The mares are in heat stimulated by the increasing photoperiod during the long days of the year. If during these months she does not become pregnant, the cycles will be repeated every 21 days on average until the days become short and the mare goes into rest for estrous cycles (anoestrus). Her heat consists of an estrous phase characterized by behavioral changes and changes in her reproductive organs to accept the male, and a luteal phase in which she is no longer receptive and prepares for gestation and, if this is not the case, repeats the mating again. cycle.
When does heat start in the mare?
Oestrus begins when mares reach sexual maturity, and this usually occurs between 12 and 24 months of age. At this time, the mare's reproductive system begins to interact with other parts of the body, hormones begin to be secreted and act, and the first ovulation occurs, with its associated physical and behavioral changes in order to be covered by the male. at the right time to get pregnant. Although the mare less than two years old is already in heat, she is not until she is 4 years old when they reach their maximum growth.
The mare is long-day seasonal polyestrous, which means that her heat appears when the daily daylight hours begin to be higher, that is, in spring and summer, presenting various heats at this time of year (they are repeated every 21 days on average). Their ovaries remain at rest the remaining months of the year, entering anoestrus, because when there are fewer hours of light, more melatonin is released by the pineal gland, a hormone that inhibits the hypothalamic-pituitary hormonal axis in the mare, which is the one that stimulates the ovaries to produce the hormonal changes responsible for ovulation.
Certain conditions make mares not in heat or very irregular in the reproductive season. These reasons may be malnutrition or extreme thinness, old age or increased cortisol from corticosteroid therapy or Cushing's disease (hyperadrenocorticism), which is the stress hormone and suppresses the mare's hormonal axis.
Estrous cycle phases in mares
Estrous cycle is the name given to the series of recurrent phases and events that are caused by the reproductive hormones of females. The mare takes between 18 and 24 days to complete all the phases, that is, in an average of 21 days the cycle will start again if she is in her season reproductive. This cycle is divided into two phases: follicular and luteal, with two stages each:
Follicular phase (7 to 9 days)
During this phase, the blood supply to the mare's genital tract increases, its walls have clear, shiny mucus, and the cervix relaxes and opens, especially around ovulation because estrogens produced in this phase are on the rise. In turn, the vagina relaxes, is lubricated and edematous and the mare is receptive to the male This is divided into two periods:
- Proestrus: lasts about 2 days, follicle growth stimulated by follicle-stimulating hormone (FSH) occurs and estrogen begins to increase.
- Estrus: lasting between 5 and 7 days, also known as estrus phase, ovulation or detachment of the preovulatory follicle, which should measure between 30 and 50 mm depending on the height of the mare. It occurs 48 hours before this stage ends. In 5-10% of cases, double ovulation occurs when two follicles develop, reaching up to 25% in the case of English Thoroughbred mares, however, double gestation in mares is a danger.
Luteal phase (14 to 15 days)
After ovulation, estrogen decreases and progesterone increases in the corpus luteum (structure formed in the ovary from the granulosa cells of the follicle, hence the name of the phase), which is It peaks 7 days after ovulation and leads to the cervix closing, becoming pale and mucus-free, and the vagina becoming dry and paler. This is so because this phase prepares the uterus to support the pregnancy, however, if it has not occurred, the mare will repeat the cycle at the end of it. In turn, this phase is divided into two:
- Metaestro: stage that lasts 2 to 3 days, where the corpus luteum is formed and progesterone increases.
- Diestro: lasts about 12 days, progesterone continues to be produced and the dominant follicle develops so that it ovulates in the next zeal. At the end of this stage, the corpus luteum produces prostaglandins, which break it down and the mare returns to heat in two or three days.
If you suspect that your mare may have become pregnant, we encourage you to read this other article on How to know if a mare is pregnant?
Symptoms of heat in mares
There are a series of signs that indicate that our mare is in heat and is receptive to mating by the male. In addition to being more restless, the mare:
- Tilt your pelvis down.
- Raises and deflects the tail to expose the vulva.
- Expels mucus and urine in small amounts to attract the male.
- Vaginal redness.
- "Vulvea", which is known as an exposure of the clitoris through the repeated movement of the vulvar lips.
- She is friendly and affectionate, she stands with her ears attentive, waiting for the male to approach her.
Each mare is unique, there are some that show very obvious signs and others that are very subtle, that's why sometimes use horses for heat in order to reveal the mare's heat.
If the mares are not in heat and a male approaches them, they are distant, they do not let them approach, they lower their tails to hide their genitals, their ears are thrown back and they can even bite or kick.
Are horses in heat?
Horses males are not in heat, since they do not go through the phases of the estrous cycle like females, but from their maturity sexual are fertile at all times. However, in the season they are more active, since the mares are also active and, when they detect a mare in heat, they develop peaks of sexual activity.
This detection is carried out through pheromones released by the mare in heat with her urine, which is thicker and more opaque than usual, through the Flehmen reaction. This reaction consists of the retraction of the upper lip when they smell the urine to detect the pheromones through the vomeronasal organ (auxiliary organ for smell in some animals, located in the vomer bone, which is located between the nose and the mouth, which allows accurately detect these compounds), along with caresses, neighs and approaches towards the mare.
For more information, you can consult this other article on How do horses reproduce?
What is the foal's heat?
This is a heat that appears between 5 and 12 days after calving, it is a very early heat that occurs when The mare has a physiological endometritis after giving birth and her defenses are suffering due to this process. Because of this, care must be taken that she is close to a whole male, especially in those that present it before day 10-11, since her endometrium is still regenerating and if a male covers her it would aggravate the mare's endometritis, which would lower fertility.
If by chance she becomes pregnant, there may be a risk to her and the foal, with abortions, dystocia, stillbirths or placenta retention, being more frequent in mares older than 12 years or those that have presented problems in the previous pregnancy.
