If you are planning to breed your mare, understanding her reproductive cycle is one of the most useful things you can learn. You may assume that because you are sending your mare to a repro vet for her breeding work, you don't need to pay attention to the details. At StallionApp we advise that you keep your own records, and pay attention to what is happening with your horse. That's not because we don't trust your vet — we do. It's because your mare is your horse, and ultimately it is up to you to know what is going on with her and to keep a detailed breeding history. When it comes to breeding horses, ignorance is not a bliss you can afford to indulge in. Understanding the process will help you make better decisions, and hopefully save you time and money.
Now, we are not saying you need to become an expert in equine reproduction (but we will encourage you to be one if you want to!). Knowing what your mare's cycle looks like can help you understand what your vet is telling you, anticipate what is likely to happen next, and keep better breeding records from one cycle to the next.
And if you've ever found yourself staring at an ultrasound screen while your vet says something like "She's a 35-millimeter follicle, grade 2 edema" and thought "Okay… what does that actually mean?" — this is for you.
The mare's cycle
The mare's reproductive cycle is approximately 21 days, although just like in humans, there is considerable individual variation.
The cycle is broadly divided into two phases:
Estrus (heat or season) — when the mare is receptive to the stallion and developing a follicle that may ovulate. This can last anything from 4 to 8 days.
Diestrus (out of heat or season) — when the mare has ovulated and her reproductive system is preparing for either pregnancy or the next cycle.
The part breeders are generally most interested in is estrus, because this is when the mare can become pregnant. This is also when everything that requires us to enter the uterus has to happen, because it is only during this period that your mare's cervix is open. If we want to enter the uterus to breed, or to run a reproductive health diagnostic such as a uterine culture or cytology, we have to do all of that while the cervix is open.
During estrus, the dominant follicle produces increasing amounts of estradiol (estrogen). Rising estrogen acts on the reproductive tract and causes several characteristic changes:
- The cervix relaxes and opens, becoming softer and more flaccid.
- The uterus develops increased edema and tone.
- The mare becomes behaviorally receptive to the stallion.
- The reproductive tract becomes more favorable to the passage of sperm.
As ovulation approaches, estrogen levels are high, and the cervix is typically at its most relaxed and open state. After ovulation, progesterone from the developing corpus luteum (this is basically what the follicle turns into after it has ovulated) becomes dominant. The cervix then closes, becomes firmer, and loses the edema associated with estrus.
What the cervix being open means practically
The open cervix during estrus allows uterine secretions and inflammatory material to drain, while the closure after ovulation helps create a more protected environment should the mare become pregnant. This period when the cervix is naturally relaxed and open is the most appropriate time for procedures such as breeding and many uterine diagnostics.
The timing of breeding, any diagnostics your vet might need to run, and any procedures such as uterine lavage to help clear debris are therefore essentially a race against the clock: you want viable sperm to be present around the time the mare ovulates, and you want the uterus to be clean and clear of fluid and other debris by the time the embryo reaches it. And you need to achieve all of this before the cervix closes. Phew!
What happens during heat or season?
A mare in estrus may show behavioral signs that she is receptive to a stallion.
She may:
- Raise her tail
- Urinate frequently
- "Wink" her vulva
- Become more interested in stallions (or geldings) — and be loud about it!
- Stand for a stallion rather than kicking or avoiding him (standing heat)
This is her behaviorally saying "I'm ready, folks, bring me a man!" But not all mares show behavioral signs of estrus, so behavior isn't always a reliable way of determining exactly where a mare is in her cycle. This is where knowing your mare, and what is normal for her, is helpful.
Some mares show very obvious signs of heat. Others are much more subtle when they are in season. Some mares are tricky and will show behavioral signs that indicate heat when they aren't at the ideal stage for breeding at all! This is why we use the ultrasound to determine estrus rather than relying solely on what the mare is doing. The ultrasound, which is done rectally as the mare's reproductive tract is very conveniently located below her rectal tract, can visually show us exactly what is happening in your mare's uterus.
Edema
Growing follicles are often what is talked about when an ultrasound is done on a mare that is within the window for breeding. But follicles alone can be misleading, as a mare can grow dominant follicles when in diestrus too. Edema in the uterus means that estrogen is rising, which means the dominant follicle is producing it — and it is this that indicates the follicle is going to ovulate and is actually breedable.
Edema in the uterus is just as important an indicator that your mare is coming into heat, if not more so. A dominant follicle without the expected uterine edema does not necessarily mean that the mare is ready to breed. Edema is an important indicator of estrogenic activity and therefore helps your vet determine where the mare is in her cycle, but it needs to be interpreted alongside the follicle and the rest of the reproductive examination.
Generally — and remember, your mare's uterus will do what it wants and doesn't always like to follow the rules — edema will rise as the follicle grows. Edema is ranked from 1 to 3, with 1 being some edema present and 3 or 3+ indicating a large amount of edema. Just to make it a bit more confusing, edema usually drops off just before ovulation. If your vet has not been tracking your mare's progress day to day, they might mistake the drop in edema that happens just before ovulation for the early stages of her heat cycle.
The follicle
Inside the mare's ovary, follicles develop during estrus. One follicle will generally become dominant and continue developing toward ovulation. This is the follicle that is going to ovulate, and your vet can see on the ultrasound the changes that take place for that follicle as your mare nears ovulation. Experience in knowing what these changes look like allows your vet to determine when it is time to order in semen and inseminate your mare.
Or at least, that's the plan. Unfortunately, mares often do not follow the textbook, and will ovulate early or "hold on" to a follicle, which can really mess up your vet's ability to time insemination. If you are keeping good records on your mare from cycle to cycle, however, you can be helpful in reminding your vet that "she went early the last two ovulations" or "she tends to hold onto them longer." Now, just because she went early or late the time before doesn't mean she will this time, but all the information we can gather on the mare and her cycle tends to help in good decision making.
You might therefore hear your vet say things like:
"She's got a 35mm follicle."
Or:
"The follicle is soft and misshapen and I think she's getting close."
The size of a follicle can provide useful information, because generally mares ovulate follicles that are larger than 30mm in diameter, so tracking the size and the rate of growth can help with ovulation predictions. The follicle will also change in shape as the mare gets close to ovulation, often going from round to more of a teardrop. It will also get softer. The appearance of the follicle, changes in the ovary and uterus, and the mare's individual history all contribute to the veterinarian's assessment of where she is in her cycle.
That's why serial ultrasounds can be so useful. A single ultrasound is a snapshot. Several ultrasounds show you the movie.
What happens when she ovulates?
Ovulation is when the mature follicle ruptures and releases the egg. We ideally want the sperm to be waiting outside the oviducts when this happens. Sperm can survive for a couple of days within the mare's reproductive tract, but the egg has a relatively short window of viability after ovulation if it is not fertilized.
The goal is to time breeding appropriately around ovulation.
With fresh or fresh-cooled semen, your veterinarian may recommend breeding at a particular point before ovulation and then monitoring the mare to determine whether she has ovulated. For example, your vet may give your mare a shot to induce ovulation when she has good edema and a follicle bigger than 35mm. Depending on the type given, your mare will usually ovulate within 36 to 48 hours after that shot. Your vet may order semen when they give the shot, the semen arrives the next day, the mare is inseminated, and hopefully she will then ovulate within the next 24 hours after insemination.
With frozen semen, timing can be even more critical because the viable lifespan of thawed sperm is shorter. Your veterinarian may therefore monitor the mare much more closely around ovulation, sometimes examining her at several-hour intervals when particularly precise timing is required.
What happens after ovulation?
Once the follicle has ovulated, the structure that remains develops into a corpus luteum (CL).
The CL produces progesterone, a hormone that helps maintain the mare's reproductive system in a state suitable for pregnancy. This is why, when your mare is having her 14-day pregnancy check, you might hear your vet comment on the size of her CL. We want a CL to be present and producing progesterone, as this helps sustain the early pregnancy.
If she is not pregnant, your mare eventually undergoes luteolysis — the CL regresses, progesterone falls, and she returns to estrus.
And the cycle starts again.
Your tracking checklist
When your vet is scanning your mare, they are gathering a wealth of information. At StallionApp we encourage mare owners to be an active part of this process and keep their own records. Not so you can challenge your vet — not at all. It's so that you can understand what your vet is telling you, and so that if for any reason your normal vet is not available and your mare needs a scan, you can be helpful in relaying what has been going on with your mare so far.
Your veterinarian may be monitoring:
- Follicle size and development
- Follicle characteristics
- Uterine edema
- Uterine tone
- Cervical changes
- Ovulation
- The corpus luteum
- Uterine fluid
- Pregnancy
The information gathered over several examinations helps your veterinarian make decisions about when to breed, whether the mare has ovulated, and whether anything unusual is happening.
And this is where good breeding records become incredibly useful.
Your mare has a history — keep it
Every mare develops her own reproductive history.
You might learn that she tends to develop follicles quickly. Perhaps she ovulates at a particular follicle size. Maybe she consistently has a short interval between ovulation and the next heat. Or perhaps she has a history of accumulating uterine fluid after breeding.
Those details can become incredibly useful when you breed her again. Instead of starting from scratch every breeding season, you have a record of what happened previously.
Over several years, this becomes much more than a collection of notes.
It becomes your mare's reproductive history.
Understanding the cycle makes breeding less mysterious
Breeding a mare can initially feel complicated. There are follicles, ovulations, hormones, ultrasounds, uterine examinations, semen shipments and an alarming number of dates to remember. But once you understand what is happening during the cycle, the process starts to make much more sense.
Your veterinarian is there to interpret what they see and make the medical decisions. Your job as the owner is to understand the process, communicate with your veterinary team, and keep good records.
And that last part is something we can help with.
StallionApp lets you keep your mare's breeding history in one place, including breeding events, reproductive information, treatments and pregnancy outcomes. Instead of trying to reconstruct what happened last breeding season from text messages, notebooks and scraps of paper, you can build a history that stays with the mare.
Because next breeding season, when your vet asks "What happened with her last year?", you won't have to say:
"Hang on. I know I wrote that down somewhere…"
With StallionApp, the answer is at your fingertips, always.
Where to go next
- Ready to start recording? Mare Management: What Breeding Records Should You Keep?
- Season coming up? Preparing for Breeding Season
- Fluid after breeding? Oxytocin in Mares
- Difficult mare? Older Maiden Mares: The Tight Cervix Problem