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The Road to a Foal: V's 2021-22 Breeding Saga | Pt. 1

 A while back, many readers expressed their interest in hearing more about my breeding adventures, so here we go.

You may recall that I decided to also breed Sierra, though I still haven't decided if I'm going to breed her this year or not.  Regardless, Vida (aka "V") (JC: Fastination) is being bred to the dutch Hunter stallion Prototype.  In 2019 and 2020 she gave me two lovely fillies by the dutch stallion Parcival, both of which sold prior to weaning.

As our barn reno gets under way, I hope to foal out my mare(s) at home in the future instead of my in-laws, but our barn is not heated.  Between that and Vida now being 2 for 2 for foaling 'early', I'm sitting on my hands until May, but that hasn't stopped me from getting down n' dirty (literally) with my new Ultrasound Machine.  I've been scanning Vida once a week for a few weeks now, so I can track her cycles, get more practice, and get familiar with my machine before breeding season really gets underway.

In my neck of the woods, many mares are what is called "transitional" in and around March.  Because we are so far north, we simply don't get adequate daylight that triggers the mares system into beginning to cycle.  Roughly 10% of mares cycle year-round, but when you consider the vast number of mares, that leaves a very miniscule amount of mares who cycle consistently for 12 months of the year.  When we first bred V in 2018, my colleage (who is a repro vet) mistakingly inseminated her while she was still transitional and while it didn't 'matter', it did cost me about $600 for nothing, so I want to avoid doing that again.

When a mare is transitional, they exhibit signs of estrus. They grow a follicle (which houses the oocyte, AKA the egg), but instead of ovulating (when said follicle * POPS * and the oocyte ventures down the oviduct towards the Uterus to meet up with the swimmers), the follicle either regresses, or turns into a different structure such as an Annovulatory Hemmhoragic Follicle (AHF) or a LUF (Lutenized Unruptured Follicle)  Both these structures are different, but look basically identical and both suck..  Either way, they act receptive to a boyfriend, but can't get pregnant and in the cases of turning into different structures, they can often persist and mess up everything for upwards of 3 months afterward.  These transitional estrus cycles are often very sporadic as well - they may not cycle on a normal 21 (+/-) day cycle, and they may only show receptivity to a stallion for a day, or sometimes they may be in standing heat for 10+ days (normal is 3-5).  Transitional mares can build Uterine Edema, which is another thing we watch when timing ovulation, which is why it is so difficult to determine if a mare is transitional or not, and when you're dealing with shipped semen you're often just hoping that they are through the transitional period, and are cycling normal. It's a bit of a hail mary early in the season in this area.

When a mare isn't transitional and is cycling normally, a variety of follicles grow at an average of 2-3mm per day, and as they begin to enter their heat cycle, a dominant follicle takes over while the growth of the smaller follicles slow.  Many mares can have multiple dominant follicles, and those follicles can all ovulate which is how we result in twins.  Fun Fact: Thoroughbreds are significantly more likely to double (or even triple) ovulate, because why should things be easy for me?

Once a follicle ovulates, the structure hemmhorages in on itself and turns into a Corpus Hemmhoragicum (CH). A CH only lasts a short period of time (like, hours) as it transitions into a Corpus Luteum (CL).  A CL is fully formed and functioning within 5 days of ovulation, and the CL is responsible for expressing Progesterone, which brings the mare out of heat.  Because of this, mares will show signs of heat and may even accept a stallion for upwards of 3 (+/-) days after ovulation, but it's important to note that a mare can still get pregnant after ovulation, the success rate just lowers as time passes.  It's important to note that there is a difference between a mare who's just a hussy, and a mare who's constantly in heat. I see so many people post online about how their mare is "always in heat", which isn't truly the case.. so because of this when it comes to breeding, the use of an Ultrasound is extremely important.

I do enlist the help of synthetic hormones when necessary, but I don't like to jump to many of them unless I really have to.  There are a variety of options available depending on what you need, but the one I use the most is Deslorelin. Deslorelin is often misconstrued as a drug that will make a mare ovulate, but this is very incorrect. The truth is, if a follicle isn't going to ovulate, it isn't going to ovulate.  That being said, Deslorelin can speed up the ovulation process provided the follicle is already on track to ovulate.  In order for this to happen, there must be Uterine Edema present, and the follicle must be at least 35mm in size to be receptive to the drug, and have every intention of ovulating on it's own accord.  Deslorelin is particularly helpful however, when working with shipped semen because it's lifespan is significantly shorter by the time it arrives, than that of a live cover breeding.  Most breeders use Fed Ex for shipping semen, and I learned the hard way in 2018 that their Overnight Guarantee does not apply to me, and it takes 2 days for semen to arrive in my greedy little hands, so timing is essential to finding success. This is magnified even more when using frozen semen, but I am currently not doing that.  Aside from Deslorelin, there are other synthetic hormones that can be beneficial when breeding, but I do my best to not use them unless absolutely necessary.

Now that you have a very brief rundown on how this all works, lets look at some images!

Above is an image taken via Ultrasound of V's Right Ovary on March 14th.  You can make out the entire outline of the ovary itself, and within it are dark circles, which are follicles (IE the structure that houses the Oocyte or Egg).  I located and measured the largest follicle, which had an average of roughly 27mm.  When measuring follicles, you should always take two measurements and find an average.  

The Left Ovary showed smaller follicles, and what appeared to be CL.  The presence of a CL would indicate two things: 1. She's no longer transitional, and 2. She's not currently in heat.  I was not able to obtain a very good photo of the CL, but truth be told they're extremely hard to make out in most cases... basically your best bet is to look at the entire ovary and find the absence of a follicle which is what I did in the image below; chances are there is a CL in it's place.  Below is the same ovary from a different angle, which shows the CL slightly better, but it's still easy to miss if you aren't looking!

All things were looking promising of V being through transition and thus cycling normally.  As hard as it is to sit on my hands, I stuck to my guns and continued to scan weekly to track her cycle.

On March 20th, V had some larger follicles developing, which would support the belief that she ovulated early-mid March.  Again, I identified the largest follicle,  however on the left ovary she had two relatively large follicles that looked like the possibility of a double ovulation in about a week.

Unlike the week prior, I was able to make out some Uterine Edema.  Uterine Edema is essentially the swelling of the uterine lining due to the surge of Estrogen, and when a mare is not in heat, it can be very difficult to detect.  When using ultrasound to create a breeding timeline, we grade Uterine Edema; some use a scale of 0-4, while others use 0-5 or 1-5 which can be a little confusing but regardless, Uterine Edema is a crucial thing to track when watching cycles and looking for Ovulation.  Uterine Edema slowly builds while the mare is in heat, then peaks and when Ovulation occurs, and it appears like an orange slice on Ultrasound.

On the 20th, I was able to identify some Uterine Edema, which I filed in the back of my mind as the presence of edema when outside of heat can indicate an infection, however with no other outward symptoms I decided to just keep an eye on it for now.  You can see this near the top of the image, where it appears pinwheel-like.  I graded this Uterine Edema a ~2 on a 0-5 scale.

The following week on March 28, it was significantly more pronounced, and I graded it a ~3 out of 5.

Upon further inspection of her Ovaries, I found something intriguing.  The larger follicle on her Left Ovary is still there, but there is another structure that is either a CH, AHF or LUF that wasn't there before.  This means perhaps I missed a larger follicle last time and she's ovulating, or somethings in the process of going sideways.  The 'fun' part about those aforementioned structures, is that you basically can't tell them apart immediately.  Aside from pulling a hormone panel, my best bet is to scan V again in a few days, as I would anyway. If it is a CH, she likely just ovulated (within 6-8 hours) and the next time I scan her, it will be transitioning or a fully formed CL depending on when I scan her next.  If it hasn't changed, it's either an AHF or LUF, and they can often persist for several weeks or even months in some cases.  If it is a persistent structure, I will likely give it a week or two to see if it solves itself, before trying to resolve it with synthetic hormones.

Oh, such fun!


  1. THANK YOU for posting this. I'm hoping things align to get B in foal this spring but I'm nervous about how much I don't know.

    1. Oh fun!!! I'll be sure to keep these coming as I can! I wont be breeding any of mine until May, so things might be a little quiet for a while!

  2. Super interesting - thanks for the info and pics!!! I've always looked at U/S screens and just seen blobs and grey squiggles. lol One of my mares is at the breeding farm this week - fingers crossed things work out.


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