Pink and Blue in the Thoroughbred Breeding Shed

Many breeders hope for a specific outcome -- either filly or colt, depending on the situation -- when they choose a match-up for their mares. 

Commercial breeders who send royally-bred mares to high-end stallions generally wish for colts. No wonder: the boys average about 20 to 25% more (with medians about 50% higher) at the all-important Keeneland yearling sale every September -- and the numbers are similar at other sales in every U.S. region and internationally. Buyers at the high end generally want colts because of stallion potential; at the low end, colts (usually soon to be geldings) are seen as more likely to run frequently and to avoid being claimed, and they thus command a significant premium.

Lifelong breeder/owners -- those whose breeding programs involve successive generations of runners -- often pray for fillies, the lifeblood of the average breeder and the only way to perpetuate the Thoroughbred female families that they've built up over decades. That gender preference becomes even more marked as a top producing mare ages, especially if previous daughters have been sold or proved unfit for broodmare service.

So, what's a breeder to do? Just cross his fingers and hope?  Or can he effect a specific outcome when breeding his mare?

The Jockey Club live-cover rules -- in case you missed it, we debated this recently --  currently make "designer matings" more difficult. It rules out lab work that would select just X or Y sex chromosomes in the sperm before (manual) insertion into the mare.The rules also prohibit in vitro fertilization, another process that would make fetal sex selection an easy process.

Timing is one factor that breeders can significantly control. A mare's hormone cycle and uterine pH levels will play some role in determining the gender of her foal.  While it is the stallion's sperm that actually provides the determining X or Y, it is a biological function of the mare to regulate which of the sperm actually reaches the egg. (The female is also responsible for which fertilized egg -- if any -- will implant for gestation.)  While the process is not completely understood, it is likely that the stage of estrous -- early or late -- and the resulting flood of hormones creates an atmosphere that is more hospitable for one or the other sex chromosomes. 

Based on studies in other mammals, a few "rules" exist:

  1. Insemination directly prior to ovulation -- i.e., having the mare covered just as estrous hits -- increases the chances that the Y-chromosome sperm will reach the egg first, as they are the faster swimmers.
  2. Insemination up to several days prior to ovulation creates a bias for X-chromosomes, which tend to have a much longer lifespan than their "male" counterparts.
  3. Acidic enviroments (low pH) are more hospitable to "female" (X-chromosome) sperm.
  4. "Male" sperm, on the other hand, prefer alkaline environments (high pH).

It is theorized that the high incidence of colts born to mares bred at certain breeding farms is due to a highly basic vaginal lavage that is performed as part of the mares' pre-cover preparation.  I haven't heard of this practice being exploited, but I have heard of boarding farms and individual mare owners going to some unusual steps to affect foal sexing, including temporarilty supplementing probiotics or adding apple cider vinegar to a mare's feed to alter her pH levels.

Have you heard of -- or tried -- any special techniques or supplements to influence whether a mare would produce a colt or filly?  Was it successful?

Related post:  Frail Broodmare? Expect a Filly

Update: Thanks to Sceptre for a few corrections that were implemented shortly after this article was posted.


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