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.



Leave a Comment:


A few years back I was looking thru a Fasig Tipton sale book when I came across a Tom Rolfe Mare.  She was 14 years old and had not had a foal in 2 years.  She was royally bred as she traced back to Northern Dancers full sister Arctic Dancer.  I then noticed she was out of an Alleged mare and that mare out of a Buckpasser mare.  I knew there was inbreeding 1x4 to Tom Rolfe, but I liked this pedigree.  I advised my owners to try and get her.  We did.....for a song...$2000.00 I thought something was wrong.  There was nothing wrong other than the 2 year stretch with no foal.  We bred the mare that spring to Lure and of course got nothing as Lure was having a big problem getting mares pregnant.  After 3 failed attempts we sent her to Fly So Free and she got in foal on one cover.  She delivered a filly the following spring and that filly has become our foundation mare.  SHe is now 8 years old, and in foal to Quiet American.  Where as her mother was a very difficult breeder, this mare gets in foal by just looking at a stallion.  She again got in foal on one cover to Quiet American.  We are hoping for a filly, but only because Quiet American is the hottest broodmare sire ging these days and a daughter of his is worth a mint if they have any pedigree at all and any running ability.  We are keeping all our mares fillies, but only because the female line is priceless and contains some of the deepest stamina you can find.  I don't mind getting a colt, but with this current bun-in-the-oven, I want a filly as Quiet American is 23 and does not have many seasons left.  

03 Jun 2009 3:22 PM

Very interesting and informative.  I remember reading that the physical condition of the broodmare also plays a role in the gender outcome of her foal;  the healthier the mare, the more likely she will throw a colt.

03 Jun 2009 3:44 PM

Thanks to Erin Ryder (of Weird Horse News fame, as well as news editor for, here is a great basic overview of Fetal Development and Growth.

03 Jun 2009 3:57 PM

I wish I saw this article a couple of months ago!!!  I have an older GSW GSPR and have yet to get a filly!!!!  I was literally heartbroken last year when I had my 3rd colt in a row.  This year the mare was bred on a Friday morning and she ovulated that do I assume a colt again????  I know I should be happy with any healthy foal and I am, but this is my last chance for a filly out of this very special mare.  THe mare was in "fair" condition at time of breeding, not her best, but not her worst either.  I'm not going to fetal sex, because if it is a colt, I don't want to be depressed for the next 11 least I can keep hope alive if I don't know..stupid yes, but hopes all I got.

03 Jun 2009 5:48 PM

I have heard of studies done on wild horses that show a significantly higher rate of colts born when the mare is losing weight at the time of conception, and a higher rate of fillies born when the mare is gaining weight at conception.  My own mare was gaining weight at the time of conception and had a filly, but statistically that is an insignificant sample size!  I thought it was an interesting coincidence nonetheless, and will pay closer attention to it in the future.  The theory behind it is that when times are good more females are born to grow the population, while when times are bad the colts are born, thus limiting the future number of births.

  • Scot's reply:  Hmmm... that's opposite of the study I cited several months ago!  The theorizing was that in bad times, producing fillies would be a survival advantage because basic biology requires many mares but only a few stallions to maintain a population.  If you can find references to the studies you heard about, please share!  It's always interesting to see similar research with divergent results/interpretation.
03 Jun 2009 7:17 PM

I had friends who had all females each year out of about 26 mares.  One year, they wanted a colt out of one of their mares and they got him.  Their strategy was to breed early in the heat for a filly and late for a colt.  It worked for them and I got the filly, as well as a colt I asked for.  I would use the same strategy if I were to breed another mare. Seems to be supported in the article above.

03 Jun 2009 8:43 PM
Pedigree Shelly

        If iI had a Broodmare with an excellent line related to such mares as La Troienne , Id definately want her to produce a filly.

  • Scot's reply:  Sounds like you're a real breeder at heart!
04 Jun 2009 5:30 PM
Pedigree Shelly

        Scot, Thanks for the compliment ! You made my day !I've been busy packing, I'm moving to Indiana (Batesville ) Not to far from bluegrass country !

04 Jun 2009 9:54 PM

This was our first year doing breeding, although we had 2 babies before this year but that was just backyard breeders, nothing fancy. But this year everything that could go wrong did. But the lady i now work for and the mare I'm Leasing is bred to one of her stallions, said for a filly, that if the mare usually has a 6 day cycle, breed on the 4th day, 5 day cycle, 3rd day, etc. This way you have a better chance of having a filly. She has learned that from a friend of hers and since both her and me would rather have a filly any day over a colt, we used this when breeding my mare. So hopefully next year I will end up with 2 fillies, but at the moment it looks like I might end up with only 1 baby next year if I'm lucky.

01 Jul 2009 4:27 PM

Recent Posts


  • Pedigree Newsletter:
    The Five-Cross Files will be featured in a new Pedigree Analysis newsletter from To sign up for this free weekly email -- or any other newsletters from The Blood-Horse -- just click here.

Recommended Reading

More Blogs