Mare Ovulation Calculator
What horse owners and breeding managers would give for a resource or utility that would allow them to precisely predict the day a mare will ovulate! While there are numerous online resources and charts that can reliably predict parturition based on the day a mare is bred, there is not yet a handy device that can be used to predict ovulation with the ease and low cost of a scanning thermometer or test strip. There are however, tried-and-true methods which owners, breeders and veterinarians routinely employ to determine when a mare will ovulate.
Mares begin to come into estrus early in the Spring, with their estrous cycle being approximately 21 days. Usually, the mare will be receptive to breeding for about five days toward the end of her cycle.
Timing and Tenacity
Most of the available literature maintains that the mare should be bred within the 12 hour period prior ovulation in order to have the best chance of getting her in foal. Unlike humans however, where ovulation might be calculated based upon the beginning or end of a woman's last menstruation, there are no similar presentations in the mare.
The mare may exhibit one or more within a set of behaviors signaling estrus (during which ovulation occurs). These include winking of the vulva, teasing response, etc. and may include changes in temperament. However, these behaviors and the extent to which they are displayed are particular to the individual mare. Some mares display few if any obvious signs of estrus, making those behaviors only marginally reliable indicators of when ovulation will occur.
Mares that exhibit no behavioral signs of estrus are called “silent heat” or “covert” mares. Usually, these mares have normal ovarian cyclic activity, but they lack the behavioral signs of being in estrus. In order to determine heat in these mares, palpation, cervical exams, or an ultrasound of the ovaries can all be used to ensure that these mares that are not displaying signs of estrus are in fact cycling.1
Observation and Optimization
Both the mare and the stallion should undergo a thorough examination by a veterinarian to ensure that both are in sound health with regard to breeding, and that there are no conditions that would pose complications or risks to the mare.
Many veterinarians and breeding managers advise that mares should be teased every other day to detect estrus (heat). It is recommended that the mare's behavioral signs are recorded throughout her cycles. Once she begins to display signs of estrus while teased, a veterinarian should be called in for an examination of her ovaries.
The veterinarian will palpate the mare’s ovaries and may use ultrasound evaluation to monitor the growth and maturity of follicles. With his or her experience as a frame of reference (and possibly knowledge of the individual mare's history), the veterinarian can establish baselines and determine a date range for likely ovulation.
With established baseline information on mares that have been cycling, it is easier for the veterinarian to determine a date range for ovulation based on follicle size (via ovarian palpation). In a breeding program, it is highly recommended that records are kept of these measurements, as this will help in determining at what follicle size the mare is likely to ovulate and the best time she should be bred. Some mares may ovulate when the follicle reaches a 35 mm, while some ovulate when the follicle is much larger.1
A developing follicle increases in diameter until one or two days before ovulation. When the follicle nears maturity, it becomes softer and changes from a spherical to a conical shape. These changes take place 12 to 24 hours before ovulation, giving the examining veterinarian information that can be used to pinpoint the optimum time for breeding.2
The mare will normally ovulate about 24 to 48 hours before the end of estrus. Many breeders elect to use the hormone hCG (human chorion gonadotropin) to induce ovulation in order to maximize chances of the mare conceiving. Custom compounding pharmacies also offer hCG in combination with other drugs, such as deslorelin, which has been shown to improve the efficacy of hCG.
Some breeders choose to wait a few months after foaling before breeding the mare again. Usually, this is to ensure that involution of the uterus (when the uterus returns to normal size post-partum) has taken place. This is especially recommended if the mare had difficulties foaling. In normal circumstances, the mare’s uterus usually undergoes complete involution three to four weeks after foaling.1
2Equinews. Ovulation in Mares.
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