Understanding your Mares Reproductive Cycle

If you own a mare, you already know that mares have moods. They have opinions. They have days when they seem to be a completely different horse from the one you rode last week. And while there are certainly individual personality differences between mares, a huge part of that variation is hormonal — predictable, cyclical, and entirely logical once you know what to look for.

Understanding your mare’s reproductive cycle isn’t just for breeders. Even if you never plan to breed her, knowing where she is in her cycle helps you plan your training, understand her behaviour, and build a better partnership. You stop fighting her on the difficult days and start working with her natural rhythm instead.

This post covers everything you need to know — from the basics of the estrous cycle to seasonal patterns, signs of heat, and when to call your vet.

The Basics: What Is the Estrous Cycle?

Mares are seasonally polyestrous, which is a scientific way of saying they cycle repeatedly during certain times of year and go quiet during others. During the active breeding season (roughly spring through autumn, depending on your hemisphere and latitude), your mare will go through a new reproductive cycle approximately every 21 days.

Each cycle has two distinct phases:

Ovulation — the release of the egg — typically occurs one to two days before the end of estrus. This is the window that matters for breeding. For the rest of us, it’s simply useful to know that the “difficult” days are biologically driven and will pass within a week.

Reading the Signs: Is She in Season?

Not all mares show their heat cycle dramatically. Some are very obvious; others show very little outward sign. Most fall somewhere in the middle. Here’s what to look for:

The Seasonal Pattern: Why Mares “Switch Off” in Winter

Unlike many domestic animals that cycle year-round, mares evolved in a seasonal world. They are long-day breeders, meaning they respond to the increasing day length of spring and summer. As days shorten heading into autumn and winter, most mares gradually stop cycling altogether — a period called seasonal anestrus.

This is completely normal. During winter anestrus, your mare is essentially having a hormonal rest. She may seem calmer, more settled, and more consistent — many people find their mares are actually easier to ride during this period, precisely because the cyclical hormonal fluctuations have stopped.

The tricky period comes in spring, as the days lengthen again. This transitional phase — roughly 60–90 days as her reproductive system “wakes up” — can produce irregular, prolonged, or erratic cycles. She may seem to be in season for two weeks straight, or cycle with very short intervals. This is normal but can be confusing if you don’t know it’s coming.

Optimal Breeding Windows

If you are breeding your mare, timing is everything. Mares ovulate towards the end of estrus, and because sperm can survive in the reproductive tract for 24–48 hours, the optimal breeding window is the 24–48 hours just before ovulation.

The challenge is knowing exactly when that is. Options include:

Teasing: Introducing a stallion (or a small pony or gelding used as a “tease horse”) and observing her response. A mare in peak heat will show strong receptive signs; one approaching the end of estrus will start to resist. Experienced breeders can get good at reading this, but it’s not precise.

Rectal ultrasound: The gold standard. Your vet can scan the ovaries and measure the developing follicle. A follicle above 35mm that is softening is close to ovulation. Most vets will recommend breeding or AI within 24 hours once this is seen.

Hormone monitoring: Less common in private yards but used on larger breeding operations — blood progesterone levels drop just before ovulation.

Common Reproductive Issues to Know About

Most mares cycle normally and without problems. But there are a few things worth knowing:

Prolonged estrus in the transitional period — covered above. Usually resolves once the season fully establishes. If it continues well into peak season, speak to your vet.

Silent heat — some mares don’t show obvious signs of estrus even when they’re cycling normally. A vet scan will confirm whether she’s cycling.

Irregular cycles — if your mare’s cycles are significantly shorter than 18 days or longer than 24 days during peak season, or if she seems to be constantly in season, it’s worth a vet check. Conditions like granulosa cell tumours (a benign ovarian tumour) can cause prolonged or absent estrus.

Mares that seem “always hormonal” — a small number of mares have ovarian issues that cause persistent hormonal behaviour. If training and management aren’t working and her behaviour is significantly affecting her quality of life, a vet examination is worthwhile.

The One Thing That Changes Everything

Track her cycles. That’s it. A simple notebook or phone note — date the start of each heat, how long it lasts, how she behaves. Within two or three months you’ll have a personal roadmap for your mare that no generic article can give you.

You’ll know that week 3 is your best training week. You’ll know that the first two days of heat she needs lighter work. You’ll stop being caught off guard by the difficult days because you’ll see them coming. And you’ll have the information your vet needs if something ever seems wrong.

Mares are not difficult. They’re just mares. And once you understand their rhythm, the partnership gets a whole lot better.