Preparing the Mare for Breeding

by Debra Ottier

Iron Horse Equine

It’s that time of year again, and here is some useful information to help plan your breeding experience. First off, ask yourself, when do I want my mare to foal? Since the gestation length of horses is 11 months, you should plan when you want to breed your mare in order to foal in an appropriate time, usually April, May or June. For example, if you want your mare to foal in June, you would plan to breed her in May.

Next, one needs to know about how a mare cycles. Mares are seasonally polyestrus breeders, which means they have several reproductive heats or estrus periods during one season. The normal cycle of a mare is approximately 21 ┬░2 days, with the following breakdown:

~ Day 0-7 Heat or Estrus Mare exhibits sexual responsiveness to stallion or teaser
~ Day 5 Ovulation Ideally breed within 12 hours of ovulation
~ Day 7-21 Diestrus Mare exhibits disinterest in stallion or teaser (kicking, aggressiveness)

Note: These are guidelines only, with individual variation.

During the winter months, mares go through a period of change where managers will view irregular cycling, followed by, in most mares, an anovulatory period or ovarian shutdown. The first transitional phase will start as daylight decreases, usually around September, with the complete shutdown occurring around October to November. This stage is a prolonged period of ovarian inactivity and is the result of the decrease in daylight associated with winter. The lower light affects secretion of hormones involved in the induction of the estrous cycle in mares.

As daylight increases, mares will go through another period of transition. This period will have a mare show signs of estrus – but NO OVULATION. It is characterized by erratic estrus behaviour. Beware!! Do not breed during the transitional stage as the mare is not producing follicles that will develop and ovulate. Signs to watch for when a mare is ready to breed are regular estrus or heat cycles lasting longer than approximately 2 days and are approximately 21 days apart. An ultrasound is the only definitive method to determine that the mare is in fact ovulating.

As spring approaches, one must take time to start recording particulars about your mares cycle, management program and nutritional status. From a nutritional standpoint, having your mare with a body condition score of 6 is ideal for breeding. This score relates to moderately fleshy with fat over the ribs being spongy, fat on the tailhead soft, fat beginning to be deposited along the side of withers, behind shoulders and along the side of neck. Research has shown that overweight mares tend to have a higher pregnancy rate and easier foaling then mares considered to be thin.

EFFECT OF BODY CONDITION SCORE (fatness) ON REPRODUCTIVE EFFICIENCY IN MARES(Henneke et al, 1984; Theriogenology 21:897)

Initial CS Final CS Conception Rate (%) Cycles per Conception
3.0 5.0 56 5.66
4.0 6.0 79 2.77
5.0 6.0 89 1.52
6.5 6.5 97 1.24
8.0 8.0 100 1.38


After regular estrous cycles have been determined, it is quite advantageous to control estrus through administration of hormonal preparations. Advantages of this include a more efficient use of stallion semen in an artificial insemination program such that a group of mares can be bred with a single ejaculate, the ability to schedule breedings to stallions of limited availability during the breeding season due to performance events, and to synchronize recipient mares to donors for embryo transfer.

Various hormone regimes have often been used to aid breeders in predicting time of ovulation and/or to help induce the mare to start normal cycling during the transitional period. Most of these therapies aim to provide an increased secretion of Luteinizing Hormone (LH), the hormone responsible for ovulation. There are nutritional supplements that will also aid the mare in stabilizing her hormone levels, to reduce sporadic behaviour and enable success.



If one wishes to breed the mare to obtain a foal as close to January 1, lights can be increased to 16 hour per day in November. The mare will start to cycle within 2 months and will ovulate within 3 months. Therefore, by following this regime, the mare will be ready to breed in February. The type of light doesn’t matter, intensity will affect the mare. The rule of thumb is if you can read a newspaper in the darkest corner, the light intensity is sufficient. Light can affect gestation length as well. Mares with increased light will foal 10-14 days earlier.


Only exceptionally well grown fillies (2-3 years of age) should be bred as their bodies are still immature and will require a higher level of nutrition for growth of themselves as well as the developing fetus.

A mare at the age of 14-16+ years are still able to conceive as eggs are produced and ovulated, but problems may lie in implantation in the uterus. Scarring or other factors on the uterine surface can lead to early embryonic mortality (EEM). Examination of the mare prior to breeding will reduce the incidence of this.


A mare’s conformation may lend the animal susceptible to infection in the reproductive tract, making implantation difficult. It is recommended to have a uterine culture and cytology done prior to breeding to ensure the mare is free from any infections which could potentially cause abortion or EEM.


Nutrition has been shown to play a large role in the reproductive efficiency in mares. A mare underweight, showing a body condition score of 4 or less has a lowered conception rate and may have difficulty foaling.

A mare with a foal at their side may not exhibit signs of estrus as they may suppress the heat to protect the foal, this is also known as lactational anestrus. Also, if separating the foal when breeding, the mare may become stressed and will not be susceptible to the stallion.


The use of steroids to suppress heat cycles during the show season will cause long term effects on the mare’s ability to exhibit heat and may even cause some mares to become more stallion like where they are very aggressive.


Some mares may ovulate more than one follicle at a time. The incidence of twinning may cause the mare to absorb both embryos reducing reproductive efficiency as their abdomen capacity and vasculature is limited for two developing fetus’. Ultrasound at day 14 to determine pregnancy and examine for the possibility of twins is recommended.


It is also recommended to keep accurate records of the mare’s activities. This will increase reproductive efficiency and lower costs as stallion owners will be better able to service the mare with this information. The following is a list of information required:

  • Details of previous estrous cycles
    • Number days in heat (avg ~7 days)
    • Number days between cycle (avg ~21 days)
  • Findings on uterine swab
  • Number of breedings before mare failed to return to estrus
  • Number of foals successfully delivered
  • Number of previous pregnancies
  • History of dystocia
  • History of early embryonic mortality
  • History of foaling injuries
  • Details of previous investigations and any treatments


  • Selection of the stallion
  • Mare exhibiting regular heat cycles
  • Negative culture obtained for all non-maiden mares
  • Nutritional status – mare body condition score minimum 5
  • Vet check obtained if mare has poor reproductive efficiency or if unaware of history


    Examined visually

    Caslicks present?


    Examined by speculum

    Does it slope forward and allow urine pooling


Examined by speculum

Presence of tears, scarring, previous foaling injuries


Rectal examination, ultrasound, culture, cytology, endometrial biopsy or endoscopic exam

Presence of endometrial cysts, bacterial infection, scarring and fibrosis, inadequate endometrial glands


Examined by rectum and ultrasound

Presence of ovarian tumour, cystic tissue, adhesion

The above checklist should aid one in preparing the mare for the breeding season. These items will help to service the mare quicker, easier and at a reduced cost to all. If one were to ship their mare to the stallions farm too soon to be serviced, the mare will not be ready to breed and time and money will be wasted waiting for the next heat cycle. At ranges of $6 -$12 per day, this can be costly.

In order to reduce risk and cost, one may consider artificial insemination with transported semen. This process is relatively easy and will reduce the risk associated with transporting a mare, especially if she has a foal at her side. It reduces costs as the mare remains at home, no mare care or transporting costs are required.

Questions? Contact Debra Ottier at Iron Horse Equine 519-823-6981or e-mail