Mares may be bred by natural cover or artificial insemination, depending on breed registry regula­tions, preference of the stallion owner and/or mare owner, location and availability of the stallion and mare, cost, safety concerns, experience of person­nel, and other factors.

Natural Cover

Natural cover (also referred to as live cover or natu­ral service) is the only breeding technique allowed in Thoroughbred mares if the foal is to be registered with the Jockey Club. Mares of other breeds may be bred by either natural service or artificial insem­ination techniques.

Pasture Breeding

In a pasture breeding program, a stallion is typi­cally turned out with a band of up to 20 to 25 mares for approximately 60 to 90 days or more beginning in April or May.

The advantages of pasture breeding are decreased horse handling, decreased expenses for personnel, and decreased breeding expenses. Disadvantages include potential for injury to the stallion or the mares, inability to detect reproduc­tive problems with either the stallion or the mares while they are turned out together, spread of disease through the herd, and lack of an accurate concep­tion date for calculation of a foaling date. It is often not possible to confirm that a given mare was even covered by the stallion.

Hand Mating

Hand mating implies that both the stallion and mare will be handled and restrained in a controlled breeding environment.

Advantages of hand breeding over pasture breeding are:

  • Breeding can be scheduled and confirmed
  • The mare can be examined before and after breeding to confirm ovulation and evaluate the reproductive tract
  • Theoretically, hand breeding offers less risk to the stallion than pasture breeding
  • A dis­mount sample can be collected from the stallion after mating to confirm ejaculation occurred and confirm the presence of live motile spermatozoa.

Disadvantages of hand breeding are:

  • Time required for satisfactory examination and teasing of mares
  • Number of personnel involved in the breeding process
  • Risk of injury to personnel in the breeding shed.

Artificial Insemination

Artificial insemination (AI) of mares involves col­lection of semen from a stallion and deposition of that semen into the uterus of a mare in heat.

The ability to cool and freeze equine semen makes it feasible to ship semen from one location to another, giving mare and stallion owners more flex­ibility in their breeding program. A majority of breed associations have accepted the use of artificial insemination, cooled-transported semen and frozen semen. Consequently, mare owners have the opportunity to breed to stallions across the country or around the world.

Artificial Insemination with Fresh Semen

When fresh semen is deposited into the reproduc­tive tract of the mare, the sperm cells will usually remain viable for at least 48 hours. Therefore, a generally recommended breeding strategy consists of depositing fresh semen into the reproductive tract at least once every 48 hours while the mare is in heat, with at least one dose of semen being deposited within 48 hours prior to ovulation. Each insemination dose should contain at least 500 mil­lion progressively motile sperm, and should be diluted at least 1:1 with a satisfactory semen extender (i.e. a skim milk-glucose type extender) containing an appropriate antibiotic. It is not rec­ommended to use “raw” or un-extended semen for AI.

Artificial Insemination with Cooled Semen

Stallion semen may be cooled and transported from the site of collection to the location of the mare or semen may be cooled to use the next day on the breeding farm.

Spermatozoa that have been cooled for 24 hours will generally remain viable for an additional 24 to 48 hours after deposition into the reproductive tract of the mare. It is therefore recommended that the mare be inseminated with cooled semen within 24 to 48 hours prior to the anticipated ovulation. In most instances, it is anticipated that only one ship­ment of semen will be sent for a given mare in a single estrous cycle. In order to coordinate the tim­ing of semen shipment, insemination and ovulation, the mare must be monitored closely by transrectal palpation and ultrasonography.

Semen can be shipped by counter-to-counter airline service and arrive later in the same day as collected or can be shipped by overnight courier service and arrive the next day.

Mares in a shipped semen program are almost always administered hCG or deslorelin to induce ovulation soon after insemination and to eliminate the need for a second semen shipment. The ovula­tion inducing agent may be administered at the time the semen is ordered, when the shipment of semen has been confirmed, or when the semen arrives.

The shipping container should be kept closed until the mare is ready for insemination. At that time, a dose of cooled semen is removed from the con­tainer, gently mixed, and inseminated into the mare without pre-warming. A small aliquot should be saved, warmed to 37° C for 10 minutes in an incu­bator and evaluated under a microscope for motility characteristics. It is expected that there will be some loss of motility during the cooling and trans­portation process. Each insemination dose of cooled semen should contain at least one billion progressively motile sperm at the time that it is packaged. A cooled semen sample would be con­sidered good if progressive motility was at least 45 to 50% after 24 hours of cooling.

The stallion manager may package and ship one or two doses of cooled semen for a given mare. A mare owner may request two doses, but the final decision is made by the stallion manager and is often dependent on the number of motile sperm col­lected from the stallion, the number of mares to be bred on the farm, and the number of cooled-trans­ported doses requested for that day. If a single dose is sent, it is generally inseminated immediately upon arrival. If two doses are sent, the mare man­ager has several options for insemination. One option is to inseminate one dose of semen immedi­ately upon arrival, keep the second dose cooled, and inseminate the second dose the next day. This tactic works well if the mare has a normal repro­ductive tract and if the semen retains adequate motility after 48 hours of cooling. If the second dose is to be kept cooled, it is recommended that a new frozen can or pack be placed into the passive cooling transport system or that the semen be placed in a small container in a refrigerator at 5 to 8° C.

A second option is to put both doses into the mare when the semen shipment first arrives. This may be beneficial in cases in which the uterus of the mare reacts to the presence of spermatozoa with a severe prolonged inflammatory response or if the motility of the stallion is known to be poor after 48 hours of cooled storage. The decision regarding how to use two doses of semen should be based on cooling characteristics of the semen and uterine health of the mare to be bred.

Artificial Insemination with Frozen Semen

There are a number of strategies for successful use of frozen semen. The optimal strategy for a given mare may depend on the number of straws or doses available and the post-thaw motility. It is generally accepted that frozen-thawed spermatozoa will remain viable for only 12 to 18 hours in the repro­ductive tract of a mare and that the equine oocyte can remain capable of fertilization for only 6 to 12 hours after ovulation. Consequently, the timing of insemination and ovulation must be very tight when using frozen semen. If only a single dose of frozen semen is available, it may be optimal to inseminate immediately after ovulation is detected.

If the amount of frozen semen available is not lim­ited, a common breeding strategy is to deposit one dose of frozen semen in the uterus within 12 hours prior to the expected ovulation and deposit a second dose within 6 to 8 hours after ovulation has been detected. Alternatively, if only dose of frozen semen is available, a common strategy is to induce a timed ovulation by administration of a GnRH agonist (i.e. deslorelin or histrelin) at 8:00 pm when the mare is in heat and has a dominant follicle of the appropriate diameter; ovulation would be anticipated 40 hours later (or at approximately 12:00 noon). The mare is monitored by ultrasound every 6 hours and inseminated immediately once ovulation has been detected. A second option is to split the single dose and inseminate the mare with one-half of the dose prior to the anticipated ovulation and inseminate the other one-half dose after ovulation has been confirmed.
Unfortunately, not all mares administered deslorelin ovulate at exactly 40 hours. Ultrasound examinations must be performed periodically after deslorelin administration as some mares ovulate earlier than predicted and others ovulate later. In addition, some mares do not ovulate at all after deslorelin or hCG administration.


The length of time that sperm cells remain viable within the reproductive tract of the mare varies tre­mendously between fresh, cooled and frozen equine semen and varies with each stallion. Fresh semen will generally remain viable for at least 48 hours, cooled semen for 24 to 48 hours, and frozen semen only about 12 hours. Therefore, the goal of breeding management of the mare is to deposit semen into the reproductive tract at a time that will maximize the probability that viable sperm cells are present at ovulation. Breeding management tech­niques that are utilized include: 1) estrous detec­tion, 2) transrectal palpation and/or ultrasonography, and 3) hormonal therapy. Although the use of cooled or frozen semen pro­vides numerous advantages, owners and breeders must realize that the time and expense required for optimal breeding management may be much greater with cooled or frozen semen compared to that required for fresh semen or natural service.