Frail Broodmare? Expect a Filly

Cold and wet weather combined with a rather hard-keeping breed of horses means that Thoroughbred caregivers must be especially conscientious about the health of their equine charges during the winter months.

In other words, winter is a rough time of year to be a Thoroughbred owner.

It becomes an even more crucial -- and difficult -- responsibility for Thoroughbred breeders, who must keep broodmares in top condition throughout the lean months.  I know from experience that some mares are just harder to manage than others -- blanketing, multiple grain rations, and high-quality hay are just the bare requirements for some mares when they're in foal or preparing to be bred. 

A few months back, I read a fascinating story on (Researchers: Mare's Condition Appears to Impact Sex of Foal) that associates the relative condition of the mare (at the time of breeding) with the eventual sex of her foal. With Thoroughbred breeding season ramping up in just a few weeks, I thought it was a good time to share the article.  In a nutshell, the author reports research that shows an overwhelming percentage of mares that were gaining condition at the time of conception produced colts, while mares that were losing condition at the time of breeding tended to produce fillies.  (Read the article for some additional analysis.)

The best practice -- and one that results in an equitable distribution of colts and fillies -- is for broodmares to maintain condition throughout the year. 

That's a tall order when you have a hard keeper.  During the winter, I feed one of my mares a spectacular quantity of feed to make sure she keeps good form.  High-protein sweet feed is augmented with mashes containing beet pulp and flax,-- fed twice daily. High-quality hay is free-fed, and supplemented with soaked alfalfa cubes. That plus blanketing in sub-30 degree temperatures does the trick for this particular mare, but I know there are much more difficult cases out there.  (Another of my mares could get fat on a single  flake of grass hay each day.  Go figure!)

Would love to hear your stories about foals you've had from mares that were hard keepers -- or your thoughts on the linked article.

(By the way -- if you found the article interesting, here's a related story:  Foal's Playfulness a Reflection of Gender, Mare's Condition.)


Leave a Comment:


I dont believe the condition of the horse has anything to do with making the gender. Its the male genes that make them such. We have had ten mares that were bred in TERRIBLE condition,came to us with their spine sticking out  and we used bran mash with Equine XTR and they had live health foals five fillies and five colts. All in great condition. I read the article and I was appauled that the writer believes that a strong colt would be able to " Spread the mares Bloodlines better than a strong filly". Thats not right.

06 Jan 2009 9:44 PM

Another thing

this is in refrence to the article about " Foal's Playfullness"

Our mares foaled around the same date  and they had plenty of friends ( colts and fillies) to play with. Didn't matter what condition mom was in, they were curious enough to seek a playmate.

06 Jan 2009 9:46 PM
Vet Tech

This actually makes sense. Nature has a way of self-regulating and if you believe the Darwinian view of 'survival of the fittest' you know that nature will adapt to any situation.  Reproduction is meant to ensure survival of the species, it makes sense that mechanisms would develop that promote continued reproductive viability of the offspring, even to the point of determining the sex of future offspring.  By the way, many animals are known to have the female actually DETERMINE the offspring's sex.  Yes, it's the male sperm that says "boy" or "girl" but the female body can produce hormones that KILL all "X" sperm or all "Y" sperm so that only the desired sperm actually reach the egg.

06 Jan 2009 10:00 PM

I don't really think I follow this. Wouldn't this make just about every foal in Florida a colt? It would also suggest that locations with colder climates like New York and Mass. generally breed fillys.

I will stay skeptic.

  • Scot's reply:  Skepticism is okay -- just take it as food for thought.  In the case of Florida vs. northeastern breeding programs... I hope that indicates that most breeding programs are able to keep their mares in "maintenance" condition, thereby eliminating any biomechanical disruption that would affect the gender bias of mares either recovering from or lapsing into poor body condition.
06 Jan 2009 10:36 PM

I'll keep that in mind. Thanks for the info on upkeep and feed.

06 Jan 2009 11:32 PM

I'm not sure about gaining or losing condition except its impact on the acidity, etc of the vaginal fluids, etc then having an impact on sperm...I have read articles on this perhaps affecting mares who tend to produce one sex over the other (even though the stallion has the actual x/y chromosone to determine sex...)

PS Hard keepers don't need more protein, they need high fat with low starch so they don't founder/ Blue Seal's Performance LS or Victory, then after foaling Blue Seal's Mare & foal.

Funny to read that you blanket at 30'...up here, we take blankets off on such a warm day!;-)

  • Scot's reply:  Good tip on protein/fat considerations for horses at risk for laminitis.  I am lucky not to have that problem with my Thoroughbred broodmares, though I do have to plan special feed for two of the riding horses -- including ALAM, a low-starch, low-grain, non-molasses, high-energy feed.  And with some hard-keeping mares in the past, I've resorted to the old stand-by of adding calorie-dense oil to their diet.  Fortunately, the hardest of my horses right now gets by with just the sweet feed/mash meals.  As for your sub-30 "warm days" -- BRRR!
07 Jan 2009 6:52 AM

OMG, that is so cool I did'nt know mares could actually make their foal be a colt or a filly. is one of my favorite websites it always has the best stuff and i've learned about lots of different ways to keep my horses healthier.

07 Jan 2009 7:52 AM
Janesville Liz

This does not make sense to me. The chromosomes contributed by the father during conception determine the sex of the fetus. Is it possible that once inside the mare, the chromosomes can tell the condition of the mare and send the little female chromosomes to be united with the egg so as not to waste the male chromosome on a weakling mare? I highly doubt it. I think this whole story is just a reporting of coincidences.

07 Jan 2009 11:12 AM


research seems to show support for the Y-Chromosome being very sensitive to vaginal acidity and more likely to suffer trauma in the vaginal canal, thereby the "gals" may be able to "swim" faster...LOL!

Now, to keep it pertinent to this a mare's acidity affected by her good or poor condition?

07 Jan 2009 1:15 PM

OOps, i mean't my comments for Janesvill Liz!

07 Jan 2009 1:16 PM

I think the author of that article made the wrong conclusion of his questionable study. If the mare determined the sex because of her inferior condition, then the strongest offspring would survive. If he's saying its almost always a filly, well, there you go. Some reptiles determine the sex of their offspring by maintaining a certain tempture. The warmer the nest,  more males are produced. Therefore, one can conclude, males are weaker and need to be coddled.  

07 Jan 2009 1:32 PM

I think the author of that article made the wrong conclusion of his questionable study. If the mare determined the sex because of her inferior condition, then the strongest offspring would survive. If he's saying its almost always a filly, well, there you go. Some reptiles determine the sex of their offspring by maintaining a certain tempture. The warmer the nest,  more males are produced. Therefore, one can conclude, males are weaker and need to be coddled.  

07 Jan 2009 1:32 PM
longtime breeder

Hmmm... interesting observation but like some of the other commenters I am not sure. My mares have always been in good condition (in both California and Colorado climates) and generally tended to produce 50/50 over their lifetimes. One mare had 5 colts in a row before ever having a filly, and ended up having 6 colts and 5 fillies total, with several repeat breedings to the same sire included (2 colts, 2 fillies). The one mare I bred who was in thin condition at breeding had a colt. And I had another mare who had 13 foals, every one of them a colt, and she had been through multiple owners with varying condition.

And since I am seeing some endorsements for specific feeds, I'd like to mention one I have had VERY good luck with on my elder horses: Nutrena Safe Choice which has an easily-crumbled pellet that does not need to be chewed. This is great for horses with missing teeth. One of my old mares, aged 28, had lost a lot of condition and I was fearing for her health--- tried this feed and within a month her bones had disappeared and in 2 months she was rounded out and had a glossy new coat to boot. She is active and energetic and still the boss of the herd at 29.

To be honest, any similar formulation and texture would work; the key is getting it to the digestive system so the gut can do its job. So a good "senior" formulation with the nutritional components and texture the individual horse needs would take care of "condition" concerns--- and the old 50/50 ratio of fillies to colts should hold true, all other aspects being the same.

This study would be really interesting applied to a very large number of conceptions, say over 1,000, in a variety of climates, and over several breeds (stallion managers could be surveyed as to condition of the mares bred). Then I think it would hold more validity.  

Patricia in far northen California

07 Jan 2009 1:41 PM

I wouldn't be so narrowminded as to discount this article.  I wonder if it is possible that mares in better condition ovulate more quiclkly than mares in poorer condition thereby introducing the egg to the faster swimming sperm (they are the first oes to the egg) and tend to be the lighter faster swimming sperm which tend to be the Y chromosome sperm producing colts.  Perhaps mares in poorer condition hold their eggs and ovulate them less quickly therby exposing the egg to the heavier, slower swimming sperm - generally carrying the X chromosome which are the slower swimmers but longer lived sperm.  I didn't see how many times the mares were bred as part of the article but that may have had something to do with it as well.  

I also wonder if poorer condition mares are less fertile in general but the ones that do make it to produce a live foal - it might be that filly foals are more resilient to be born than colts.  Female progeny tend to be stronger than male progeny so the ones that make it on poorer condition mares tend to be fillies.

07 Jan 2009 1:50 PM

The operative word here is, appears.  No definitive study using a large test group of both types.  My personal experience tells me this is nonsense.  But if it make you feel better then go for it.  

I feel beet pulp is a great feed additive but I don't consider it a mash.  Contrary to popular believe beet pulp does not require hot or cold water before serving.  As long as the horse has plenty of water available.  If you are not humanizing your operation and throwing blankets on when the temps drop below 50. And your horses are in good health they will developed a thick winter coat that will more them keep them comfy in sub freezing weather.  Bar wind-chill.  Let their manes grow out as nature intents.  Keeps their necks warm in the winter and cooler in the summer along with shedding water.  I feel that a fat additive is important.  As any winter mountain climber will tell you.  They burn a lot of calories keeping warm.  We use Buck-Eye Ultimate Finish.  We also feed it to our fox hunters, steeplechase and flat horses.  An excellent product.  High quality hay is term people often use but don't define.  Just because the hay has the look does not mean it is high quality without having it tested.  A good quality hay will have a "Relative Feed Value" RFT of over 100. But that is not to say that hay in the high 70's is not worth feeding.  Unless that is all the horses are living on.  Most mares getting 9-10 lbs (winter months) of quality feed a day will do just fine on lesser hay.  Lots of hay of any quality is very important during colds spells.  Its the fuel that stokes the furnace during digestion and keep their body core warm.  Of course there is always the special needs mare that has to be dealt with accordingly

07 Jan 2009 2:02 PM
catnip lane

i have to agree with you Scott about blanketing.  I find that my Thoroughbreds here in Lexington maintain their condition much better when I blanket them during cold spells.  Each field has a run in shed (which they all use) and some are put up in the barn at night.  They still do better with the blankets in my opinion.  It makes sense if you're looking at calories used to keep warm.  It's labor intense to use the blankets but I think it's worth it.  The only one I don't blanket is a very fat Paint gelding....

  • Scot's reply:  *grin*  My Thoroughbred mares/foals and my off-the-track gelding benefit from their blankets, but I couldn't imagine putting one on the Haflinger gelding or the Mustang.... 
13 Jan 2009 6:27 PM
The Five-Cross Files

With some matings, you hope for a colt, with others a filly. But what affects foal gender -- and can Thoroughbred breeders influence the process?

03 Jun 2009 1:53 PM

Interesting theory, but this assumes that mares which produce fillies year after year are in poor condition each time?  Or how about the ones which always produce colts? If the mare's pH level has more to do with it than anything else, then how do you explain the mares which have an equal number of colts and fillies?  Seems to me that you have a 50-50 chance each time and that's it.  Now if you had some hard facts to document the pH level for mares which produce one sex as opposed to the other, then you might have a valid conclusion rather than speculation.  I have bred mares which produced all fillies or all colts to date, but then there are the 2 which produced both flavors over time. I think it's pure chance.

04 Jun 2009 1:10 PM

Here are my thoughts. I think these conclusions are ment more for wild horses who arnt on anything more than grass.

  It is the stallion who donates the portion of the DNA that determins the sex, but it is the mare who carries the embryo to become a future progenator for the species, so what is best for the species is what will prevail. Now that being said. I would agree that there could be hidden signals within the mares system that would help one over the other and vice versa, but it probably does not play a big part over all. In humans male offspring tend to be the weaker, and are more often aborted. (Im saying it is only more often, not a rule by any means) So, I am thinking that fillies must be the same. More things have to be right for a healthy filly. Female embryos tend to have a lower tolerance to high glucose levels so I think fillies are the harder of the sexes for the mare and the species. Since the female DNA coding is the same as male but with extra, who is to say that the embryo couldnt dispose of the "harmful" coding to survive. Hense survivel of the fittest, which would now be a healthy little boy.

 Since high glucose usually happens in good rain seasons, thats why they notice more mares with colts at their side than fillies. And in a less boutiful year there will be more fillies than colts.  

21 Aug 2009 7:46 PM
Sexed Semen for Stallions: Thoughts?

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