With the Northern Hemisphere breeding season fast approaching, I am seeing more and more comments on various social media platforms that make my eyes roll back in my head.  With the invention of globalized news and social media platforms, it appears as though every one is an expert in the world of breeding, raising, and owning horses, and although their opinions from the outside weigh nothing in the actual breeding selections or foaling outcomes of the breeders and owners of these great horses, they type nonetheless.

But then I realize I should not be frustrated, as I am different from the masses.  And even just ten years ago, I would have known no different from their opinions.  I was also an outsider to the game, and an ignorant one at that. Farm management of commercial establishments changed much of that, but that I went to graduate school.  And my education at the University of Kentucky’s Gluck Equine Research Center opened my eyes.

Here are just a few of the things that I learned.

(DISCLAIMER – These are just my opinions, based on science. And no, I am not a vet.  Just a pensioned farm manager with a PhD.)


One: Breeding a small mare to a big stallion will not give you a big foal.

This was discovered through a really awesome (and convincing) study done in the beginning of last century.  The researchers bred pony mares to draft stallions, and draft mares to pony stallions. None of the pony mares died during parturition to what should have been a monstrosity of a foal, and more surprisingly, the fetuses that resulted mimicked the mares size – not the sires.  The draft mares produced draft sized foals, and the pony mares delivered pony sized foals.

draft mare

And this is because fetal stress is imperative for parturition.  The fetus dictates when it is done cooking, and once there is not much more room for it inside that uterus, it begins to secrete cortisol – aka the hormone of stress.  This cortisol production then triggers the cascade of hormones needed for parturition to ensue from the mare, and she begins to produce that essential oxytocin, prostaglandins, and other steroids.


Fowden et al. (2007) “The Endocrinology of Equine Parturition” 

This is why the maiden mare tends to produce the smallest foal – her uterus has not been stretched out by prior foals, and therefore is its smallest. As the mare produces more foals, each one stretches the uterus a bit more, and the fetus can grow a bit larger.  This will generally increase as the years pass by, but there is a threshold or plateau, because we also know that the uterus ages as the mare ages.  Age correlates with endometrial quality – and so after a certain period, we will tend to see this fetal size drop back down.  This is due to the glands of the uterus not being able to pass on those essential nutrients and hormones at quite the level they had in the past, and the foal therefore doesn’t get as much of what I call “happy juice.”

Now obviously, there are outliers to this principal. We will have maiden  mares with large uteruses, and we will also have older mares who have GREAT endometrial quality and continue to produce beasts, but in general, a mare will produce a smaller foal first, and this will increase in size for the next 5-7 years where it will then plateau.  Following this, the mare will produce smaller foals in her late teens and early twenties.

Aka – Size Matters.  At least for the female in the equation.


Two: Regumate is the most overused drug in equine reproduction.

GASP! Don’t say its so, said every farm manager I ever said this to.

Equine pregnancy is both simple and complex at the same time – and we are learning more and more about what hormones are necessary for it every day. Here is what we currently know in a nutshell:

For the first 35 days, the corpus luteum (CL) that is produced after ovulation creates progesterone, and this supports pregnancy – I tell my students, it literally stands for PRO-GESTATION. If the pregnancy is healthy and developing correctly, it will produce endometrial cups around day 35, and these secret equine chorionic gonadotropin, or eCG.  This eCG mimics the ovulation-inducing luteinizing hormone (LH) and causes accessory CL’s to develop.  These produce additional progesterone for upwards of 100-150 days, and at this stage of pregnancy, we consider it to be of ovarian control – as it is the CL’s on the ovaries that are supporting the fetus.

But this shifts around day 100, as the endometrial cups are eaten away by cells of the immune system, and stop producing that awesome eCG. Simultaneously, the fetus has matured tremendously, and the fetal gonads (ovaries or testis), the fetal adrenal, in addition to the placenta are now quite functional and can produce hormones themselves.  But this also coincides with a drastic decrease in progesterone, and an increase in estrogens – which is shown on this image that was published by Twink Allen.


Allen et al.png

Allen et al. (2002) “The influence of maternal size on placental, fetal, and postnatal growth in the horse” Journal of Endocrinology

Now right before parturition, this does shift, and we see a surge in progestins, or metabolites of progesterone..  But for the most part, during mid to late gestation, progesterone is negligible.  And I know, I know, you will say “but I sent out bloodwork for my mare, and she had a LOT of progesterone at 250 days!”

Well, good for you. But sorry, what you’re seeing is not progesterone, but the progestins.

Unless the laboratory that you sent your sample off to is using a technique called “Mass Spectrometry”, what they are detecting is basically anything that looks like progesterone due to cross reactivity.  And while progesterone is quite low during this time in gestation, we do see a lot of PROGESTINS – specifically DHP and 5a-20P – in mid gestation.

So what does that mean to all of you farm managers who are pulling progesterone levels after 150 days gestation and then treating the mares with “low” progesterone with Regumate?  It means you’re literally squirting liquid-nothing in your horses mouth, because we SHOULD see low levels of progesterone at this point.

Exceptions to this rule?  If your mare has a compromised cervix, Regumate will still help to keep it high and tight – which we want.  And if your horse is diagnosed with Ascending Placentitis, Regumate is still recommended as part of the treatment.

But otherwise?  For a normal pregnancy in a normal mare?  Stop thinking your mare NEEDS progesterone.  What she NEEDS is for you to spend your money more wisely.  Like on carrots. Or cookies. (I’m kind of kidding, she is probably not).


 Three:  Antibiotics are not always necessary post-breeding.

This was the topic of my dissertation, and to this day I treat myself with omeprazole for the ulcer that both veterinarians and farm managers cause me.

And I used to be one of you.  We would breed a mare at 2 or 6 PM, and ultrasound for ovulation the following morning at 7 AM. On one farm, we simply infused EVERY mare with penicillin and gentamicin immediately after scanning her, and on another, we  infused anything that had even trace amounts of fluid in their uterus at that time.

And then I went to graduate school, and learned about this thing called breeding-induced endometritis, a disease that my advisor Dr. Mats Troedsson was considered The Godfather of – and what he taught me was both fascination and infuriating.

I learned that mares will naturally have an immune response to the deposition of semen into their body.  This is because everything from bacteria to sperm, and even STERILE solutions, are considered foreign to the immune cells that are floating around the body just waiting for battle.  So when we put 100mLs of semen  – consisting of sperm, gel, seminal plasma, and even some bacteria – the immune system charges with bayonets extended.

And we need it to.  Because only one sperm is necessary for fertilization, and the other billion need to go away.  So the immune system tells the body to not only bind to and eat the excess sperm and bacteria, but it also tells the uterus to contract, and the fluid, cells, and other unnecessary garbage to be sent back out the cervix.



Robertson et al. (2014) “Seminal fluid and adaptation for pregnancy -Comparative Biology for Mammalian Species” Reproduction in Domestic Animals

In the normal mare, this takes between 24 and 36 hours to accomplish.  But in the abnormal, or unhealthy mare, this can take even longer than 4 days (96 hours).

The problem is, we usually don’t know which mare is truly abnormal, because we rush to treatment the minute we see fluid.  And this fluid might be perfectly normal, and more importantly, is probably not even caused by dangerous bacteria.  So not only are the antibiotics not helping – as they will not assist in the digestion of sperm, but they are also assisting with the ever-increasing resistance that we are already causing in our every day bacteria.

And this could be outright dangerous.

Now, physiologically, the embryo lives within the oviduct for 5.5-6 days after fertilization, and the cervix stays relatively open for 24-48 hours post-ovulation.  So we have a grace period to get that fluid out before it is actually hindering the success of your future pregnancy.  Therefore, I’ve learned to slow my roll.  Take a culture AND cytology of the fluid (double guarded), give oxytocin while waiting for culture results, and then treat according to plan.

These mares that are innately diseased and have a dysfunctional immune response will not always be helped by shoving more things into their uterus.  This just causes the immune system to begin its job over and over, and therefore you can actually prolong the time it takes to clean them up.  On the other hand, oxytocin is given systemically and therefore doesn’t exasperate that.

My motto? When in doubt, oxytocin it out. 

Now, there are obviously extenuating circumstances in this regard as well, and a good vet will know to take it case by case.  But the best thing you can do is have a full history of your mare, do full diagnostics on her, and go from there.  And if you do get too anxious waiting for results, and are desperate to begin treatment – think about turning towards an immunomodulator, or a mucolytic, or even simple dilution (lavage with LRS/saline) before immediately turning towards antibiotics.  Our future generations will thank you.


Four: Foal heat breedings are not a simple yes or no question.

I have heard a lot of farm managers say “we don’t do foal heat breedings” or “we have no problem with foal heat breedings” and I now roll my eyes and/or giggle when I hear this.

Foal heat is what we in the industry call the first estrous cycle after the mare delivers her foal.  These tend to occur roughly 7-10 days after parturition, and I always teach my students in my Reproduction class that this leads us to some simple math. Because while historically foal heat breedings led to a lower pregnancy rate, with the invention of ultrasonography and diagnostics, we have much great success as long as the math is both completed and adhered to.

Because immediately after parturition, the mare experiences a process where her uterus must cleanse itself and return back to its normal elasticity after carrying a 120 lb foal, and we call this process involution. On average, this takes 14 days after foaling.

And as I’ve already mentioned, the embryo lives in the oviduct for the first 5.5-6 days after fertilization.

So simple math – if she ovulates at 7 days post foaling, then the embryo will arrive to the uterus when, and to what?

7 + 5.5  = 12.5 days

If involution truly requires 14 days in your mare, then that embryo is arriving to a pretty volatile location.  A fairly damaged endometrium, potentially some leftover fluid full of immune cells, and it will therefore probably not survive.

But if the mare waits and doesn’t ovulate until 10 days post foaling, we have some different math.

10 + 5.5 = 15.5 days

With this mare, if involution truly occurs within 14 days post foaling, will have a much happier and healthy uterine environment for that embryo, and the embryo is much more likely to survive. Embryo

14 Day Embryo, as taken from Ginther et al. (1998) AAEP PROCEEDINGS





So the rule of thumb for foal heat breedings?  A) Monitor involution, and B) Attempt on ovulations that occur further along post foaling. If the mare appears to be ready to be bred greater than 10 days post foaling, your pregnancy outcome can be quite successful.



Five: Stallions will limit their own book size

I do slightly hate teaching this aspect to my students, because I truly do believe that we are pushing it with stallion book sizes as they creep upwards of 200+ mares per breeding season, but with that being said, I try to teach the science and not the opinion (which I fail at doing. Often).

I hope that it is generally known that stallions produce sperm, and sperm is required for fertilization, and fertilization then (hopefully) results in a pregnancy.

One way that we can predict how many sperm a stallion will produce in a day is by measuring that specific horses testicular volume, and applying it to a simple mathematical equation.

Testicular Volume (TV) = 0.52 x L x W x H

Daily Sperm Output (DSO) = (0.024 x TV – 0.76) x 10^9

A normal stallion that is post pubertal but who has not undergone any testicular degeneration caused by age will produce between 3-8 billion sperm per day, meaning that if we intend to utilize a 500 million sperm “breeding dose” that the average stallion could breed 8-16 mares per day successfully, which we obviously do not do.


Linear relationship between testicular volume and daily sperm output.

What we do instead is take this testicular volume calculation and combine it with knowledge of the horses history and the farms management, taking into consideration things like libido, prior seasonal pregnancy rates, per cycle conception rates, and then adjust book size with that knowledge.



Equine Reproduction: Chapter 293 “Breeding Efficiency” by Laura Nath

You can see in this table that a stallions daily sperm output doesn’t always correlate with their pregnancy rate, and that it takes numerous variables to properly predict a book size.

Stallion 1 and Stallion 4 have similar DSO, but differ greatly in their pregnancy rates, leaving them with considerably different seasonal pregnancy rates.  Stallion 1 still competently fulfilled his book size of 200, while Stallion 4 only achieved a 79% pregnancy rate for a smaller book size.

And at the very top of the predictor for book size is, quite simply, libido.

We can have a stallion who has great DSO and pregnancy rates, but if we cannot coerce him to breed a mare, his entire season will slow down.

And breedings, or trips to the shed, do not pad the pockets of the stallion owners – viable foals who stand and nurse do.  So while it might seem fully business oriented to increase a stallions book size to 200 mares, unless the stallion can withstand that lifestyle with  a high libido, great DSO, and successfully “stop” mares, no one is profiting – neither mare nor stallion owner.

It is expected that a stallion should have at least an 80% live foal rate, and if this drops, book sizes should be adjusted.  Breeders can look up this number for stallions they are interested in using the Jockey Clubs Fact Book.

But the greatest take away is that it is not only bad management, but bad business, to set a stallions book size to a number that is unsustainable for him.  While it might alarm the masses when American Pharoahs first book was 208 mares, his 2017 foal crop was an almost 80% viable foaling rate – which is right on target for assessment of a good book size.


American Pharoah

I can’t reiterate any more that an improperly set book size rarely hurts the stallion, it only hurts the time spent of the providers and the bank accounts of the owners.  The stallion will simply stop breeding before his sperm output ever truly dissipates, and will show his disinterest quite readily.  And if he won’t breed, he can’t service mares, and if mares don’t get serviced, they can’t get pregnant.  And if mares can’t get pregnant, they can’t have foals.  And if they can’t have foals, stallion owners don’t get paid.

Simple economics guys.  Simple economics. 

On a daily basis, I am amazed at the simple knowledge I have learned during the course of my doctorate, and yet at the same time, I am usually just as disturbed by how little of this education is both given to and absorbed by the general public.  How often our breeders, their managers, and even their veterinarians, are ignoring the basic science.

Obviously, there are outliers to each of these principals, and if you breed enough horses over enough years, you will see the exception to the rule.  But you can never lose with education, because it gives you the ammunition to create a fully formed opinion instead of a shot in the dark, and for that, I will try to keep firing away.




14 Comments on “Five Things I Learned In Graduate School That I Never Knew As A Farm Manager

  1. I love the science. Should have persevered with that biochemistry degree.

    Glad to see another post too

  2. This was so interesting! My brain hurts and I like it hahah.

    I have a question that I would love your opinion on if you have one! A good friend of mine bought a horse that was a twin (there were apparently ultrasounds done in pregnancy and this was missed – but both were born healthy which is a miracle). My friend is wondering if she should expect stunted growth or any other issues. I have been doing some reading online but haven’t found much. The horse my friend bought is 3 years old, gorgeous (she’s by bon balou) and seems happy and healthy. I’m curious if you have any experience with twin pregnancies or any thoughts 🙂

    • If they survive pregnancy, one usually is dysmature at foaling, but not always. If both foals were healthy at parturition and developed normal, they should be fine. Radiographs of joints as they develop will usually give you a good indication of if they are healthy—but that’s the same for any other Horse as well!

      In a nutshell, if the Horse is healthy at the age of 3, he/she should be A-OK!

  3. Thanks again for a wonderfully written and educational piece! I always look forward to anything you write!

  4. Great read! With regards to the point about foal size, do the offspring of the pony/draft mares & stallions finish at different sizes as mature horses?

  5. Studying Equine Science Bachelors Degree in Australia and we learnt all of this in our reproduction subjects! Fantastic info and the world of horse reproduction is quite an interesting one that’s for sure. 🙂

  6. I think big farms enjoy wasting money. My vet pulls blood when the mare is declared in foal, either she needs regumate or she doesn’t. End of story.

    • Wait-so who is wasting money? The farm or the vet? Sounds like the farms just trust their vet.

      Also-you can check progesterone levels at first pregnancy detection (usually 14-16days post ovulation). We tend to use 4ng/ml P4 as the cut off for adequate P4 to support pregnancy.

  7. Thank you for making my brain feel like jello….but I LOVED this piece. It flirts with myth busting and really puts common misconceptions to the forefront. Much appreciated.

  8. As someone who owns a horse, and left the veterinary field for that of pharmacy, I got a kick out of the omeprazole comment. Thank you for the humour and thank you for putting out some research-based information, both of which the Internet seems to be lacking.

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