Horse Reproductive Processes

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Reproductive Processes

Horses, like all mammals, give birth to live offspring. These offspring are nourished with their mother’s milk. Horses start to mature, or reach puberty, between 12 and 18 months of age. However, they are generally considered to be fully mature adults at four to five years of age, depending on the breed of the horse. Fillies will have their first ovulatory cycle when they are about 12-15 months of age, or slightly older. Even though young horses are capable of reproducing, it is not advisable.

Both mares and stallions should not start breeding until they are at least three years old, as early breeding can affect their growth. At that point, mares can be bred to produce one foal a year. A stallion can breed to produce many more – as many as 200 foals a year. It is advisable to keep stallions and fillies separate once they have reached one year of age. Even though young horses can reproduce, it is not advisable to have them do so. 


The main goal of controlled reproduction is the birth of a healthy foal. Therefore, it is important to understand how reproduction occurs. 

The mare’s role in the reproductive process involves:

  • Producing ova, or eggs
  • Providing housing and nourishment for the developing embryo, which becomes a fetus
  • Giving birth to the fetus
  • Producing milk for the nourishment of the young foal.

The male parent of a horse is a stallion and is commonly known as the sire. The female parent is the mare and is commonly called the dam. Their offspring is called the foal.

The mare’s reproductive tract includes the vulva, vagina, cervix, fallopian tube, uterus and ovaries.

  • The vulva is the external opening of the urogenital tract. It is part of the birth canal and the area where urine is voided.
  • The vagina is also part of the birth canal and is where semen is deposited.
  • The cervix is approximately 10 cm in length and lies between the vagina and the uterus. It relaxes when the mare is in heat and closes when she is not or when she is pregnant.
  • The oviduct, also called the fallopian tube, transports sperm and ova, or egg, to the site of fertilization, which occurs in the upper one-third of the fallopian tube. The fertilized ovum is then transported to the uterus. 
  • The uterus, or womb, is where the major development and nourishment occur. The uterus consists of two separate uterine horns. 
  • The ovary is the primary sex organ of the mare. The ovary produces the ovum to be fertilized and produces the estrogen and progesterone hormones.

A mare’s reproductive activity is seasonally polyestrous, which means that she has a reproductive season and a non-reproductive season. Both are influenced by light.

Once the mare has started to cycle normally, she has an estrous cycle of about 21 days, plus or minus 3 days. The cycle is composed of the estrous period, which means she is in heat and producing ova, and the diestrual period, which means she is out of heat. The estrous period usually lasts for four to seven days. It ends when the mare ovulates and releases the ovum into the oviduct. The normal diestrus period lasts 14 to 17 days. 

In more northern latitudes, like Canada, mares normally do not cycle during the late fall and winter months. As daylight hours in the early spring increase, they start cycling again. They will go through a transition period during this time before they begin to cycle normally.

The mare will have multiple heat periods that tend to be concentrated in late spring and summer. This is the normal breeding season for horses. In April, about 40 percent of mares are ovulating, increasing to about 70 percent in May and over 90 percent in June, July and August. Broodmares, or mares used for breeding, require additional calories in their diets.

Hormones are substances produced by a ductless gland and transmitted via the bloodstream to another organ, where they trigger a physiological response: 

  1. The estrous cycle is controlled by hormones. A follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are produced by the pituitary gland.
  2. FSH is released into the bloodstream and travels to the ovaries to cause the development of a follicle that contains an ovum.
  3. Estrogen is then released into the bloodstream.
  4. Estrogen in the blood cause LH to be released and act on the brain to put the mare into the physical state of being in estrus, or having an estrous cycle. Estrogens also act on the fallopian tube, uterus and cervix to prepare the reproductive tract for pregnancy.
  5. LH helps the maturation and ovulation of the egg-bearing follicle. Ovulation occurs when the mature egg leaves the follicle and begins its trip into the uterus.
  6. After ovulation, the level of estrogen drops and what’s left of the ovulated follicle are converted to form a corpus luteum (CL), which is a temporary glandular mass, also called yellow body. Cells in the CL secrete the progesterone hormone. This hormone shuts down the estrous hormones and sets the stage for pregnancy.
  7. If the mare is pregnant, progesterone will maintain the pregnancy by suppressing other hormonal activity.
  8. If the mare does not become pregnant, she will come back into heat.

Estrus can be detected by teasing the mare with a stallion or rectal palpation. Teasing means that a stallion is led near the mares. In the presence of the stallion, the mare will show behavioural signs of estrus, such as urination, squatting, winking of the vulva, and seeking out the stallion. Mares should be teased daily with a stallion to determine estrus. If a mare does not respond to teasing on the first day of estrus, the mare should be teased with a second stallion to be sure that the mare is not in heat. Additionally, estrus can also be detected through ultrasonography, where a veterinarian can determine follicle size and uterine changes indicative of estrus.

The estrous cycle is characterized mainly by the mare’s behavioural responses to the stallion. Estrus lasts an average of four to seven days. The mare will show behavioural signs such as:

  • Squatting with the hind legs spread apart and tail raised
  • Frequent urination
  • Exposure of the clitoris by contracting and relaxing the vulva, called winking
  • Leaning into the stallion
  • Seeking out the stallion.

During the diestrus part of the cycle, the mare will reject the stallion with behaviour such as:

  • Tail switching
  • Squealing
  • Striking, biting and kicking.

The stallion’s role in the reproductive process involves:

  • Producing reproductive cells known as sperm or spermatozoa and the male sex hormone testosterone
  • Introducing sperm into the female reproductive tract to fertilize the egg and produce a foal.

The stallion’s reproductive organs include the testes or testicles and the penis.

  • The stallion has two testes or testicles, suspended by a spermatic cord that extends from the abdomen to its attachment on the testicle. The testicles are ovoid, which means they are shaped like an egg, and are housed in the scrotum, which is an out-pouching of skin. The stallion’s testicles are formed within the abdominal cavity and descend into the scrotum between 30 days before birth and 10 days after birth.
  • The scrotum is made up of two scrotal sacs, which are separated by a septum. The scrotal sacs are located on either side and behind the penis.
  • The penis is the male organ of copulation and has three parts. The root attaches the penis to the pelvis with two strong ligaments and a pair of muscles. The body or shaft is the main portion of the penis. The glans penis is the enlarged front end of the penis. 
  • The urethra, a tube the length of the penis, conducts urine and semen to the outside.
  • The body of the penis is formed by the corpus cavernosa, which are sponge-like columns of erectile tissue that become engorged with blood during erection.
  • The epididymis stores sperm as it matures, and transports sperm from the testes to the deferens duct. The deferens ducts transport sperm from the epididymis to the area of the sex glands. The vesicular glands are sex glands and add gel to the ejaculate. The bulbourethral, vesicular and prostate glands add fluids to the ejaculate. 
  • The retractor penis muscle keeps the placid penis in the sheath and relaxes during erection.

As with a mare, the reproductive performance of the stallion is influenced by the season of the year. During the winter months, sperm production is about half that of spring and summer. The age of the stallion can also influence sperm production.

Stallions should enter the breeding season in excellent condition, as some may lose weight and condition as they breed. If they lose weight or condition, extra calories should be added to their diet. Stress can also affect the fertility and performance of the stallion.

LINK: For more information about horse anatomy and physiology, visit the Ontario Ministry of Agriculture, Food, and Rural Affairs or The Horse website. 

What does the Code of Practice for the Care and Handling of Equines say about responsible breeding and soundness?

“Many welfare problems can be prevented through responsible breeding. Responsible breeding:

  • Is purposeful rather than accidental or indiscriminate
  • Is managed by owners and handlers that are trained and knowledgeable
  • Involves careful selection of a mare and sire that are proven in their field, have good conformation and temperament, are healthy and free from known hereditary conditions that will impact the welfare of the offspring
  • Is based on comprehensive criteria for breeding, including past reproductive performance, age, size of the sire and mare
  • Produces offspring that have a known market or purpose.

Mares have special care requirements during pregnancy, foaling and the post-foaling period. A young foal also requires special care. The decision to breed should be carefully considered. Breeding can be very expensive and is not without risk to the mare. If there are complications during gestation, costs can increase substantially. The horse market is unpredictable, and there is frequently an oversupply of average horses for sale. If you do not wish to sell the offspring, it is crucial to be aware that horses can live in excess of 30 years, which may be longer than your ability to care for the animal.”

The following requirement is identified in the Code of Practice:

Do not breed horses unless you are familiar with and able to provide the basics of care as outlined in this Code for both the mares and foals.

These recommended practices are also provided in the Code of Practice:

  1. Seek advice from a veterinarian or experienced breeder prior to breeding a horse. 
  2. Plan for what you will do if you are unable to sell the foal or if you are no longer able to care for the foal. 
  3. Breed only if the foal has a known market or purpose.

“It is important to ensure that the mare is assessed by a veterinarian, professional breeder or technician at a breeding facility prior to breeding to ensure she is physically fit and healthy for the pregnancy.

A breeding soundness examination is recommended in order to detect reproductive abnormalities. The examination is particularly important for mares that have never been bred before, previously lost a foal, or have failed to conceive in the past.”

These recommended practices are provided in the Code of Practice: 

  1. Arrange for a breeding soundness examination to be done by a veterinarian proficient in equine reproduction. 
  2. Select stallions with appropriate body weight and size for the physical development and size of the mare, when natural breeding is used.
  3. Ensure mares have a body condition score of at least 5 out of 9 before breeding. 
  4. Treat mares for reproductive abnormalities before they are considered again for breeding in subsequent seasons.

What does the Code of Practice for the Care and Handling of Equines say about foaling?

“The physical changes indicative of impending parturition (foaling) may include:

  • Development of an udder
  • Distended teats
  • Softening of the tail head
  • The presence of a waxy substance on the end of the teats
  • Elongation of the vulva.

Some mares will foal without showing any of the above signs; therefore, it is essential that mares close to foaling are closely observed.

Most mares will foal unassisted; however, it is critically important to be knowledgeable about the foaling process so that a problem can be promptly identified and addressed. Survival of the foal and/or the mare is improved by rapid intervention when foaling difficulties arise. Contact a veterinarian or experienced personnel promptly when abnormalities during foaling occur, and be prepared to report relevant timelines (e.g., minutes since water broke). Normal foaling takes less than 20 minutes from the time the water breaks. If after 10 minutes of active labour the foal is not visible, provide appropriate intervention without delay.

Individual mares tend to show similar signs of impending parturition and follow similar timelines from one breeding season to another. When reviewed in subsequent breeding seasons, the records also provide valuable information to better predict foaling of individual mares.”

The following requirements are identified in the Code of Practice:

A plan must be in place for the foaling process, including a plan for getting prompt expert advice or help if needed.

Mares close to foaling must be observed at least twice a day for health, well-being and signs of foaling. 

These recommended practices are also provided in the Code of Practice:

  1. Consult a veterinarian or experienced breeder to become knowledgeable in the foaling process and how to provide appropriate assistance when the mare is having difficulty foaling.
  2. Ensure mares foal on clean pasture or in a large, clean box stall.
  3. Ensure a veterinarian or experienced breeder is available to attend the foaling at the first sign of difficulty.
  4. Arrange for the veterinarian to examine the post-partum mare, primarily if they did not follow the normal foaling progressions. The placenta should pass within three hours and be saved so it can also be examined.
Excerpts from the Code of Practice for the Care and Handling of Equines (2013) have been used with permission, Equine Canada and the National Farm Animal Care Council. The process for the development of Codes can be accessed through the National Farm Animal Care Council.

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