallions had better fertility in the Southern Hemisphere in comparison to the North.

Once finished with their sexual adventures in the South, these valuable stallions then hop back onto their jet planes and return to be bred for the Northern Hemisphere breeding season. So think of it as California Chrome and American Pharoah are taking a five month vacation in the sun.

Shuttled stallions exist in a constant state of summer. Longer days. Green grass. 5 star meals. And a line of women, I mean mares, waiting to be satisfied. It sounds like a pretty darn good life. Shuttle on.

About the Author

Carleigh Fedorka is a Ph.D. student at the University of Kentucky’s Gluck Equine Research Center. A Pennsylvania native, she moved to Kentucky after graduating from St. Lawrence University and has worked closely in all aspects of the thoroughbred industry. She spends her free time eventing as well as training, selling and rehoming OTTBs. Read more about her horse life at her blog, A Yankee in Paris.