What Happens During a Mare's Breeding Cycle With Fresh-Cooled Semen?

If you are breeding a mare for the first time, you might feel that understanding the process of breeding a mare with fresh-cooled semen is a little daunting. And yes, your reproductive vet or whomever you have managing your mare's breeding will likely explain the process, but many of us don't want to hammer our busy vets with questions. We wrote this article so that you have a reference you can call on anytime, and we hope to explain the process to you and answer most of your questions.

First things first: the pre-breeding repro exam

We recommend that at the start of the season you wish to breed your mare — or even better, at the end of the season of the year before you wish to breed — you have your reproductive vet do a reproductive exam on your mare. This will involve an ultrasound to look at her uterus and ovaries, but we also highly recommend that you request that a culture and a cytology are done on your mare.

"Wait a second," you might be thinking, "why do I need a culture done on my mare if she has never been bred before?"

Trust us, just because a mare is a maiden doesn't mean that she can't have a uterine infection. You will save so much time and money by having a culture and a cytology done before you attempt to breed her — because if she does have a uterine infection, the chances of her getting pregnant or holding a pregnancy are slim.

Remember, you can always put a light mask on your mare or put her under lights starting December 1 if you want her to start cycling early, so that you can get started breeding her earlier in the year too.

Breeding time!

Now imagine that you have a clean and clear from the repro vet in your mare's breeding soundness exam, a clean culture and a clear cytology. Things are looking good for you to breed your mare the next time she cycles.

Your vet will likely ask you to look for behavioral signs of estrus if your mare demonstrates those when she comes into heat. They will likely ask you to watch for that and bring her in. If you do not have a mare that shows any behavioral signs, your vet will try and assess where she is in her cycle via an ultrasound, and from that give you a timeframe to bring her in for a check.

Let's say you notice that your mare is coming into heat, and you arrange to take her to the repro vet for breeding. You drop her off and leave her there. But you may wonder, what is actually happening?

Some other questions in your mind might be: How many times should your mare be ultrasounded? When does the semen get ordered? Why does your vet sometimes tell you to order semen when the mare hasn't even ovulated yet? And what happens if she doesn't ovulate when everyone expects her to?

If you are new to breeding, understanding the basic process can make the whole thing considerably less confusing.

This article is about one normal breeding cycle using fresh-cooled semen, where your mare is being managed by a reproductive veterinarian. There are many variations on this process, and mares don't always read the textbook. Your veterinarian will make the decisions for your particular mare. But this should give you a good idea of what a typical cycle looks like. We are also going to assume that you have already chosen the stallion, and have the stud contract ready to go.

Side note: if you are the person tasked with communicating with the stallion owner, we suggest that you give them a heads up when your mare is being taken to the repro clinic. Some stallion stations prefer only to be communicated with when semen is needed, but a lot appreciate a heads up.

Why does fresh-cooled semen require so much management?

Fresh-cooled semen gives us a pretty established 48-hour window of time in which to breed a mare. CSU recommends inseminating mares with cooled semen within approximately 24–48 hours before anticipated ovulation. Research has also found good pregnancy rates when cooled semen is inseminated from 24 hours before to approximately 12 hours after ovulation.

Most stallions' sperm will be viable for about 48 hours after being collected. Of course there is variation, and of course some stallions last longer and some not so well, but generally stallion semen is collected, processed, cooled, shipped out to arrive at the mare destination the next day, and will last well for 48 hours — but possibly up to 72.

Breeding with fresh-cooled semen is all about timing. Your vet needs to ultrasound your mare to try and determine when she is going to ovulate, as they want to inseminate as close to ovulation as possible.

The first ultrasound

Your vet will ultrasound your mare and try to answer the following questions:

• Is the mare in estrus?
• Is there a developing dominant follicle?
• How large is the follicle?
• What does the follicle look like? Is it round in shape, irregular or teardrop shaped?
• Is there uterine edema?
• Is there any fluid in the uterus?
• Is there a corpus luteum indicating that she has recently ovulated?

If your vet determines that yes, she is in heat, and yes, she has a breedable follicle developing, then it is likely they will order semen to be collected the next day, so it will arrive the day after. So if you took your mare in on a Monday, and your vet determined she is ready to breed, they will order a Tuesday collection to ship overnight, with the view to breed her on Wednesday.

What is a breedable follicle?

Your vet is looking to see two things: uterine edema, which indicates the estrogen is rising, and a dominant follicle that is 35mm or larger. If your mare has both of these things, she is in the window of being able to be bred.

As estrus progresses, estrogen from the developing follicle produces the characteristic edema in the uterus. The follicle also changes as it approaches ovulation. Its size, shape and texture can all provide information about how close the mare may be to ovulating. When your vet sees both of these things together, and all else looks good — no fluid in the uterus or anything else that would indicate a problem — they will likely want to order semen.

This is the part that can seem particularly strange to a new breeder. You may get a phone call from your repro vet saying:

"She's ready. Order the semen."

And you think:

"But she hasn't even ovulated yet!"

With fresh-cooled semen, we generally want viable sperm to be present in the mare's reproductive tract before ovulation occurs. The semen therefore needs to be ordered with enough time for the stallion to be collected, the semen to be processed and cooled, and the shipment to reach your veterinarian so he or she can inseminate your mare before the mare ovulates.

Once we have ordered semen, how do we know she will ovulate?

We don't. Welcome to the fun of breeding!

We can order semen, give the mare an ovulation induction shot, and hope hope hope that she cooperates and ovulates exactly when we want her to. Most of the time she will, but a frustrating amount of the time, she won't.

Your vet will likely give your mare an ovulation induction shot of something like hCG or deslorelin when they order semen. These work to induce ovulation within 36–48 hours, so most vets will give this shot the day that they order semen. So if your mare looks ready on Monday, and the vet orders a collection for Tuesday that will arrive on Wednesday, we want the mare to ovulate Wednesday evening. If a shot of deslorelin generally induces ovulation 48 hours later, giving the shot Monday evening will hopefully mean she ovulates after the semen arrives on Wednesday.

And then there is FedEx

If this sounds complicated, there is another factor we have to consider.

FedEx is most commonly used to ship semen overnight in the USA. But whatever country you are in will likely have a similar overnight shipping service. And most of them are… underperforming… in terms of actually getting shipments consistently delivered within the 24-hour timeframe they promise.

You will hear breeders, stallion stations and reproductive vets bemoan FedEx a lot. The pain is real. Imagine all the hard work that goes into coordinating this timing-crucial task of breeding a mare with shipped fresh-cooled semen, and you get all that spot on only to have FedEx lose the semen. Or deliver it on Friday when it was supposed to be there on Wednesday. The semen is likely dead by then, and your mare has very likely already ovulated.

Side note: if the semen arrives just after ovulation, there is substantial research that shows inseminating a mare up to 16 hours post-ovulation can result in a pregnancy. In fact, timing insemination between 6 and 12 hours post-ovulation has quite favorable results. So all is not lost if the semen shows up late but within that window.

Best case scenario

The best case scenario is that your vet orders semen, gives your mare an ovulation induction shot, the stallion is collected the next day, the semen looks great, the semen arrives with you the day after, your mare is bred, and your mare ovulates.

Let's hope this is exactly what your first breeding experience is. Then what?

Your vet will likely want to ultrasound your mare the day after ovulation just to check that she hasn't accumulated any fluid that might need a uterine lavage or some other intervention to clear. There is a large amount of variation in how different reproductive specialists manage pre- and post-breeding. Some will do a lavage as part of a post-breeding protocol, some will give oxytocin to all mares after ovulation to assist uterine clearance, whereas others will only give this if there is fluid present.

Having an experienced repro vet that you can trust is a huge part of this process for the first-time breeder. Pay attention, ask questions, and take notes — not because you are trying to second guess your vet, but because you are invested in this process and want to learn as much about it as possible. You also need to keep detailed logs, as these will become useful to you later down the line.

How many ultrasounds will she need?

There isn't a magic number.

A mare that is early in her cycle might only need to be checked every day or two initially. As she gets closer to ovulation, examinations generally become more frequent because the veterinarian needs to narrow down the timing.

Serial ultrasound examinations are a normal part of managing a mare for cooled semen. CSU describes close monitoring by transrectal palpation and ultrasonography as necessary to coordinate semen shipment, insemination and ovulation.

So, as a very general example, your mare might be examined:

Ask your reproductive vet about their protocol. Again, not to second guess them, but to learn. An experienced reproductive veterinarian will have reasons for everything they do. When you give your mare over to a reproductive specialist, it is their experience in the field that makes them good at their job. The more you can learn from them the better.

Your mare might need fewer examinations. She might need more. If she is developing a follicle unusually quickly, your veterinarian may want to see her twice in one day. If she is taking her time, you might have several days between important events.

And then comes the pregnancy check

Once ovulation has occurred, the waiting begins.

The first pregnancy ultrasound examination is generally performed around 14–16 days after ovulation, to look for an embryonic vesicle. It is possible to check earlier, but it is also a lot more likely that a pregnancy will be missed before 14 days, so most repro vets like to do this between days 14 and 16.

You don't want to wait much later than 16 days, because if your mare has twins the vet will want to know that early on. If pinching a twin is required, this is easier to do not much later than day 16, because the embryos are still mobile in the uterus until then. Once they have attached themselves in the uterine lining around day 16, it can be difficult to pinch if they have settled close to one another.

If she is pregnant, congratulations. The next thing to schedule is the heartbeat check at 25 days.

If she isn't, your veterinarian will look at where she is in her next cycle and work out what to do next. They may want to run a culture and cytology to check that she doesn't have a uterine infection that caused her not to take.

Even with good breeding management, a mare does not have a 100% chance of becoming pregnant every cycle. Published pregnancy rates for cooled semen vary considerably depending on the mare population, stallion, semen quality and management system. One large field study found an overall per-cycle pregnancy rate of 66.9% for cooled-shipped semen, while other published reports have been higher.

A single unsuccessful cycle does not necessarily mean that your mare has a fertility problem, or that the semen wasn't good, or that your vet did something wrong. Often it is just one of those things. That said, keeping good records on the process will help you figure things out if you end up breeding multiple attempts and your mare still isn't taking.

So what does a "normal" cooled-semen cycle look like?

If everything goes according to plan, it might look something like this:

1. Your mare comes into estrus. Your veterinarian examines her and finds a developing follicle and appropriate uterine changes.
2. She is monitored as the follicle develops. Ultrasounds may initially be every day or two and become more frequent as ovulation approaches.
3. The follicle becomes mature. Your veterinarian considers the follicle's size and appearance, uterine edema and the mare's individual history.
4. Semen is ordered. The timing is based on the mare's expected ovulation, the stallion's collection schedule and the time required to ship the semen.
5. Ovulation may be induced. Your veterinarian may administer hCG or deslorelin to help make the timing more predictable.
6. The semen arrives. It has been collected, evaluated, extended and cooled for transport.
7. Your mare is inseminated. The goal is to have viable sperm present around the time she ovulates.
8. Your veterinarian confirms ovulation. The mare is examined again to make sure she has ovulated and to check for problems such as post-breeding uterine fluid.
9. Pregnancy is checked. Around 14–16 days after ovulation, your mare is examined for pregnancy.

And that's it. At least, that's what happens when the mare behaves herself.

What if she doesn't follow the textbook?

This, unfortunately, is likely.

• Sometimes the mare doesn't start cycling when you think she is going to.
• Sometimes the follicle grows more quickly than expected.
• Sometimes it grows slowly.
• Sometimes, especially early in the season, she will be having a transitional cycle — which looks like a regular cycle at first but is not breedable.
• Sometimes the mare develops a beautiful big follicle and then refuses to ovulate.
• Sometimes she ovulates before the semen arrives.
• Sometimes the semen shipment is delayed.
• Sometimes the semen motility is low when it arrives (again, not necessarily the stallion station's fault).
• Sometimes she develops fluid in the uterus post-breeding.
• Sometimes everything is perfect but the mare isn't pregnant at the 14-day check.

How much does all of this matter?

It can feel like a lot when you are new to breeding.

You may find yourself making phone calls about semen while simultaneously trying to understand what a 38mm follicle means, driving your mare back to the vet for another ultrasound, and wondering why everyone is suddenly very interested in what her uterus looks like.

Fresh-cooled semen gives mare owners access to stallions that may be hundreds or thousands of miles away. It gives us enormous choice in breeding programs. The trade-off is that the mare's cycle has to be managed around the lifespan of the semen and the timing of ovulation.

That is why good reproductive veterinary management matters.

What should you, as the mare owner, keep track of?

Even if your veterinarian is managing the entire breeding process, you should keep your own records.

Over time, this becomes incredibly useful.

You may discover that your mare tends to ovulate quickly after reaching a particular follicle size. Perhaps she consistently needs treatment for post-breeding fluid. Perhaps she tends to have a longer estrus than most mares.

Your veterinarian will still make the medical decisions, but you now have a history of your mare that you can carry from one breeding season to the next.

And when your veterinarian asks next year — "What did she do last year?" — you won't have to try to remember. You'll have the record.