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How Often Do Horses Go Into Heat & What Age Do They Stop?

Last updated: September 8, 2023

By: Miles HenryFact Checked

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Thinking of breeding horses? Recently, we decided to replace an older broodmare with a young prospect., and a big question popped up: When do horses go into heat, and when do they stop?

Most mares start their heat cycle before they turn two and usually stop by the age of twenty. Each heat cycle lasts about three weeks. Things like age, where the horse lives, and the time of year can change this. But remember, every horse is different.

Did you know a mare’s heat cycle can influence the success of breeding? Many horse owners are in the dark when it comes to understanding these cycles. If you’re thinking of breeding, you’re about to unlock some essential insights that can make all the difference.

Picture of two horses playing.

What is the Estrous Cycle?

When talking about horse breeding, you’ll often hear about the “estrous cycle.” But what is it exactly? Let’s break it down. Just like humans, female horses have a cycle that prepares their body for a potential pregnancy. This cycle is nature’s way of making sure everything is timed right for a baby horse, or foal, to be born.

Phases of the Mare’s Heat Cycle:

  1. Proestrus:
    • Think of this as the “warm-up” phase. The mare’s body starts getting ready for the main event, which is ovulation (releasing an egg). During this time, her body produces hormones that make the lining of her womb thicker and ready for a potential pregnancy.
  2. Estrus:
    • This is often called the “heat” phase. It’s when the mare is most receptive to a male horse, known as a stallion. Her body releases the egg during this phase, hoping it will meet up with sperm and start the process of creating a foal. This phase is the best time for breeding, and it lasts about 5 to 7 days.
  3. Metestrus/Diestrus:
    • After the heat phase, the mare enters a “cool-down” period. If she’s pregnant, her body begins the early stages of supporting the growing foal. If not, her body starts to reset and prepare for the next cycle. This phase can last up to two weeks.
  4. Anestrus:
    • This is the mare’s “off-season.” During certain times of the year, especially winter, mares might take a break from their heat cycles. It’s nature’s way of ensuring that foals aren’t born during harsh conditions. During anestrus, the mare won’t show signs of heat and isn’t interested in breeding.

By understanding these phases, horse owners can better plan for breeding and ensure the best care for their mares.

Frequency of Heat Cycles in Mares

If you’re considering breeding your mare or simply curious about her reproductive patterns, it’s important to understand the frequency of her heat cycles. Let’s explore how often these cycles occur and what factors might influence them.

Generally, mares go into heat, or estrus, every 21 to 23 days. This means if you’re observing your mare closely, you might notice signs of her being in heat roughly once a month. However, this isn’t a strict rule, and there are some factors that can change this timing.

Factors Affecting the Frequency:

  1. Seasonal Changes and its Impact on the Estrus Cycle:
    • Mares are seasonally polyestrous. This fancy term simply means they have multiple estrus cycles during specific seasons, usually spring and summer. As the days get longer, mares are more likely to go into heat. In contrast, during the shorter days of fall and winter, many mares enter the anestrus phase, pausing their heat cycles.
  2. Breed and Genetic Differences:
    • Just as different breeds have various traits like size or color, they might also have differences in their reproductive cycles. Some breeds might start their reproductive years earlier or have slightly altered heat cycle lengths. It’s always good to learn more about your specific breed to better understand its unique characteristics.
  3. Health and Nutritional Conditions:
    • A mare’s overall health and what she eats play a role in her reproductive cycle. Mares that are underweight or malnourished might have irregular cycles or even stop cycling altogether. On the other hand, illnesses or certain medications can also affect the regularity of her cycles. If you’re planning to breed, ensuring your mare is in good health and receiving proper nutrition is crucial.

Knowing these factors can help horse owners provide better care for their mares, especially if they’re considering breeding. Remember, always consult with a veterinarian if you have concerns about your mare’s reproductive health.

Picture of a horse and her baby foal.

Signs of a Mare in Heat

Whether you’re planning to breed or simply want to understand your mare better, recognizing the signs of her being in heat is key. Mares give off both physical and behavioral signals that they’re in the estrus phase. Let’s take a closer look:

Physical Symptoms:

  1. Vulva Changes:
    • During the heat cycle, you might notice that your mare’s vulva appears more relaxed or swollen. This change is a natural response to the rising levels of certain hormones in her body.
  2. Mucous Discharge:
    • Don’t be alarmed if you observe a clear or whitish mucous discharge from your mare’s vulva. It’s a typical sign of estrus and is the body’s way of cleaning the reproductive tract and providing a suitable environment for sperm.
  3. Frequent Urination:
    • If your mare seems to be urinating more often than usual, it might be an indication that she’s in heat. This isn’t because she’s drinking more water but is actually a behavioral sign linked to the estrus phase.

Behavioral Changes:

  1. Increased Attention to Stallions:
    • A mare in heat is more receptive to male horses. She might show more interest in them, trying to get closer or even nuzzle them.
  2. Restlessness:
    • You might observe that your mare seems more restless or agitated. This could manifest as pacing, being more alert, or showing a general sense of unease.
  3. Vocalizations:
    • Some mares become more vocal during their heat cycle. They might neigh or whinny more frequently, especially when they’re near stallions.

Why Recognizing the Signs is Important:

Being able to accurately spot when your mare is in heat can be the difference between a successful breeding attempt and a missed opportunity. Not only does it ensure the best chances of pregnancy, but it also helps in preventing unwanted breedings.

Plus, understanding your mare’s behavior can improve the bond between you and your horse, as you’re more in tune with her natural rhythms. By being observant and understanding these signs, horse owners can better care for and connect with their mares.

Picture of a foal and its mother.

Optimal Breeding Age for Mares

Breeding is a significant decision for horse owners. One of the key factors to consider is the age of the mare, as it plays a vital role in fertility and the overall health of both the mare and her offspring.

Age Range When Mares are Most Fertile:

Mares are typically most fertile between the ages of 4 and 15 years. During this period, they often have regular estrus cycles, and their reproductive systems are functioning optimally. While mares can and do conceive outside of this age range, their peak fertility lies within these years.

Risks and Considerations When Breeding Younger Mares:

  1. Physical Maturity: While most fillies experience their first estrus cycle by age two, this doesn’t necessarily mean they’re ready for breeding. Their bodies are still growing and maturing. Breeding too early can pose risks to both the mare and her foal.
  2. Emotional Maturity: Young fillies might not yet have the emotional maturity to mother a foal. This could lead to issues like rejecting the foal or not knowing how to care for it properly.
  3. Reproductive Complications: Younger mares are more prone to complications during pregnancy and foaling. These complications can be stressful and potentially costly for owners.

The Relationship Between Age and Fertility in Horses:

As mares age, their fertility naturally decreases. After about 15 years, the decline in fertility becomes more noticeable, and by the age of 20, many mares have significant difficulty conceiving. This reduced fertility can be attributed to several factors:

  1. Egg Quality: Just as in humans, the quality of a mare’s eggs decreases with age. Older mares might produce fewer viable eggs, making conception less likely.
  2. Uterine Health: The health of a mare’s uterus can decline over time, especially if she’s had multiple foals or infections in the past. An older uterus might not support a pregnancy as effectively as a younger one.
  3. General Health: As mares age, they might face other health issues that can impact their ability to carry and birth a foal safely.

While horses have a broad age range during which they can conceive and give birth, their peak fertility is between 4 and 15 years. It’s essential to weigh the risks and benefits of breeding at different ages and always consult with a veterinarian to make informed decisions about your mare’s reproductive health.

When Do Mares Stop Going Into Heat?

As mares age, their reproductive systems undergo changes. One of the most notable changes is the eventual cessation of their heat cycles. Understanding this transition can help horse owners make informed decisions about breeding and the overall care of older mares.

General Age Range for the Cessation of Heat Cycles:

While there’s variability among individual mares, most mares start to experience irregular heat cycles or cessation of cycles between 18 to 24 years of age. By the age of 25, many mares have completely ceased going into heat, though exceptions do exist.

How Age Affects the Regularity and Predictability of Heat Cycles:

  1. Decreased Regularity: As mares approach their later years, the regular 21-23 day heat cycle can become less predictable. Some mares may skip cycles altogether or have longer intervals between them.
  2. Reduced Fertility Signs: Older mares might show fewer or more subtle signs of being in heat. Their physical and behavioral symptoms might not be as pronounced as in their younger years.
  3. Anestrus Duration: During the non-breeding seasons, older mares might have prolonged anestrus periods (times when they don’t cycle) compared to their younger counterparts.

Tips for Managing Older Mares in a Breeding Program:

  1. Regular Vet Check-ups: Schedule consistent vet visits to monitor the reproductive health of your mare. Veterinarians can offer insights into her fertility status and advise on whether continued breeding is advisable.
  2. Manage Expectations: Understand that fertility rates drop as mares age. It might take longer for an older mare to conceive, or she might not conceive at all.
  3. Optimal Care: Ensure older mares have a balanced diet, regular exercise, and preventive health care. Good overall health can have a positive effect on whatever reproductive potential remains.
  4. Consider Alternatives: For mares with valuable genetics, consider embryo transfer. This procedure allows the mare’s embryo to be placed in a younger surrogate mare, reducing the physical strain on the older mare.

While mares generally cease their heat cycles in their late teens to early twenties, each mare is unique. If breeding an older mare is a consideration, it’s essential to be informed, patient and prepared for challenges. Regular consultation with a veterinarian will provide the best guidance for managing older horses in a breeding program.

Picture of a mare and her foal. Knowing when horses go into heat is paramount to breeding success.

Enhancing Breeding Success

Breeding horses can be both rewarding and challenging. To increase the chances of success, it’s essential to consider a multifaceted approach. Here’s a closer look at the elements that can significantly influence breeding success:

Role of Veterinary Consultations and Pre-Breeding Check-ups:

  1. Assessing Reproductive Health: Regular veterinary check-ups can identify any issues or abnormalities in the mare’s reproductive system. This includes checking for infections, cysts, or any other structural abnormalities.
  2. Timing is Key: Veterinarians can guide on the best time to breed by closely monitoring the mare’s cycle, ensuring higher chances of conception.
  3. Semen Quality: If using artificial insemination, vets can evaluate the quality of the semen to be used, ensuring it’s viable for breeding.

Modern Tools for Monitoring Estrus:

  1. Hormonal Tests: Blood or urine tests can measure hormone levels, such as progesterone and estrogen, to pinpoint where the mare is in her cycle.
  2. Ultrasonography: This imaging method provides a non-invasive way to observe the mare’s reproductive organs, particularly the ovaries. It can show the growth of follicles, the presence of an ovulatory follicle, or if the mare has ovulated.
  3. Behavioral Observation: Modern technology also includes devices that monitor the mare’s behavior, logging any changes that indicate she’s in heat. Combining tech with traditional observation can provide a comprehensive picture of the mare’s cycle.

The Significance of Good Mare Health and Nutrition:

  1. Overall Health: A mare in good health is more likely to have regular estrus cycles and conceive successfully. Ensure she’s free from illnesses, stress, or any underlying health issues.
  2. Balanced Diet: Proper nutrition plays a pivotal role in reproductive success. Mares should receive a balanced diet with adequate vitamins and minerals to support the demands of pregnancy.
  3. Body Condition: A mare’s body condition can influence her reproductive potential. Both underweight and overweight mares can experience difficulties in conception and carrying a pregnancy to term. Maintaining an optimal body condition score is crucial.

Enhancing breeding success involves a combination of regular veterinary consultations, leveraging modern monitoring tools, and ensuring the mare is in prime health and nutritional condition. By paying attention to these factors, horse owners can optimize the likelihood of a successful and healthy breeding outcome.

Key Takeaways for First-Time Breeders

Breeding a mare for the first time can be a journey filled with excitement and uncertainties. As you embark on this adventure, here are some pivotal lessons to guide your way:

  1. Timing is Everything:
    • Understanding the mare’s heat cycle is foundational. Recognizing when she’s in estrus maximizes your chances of a successful mating. Whether it’s through physical symptoms, behavioral signs, or advanced monitoring tools, being in sync with your mare’s cycle is paramount.
  2. Patience is a Virtue:
    • Breeding isn’t always a one-time success. It may take multiple attempts before a mare conceives. Be patient, adjust your approach as needed, and always keep the well-being of the mare in mind.
  3. Observe, Observe, Observe:
    • Mares often give clear signs of where they are in their cycle. Familiarize yourself with these signs, and trust your observations. Even with technology, a breeder’s intuition and observation skills are invaluable.
  4. Seek Professional Guidance:
    • While there’s much you can learn and do on your own, never underestimate the importance of professional advice. Regular consultations with a vet can guide your decisions, clarify doubts, and provide actionable insights.
  5. Never Stop Learning:
    • The world of equine reproduction is vast and ever-evolving. Stay proactive in your learning. Attend workshops, read up on the latest research, and engage with other breeders. Knowledge is your ally in ensuring a successful and healthy breeding process.
  6. Expert Advice is Gold:
    • Whether it’s from seasoned breeders, veterinarians, or equine nutritionists, tap into the wealth of knowledge around you. Their experiences and expertise can help you navigate challenges and make informed choices.

Remember, breeding is as much an art as it is a science. With dedication, observation, and the right support, first-time breeders can embark on this journey with confidence and care.

horse riding edited

Managing and Suppressing a Mare’s Heat Cycle

For various reasons, horse owners might want to manage or even suppress the heat cycle of their mares. This could be due to behavioral issues related to estrus, scheduling of competitive events, or other specific needs. Here’s a guide on how this can be achieved and the considerations involved:

1. Why Suppress the Heat Cycle?

  • Behavioral Changes: Some mares exhibit pronounced behavioral changes during estrus, which can be challenging for riding or training. This includes mood swings, reduced performance, or heightened sensitivity.
  • Competition Schedules: For competitive mares, being in heat during an event might not be ideal. Suppressing the cycle ensures consistent performance.
  • Medical Reasons: Occasionally, mares might have medical conditions, like ovarian cysts, which can be alleviated by suppressing their cycle.

2. Methods of Suppression:

  • Hormonal Treatments: One of the most common methods involves administering synthetic hormones that interrupt the mare’s natural cycle. Progestin is one such hormone used.
  • GnRH Vaccines: These are used to inhibit the secretion of gonadotropin-releasing hormone, subsequently suppressing estrus.
  • Ovariectomy: A more permanent solution involves surgically removing the ovaries, but this is typically reserved for severe medical conditions.

3. Natural Approaches:

  • Light Control: Mares are long-day breeders, meaning longer days stimulate their reproductive cycle. By controlling light exposure, one can influence the mare’s cycle.
  • Dietary Adjustments: Some believe specific dietary supplements or herbs can help modulate a mare’s behavior during her cycle, although scientific evidence is limited.

4. Considerations Before Suppression:

  • Consultation: Always consult a veterinarian before deciding to suppress a mare’s heat cycle. They can guide on the best method suited for the mare’s individual needs.
  • Side Effects: Hormonal treatments might have side effects. It’s essential to be aware and monitor the mare closely during treatment.
  • Reversibility: Some suppression methods can be reversed, allowing the mare to return to her regular cycle, while others might have long-term impacts.

5. Monitoring and Adjustments:

  • After beginning any suppression method, continuously observe the mare for any behavioral or physical changes. Regular vet check-ups are also recommended to ensure the mare’s health and well-being.

Suppressing a mare’s heat cycle can be beneficial for various reasons, but it’s a decision that should be made with ample information and professional guidance. It’s not just about stopping the cycle but ensuring that the chosen method aligns with the mare’s overall health and well-being.

Below is a helpful YouTube video that explains when horses first go into heat.

Conclusion: How Often Do Horses Go Into Heat

Embarking on the journey of breeding a mare for the first time is both a rewarding and complex endeavor. The intricacies of understanding a mare’s heat cycle, recognizing the signs of estrus, and determining the optimal breeding age are essential components to grasp.

Coupled with the significance of good health and nutrition, and the advantages of modern tools for monitoring estrus, breeders are equipped with a comprehensive toolkit to enhance their chances of success.

However, as with any journey, the path to successful breeding is often paved with challenges, requiring patience, keen observation, and an unwavering commitment to learning. If you’re a first-time breeder or even a seasoned one, always remember the value of expertise.

No book or article can replace the tailored advice and insights a veterinarian or equine specialist can offer. Dive deeper into this fascinating world of equine reproduction, consult professionals, attend workshops, and continually seek to expand your knowledge.

In the vast field of equine breeding, every mare is unique, and every experience is distinct. So, equip yourself with knowledge, surround yourself with expertise, and embark on this rewarding journey with confidence. Your dedication and commitment to understanding and caring for your mare will surely pave the way for breeding success.

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