Last Updated on October 23, 2022
Understanding the estrous cycle in horses can be key to understanding the behavioral and physical changes that mares go through during this time. If you’ve got a moody mare, or you want to know when to breed your horse, let’s find out everything you need to know about the estrous cycle in horses!
What Is The Estrous Cycle In Horses?
The estrous cycle in horses is the name given to describe the reproductive period of a horse. Female horses are seasonally polyestrous – this means that their estrous cycle only takes place when the daylight period of the day is the longest, during the spring and summer. They will gradually stop cycling in the fall and most mares do not have an estrous cycle at all during the winter.
The estrous cycle in horses consists of a series of physical and hormonal changes that take place over a set period of time, and it is split into two distinct phases.
The first of these is called diestrus. This is the period where the mare is not fertile and will not be receptive to being rated by a stallion. During this time, gradual changes are occurring within the reproductive tract to prepare the mare for the next stage of her cycle.
The second phase of the estrous cycle in horses is called the estrus phase. This is when the mare is receptive to being mated as she is fertile and able to conceive. You will commonly hear horse owners referring to this as the mare being ‘on heat’ or ‘in season’, and it is highly likely you will see some behavioral and physical changes in your mare during this time.
How Long Is A Mares Heat Cycle?
Most mares have a very consistent reproductive cycle, which is normally three weeks in length. This makes it easy to predict when a mare is going to come into season.
During this cycle, the mare is normally in diestrus for around 15 days. She will then be in estrus or on heat for around six days. The cycle length will be more variable towards the start or end of the reproductive season.
If you are considering breeding from your mare, you will need to be able to identify the signs that she is coming into estrous. Some mares show very obvious signs, while others are much more subtle. Knowing when she is most fertile will help maximize the chances of successful mating and conception.
What Does A Mare In Estrus Look Like?
When a mare is in estrous, she will undergo both physical and behavioral changes. These are caused by increasing and decreasing levels of reproductive hormones in the body. During the estrous phase, a viable egg is released from the ovary ready to be fertilized, and the mare needs to be able to indicate this to any potential mating partners.
Externally you may not notice a huge difference in your mare when she is in estrous. Some horse owners may observe that the vulva of the horse becomes slightly swollen, but this is not the case for all mares.
However, internally there are many changes taking place in female horse genitals to enable the mare to be successfully mated. The cervix, a tight seal at the entrance of the uterus, will become relaxed. The uterine wall will become firm and the ovaries will prepare to release an egg for fertilization.
If you are intending on breeding from your mare, your veterinarian may perform an ultrasound scan of the mare’s reproductive system. This will enable them to observe these changes taking place and select the ideal time for the mare to be mated with the best chance of success.
The more traditional way of determining when a mare is ready to mated is to observe her behavioral changes. Most mares are very good at demonstrating when they are and are not willing to be covered by a stallion. Some will show these signs even when a male horse is not present, whilst others will need to be teased by a stallion first.
When a mare is in the estrus phase, she will raise her tail and ‘wink’ the lips of her vulva. She may expel small amounts of urine frequently, and will allow a male horse to mate with her. If a mare is not fully in estrus, she will squeal and kick at the stallion if he tries to mount her.
Do Female Horses Get Periods?
A period, or overt menstruation, is the term for when a mammal expels bloody tissue from the reproductive tract at the end of the reproductive cycle if a successful mating has not taken place. This occurs when the lining of the uterus – the endometrium – is shed in preparation for the next reproductive cycle. Overt menstruation primarily occurs in humans and primates such as chimpanzees.
Most mammals, including horses, have a process called covert menstruation. This means that the endometrium is reabsorbed by the body at the end of the reproductive cycle. This means that female horses do not get periods, as all the bloody tissue is retained inside the body.
If you ever see blood on a female horse genitals, this is not normal and you should seek veterinary advice as soon as possible.
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Summary – Estrous Cycle In Horses
So, as we have learned, the estrous cycle in horses coast during the spring and summer months when the daylight time is longest. Most mares have an estrous cycle that is 21 days long, and she will be fertile and receptive to mating for around six days of this cycle. This fertile period is often referred to as the mare being on heat, and you may see significant behavioral and physical changes in your mare during this time.
We’d love to hear your thoughts on the estrous cycle in horses! Do you find it particularly difficult to tell when your mare is in season? Or perhaps you’ve got some questions about how to help a mare that has mood changes during her estrous cycle? Leave a comment below and we will get back to you!
Kate Chalmers is a qualified veterinary nurse who has specialized in horse care for the vast majority of her career. She has been around horses since she was a child, starting out riding ponies and helping out at the local stables before going on to college to study Horse Care & Management. She has backed and trained many horses during her lifetime and competed in various equestrian sports at different levels.
After Kate qualified as a veterinary nurse, she provided nursing care to the patients of a large equine veterinary hospital for many years. She then went on to teach horse care and veterinary nursing at one of the top colleges in the country. This has led to an in-depth knowledge of the care needs of horses and their various medical ailments, as well as a life-long passion for educating horse owners on how to provide the best possible care for their four-legged friends.
Kate Chalmers BSc (Hons) CVN, Dip AVN (Equine) Dip HE CVN EVN VN A1 PGCE