Best Management Practices for Breeding a Mare

Breeding Management Practices for Mares

Breeding Management Practices for Mares

Contributed by Washington State University

Breeding season for horses usually occurs in the spring. This makes winter a good time for breeders to plan for the upcoming season. Though people have many reasons for breeding a horse, whether for commercial purposes or as a hobby, there are some important factors that all breeders should consider before getting started.

The mare’s age is important, as well as the number of pregnancies and foals she has produced in the past. Older mares that have not had a foal may have more difficulty becoming pregnant. Mares with a history of abortion or pregnancy with twins may be categorized as high risk. Also consider the mare’s health and if she is in good body condition. Does she have any potentially heritable disorders? Does she have any musculoskeletal problems that carrying a foal would exacerbate, such as lameness due to tendon injuries or arthritis?

“Another question horse breeders should ask themselves is why they want to breed this mare, in particular, when so many unwanted horses are out there,” said Dr. Lisa K. Pearson, a Washington State University (WSU) large animal theriogenology resident. “If an owner would be just as happy adopting or rescuing an adult horse for training, showing, or pleasure riding, then perhaps bringing another horse into the world would not be to their advantage. Also, will the owner accept responsibility for the foal for its lifetime? If the foal is to be sold, will there be provisions that if that new owner cannot keep it, it can be returned to the breeder? If it is to be raised and used by the breeder, will they provide for it regardless of the foal’s potential use as an athletic animal? All of these questions should be addressed before a mare is bred.”

Breeding Soundness Evaluations
Once the decision is made to proceed, it is essential that the owner involve a veterinarian from the very beginning. First, a breeding soundness evaluation (BSE) should be performed to assess the mare’s reproductive potential. This important examination has several components. Mares undergo a physical examination, including an assessment of the conformation of the mare’s perineum (the rectal and vulvar area).

broodmare and foalHorse breeders should ask themselves why they want to breed their mare when there are so many unwanted horses out there.

Transrectal palpation and ultrasound of the reproductive tract are also performed to assess where the mare is in the estrous cycle and to evaluate any pathology, such as abnormalities of the ovaries, uterus, cervix, vagina, or bladder. Mares are seasonally polyestrous, meaning that they have a distinct breeding season.

“Ideally, we’d like to see mares bred from March to June but there are techniques for extending this window,” Dr. Pearson said. “Some mares may cycle year-round.”

A BSE also includes sample submission for laboratory analysis. Uterine culture and cytology are used to screen for infection or inflammation, and a uterine endometrial biopsy can provide information regarding the potential of the uterus to carry a foal to term.

“Owners should schedule the BSE with their veterinarian early in the season, as mares may need treatment for any diagnosed conditions,” Dr. Pearson said. “Additionally, starting early allows for several cycles to pass to try and establish a pregnancy. The BSE is a very important and common examination that is performed here at WSU on a regular basis. One advantage we have here is that we have a laboratory on-site to examine samples, so we can get results faster than those who have to ship them to a laboratory.”

Tracking a Mare’s Cycle
Once a mare is judged reproductively sound, the next step is tracking her cycle. This is best done using ultrasound to examine the ovaries and uterus. One examination will not provide enough information to know when a mare should be bred. Typically, examinations are performed several days apart until a dominant follicle is established on one or both ovaries. Thereafter, daily ultrasound examinations may be needed to accurately time insemination. Insemination can be accomplished through artificial insemination using fresh cooled semen or frozen semen, or by live cover.

“Assuming we are using fresh cooled semen,” Dr. Pearson said, “once the mare has a sufficiently large follicle, she can be induced to ovulate by injecting commercial hormone preparations.
Ovulation will usually follow within 36 hours. Insemination is timed around ovulation, and usually two doses of semen are administered, once before and once after ovulation. Ultrasound examinations are performed after insemination to ensure that ovulation occurred, and to assess the amount of post-insemination inflammation in the uterus.”

“All inseminations cause inflammation,” she said. “Some mares, however, tend to have more severe reactions than others, causing the uterus to accumulate fluid. If this fluid remains when the embryo travels down the oviduct to the uterus, the embryo will not survive. Veterinarians can lavage or flush the uterus with sterile fluid solutions and administer the hormone oxytocin to help remove fluid from the uterus.”

transrectal ultrasound broodmarePhoto: Henry Moore Jr. BCU/WSU

A transrectal ultrasound can help assess where a mare is in the estrous cycle and evaluate any potential abnormalities.

A pregnancy diagnosis can be made 12 to 14 days after breeding.
Ultrasound is used to visualize an embryonic vesicle. If the mare is not pregnant, she will return to estrus usually within one week.

A mare’s cycle is 21 days long, so she may be developing a new dominant follicle which can then be tracked. A mare can also be “short-cycled” if she is not pregnant, meaning that an injection can be given to hasten the next heat. This typically causes an advancement of approximately five days.

If the mare is pregnant, the veterinarian will check for the presence of one or two embryonic vesicles, and if two are present (twins), the veterinarian will discuss options with the owner.

Twins have a very high rate of abortion, and twins that survive to birth are usually smaller, weaker, and may require extensive medical care.

If one vesicle is seen, the veterinarian will make a schedule with the owner for subsequent evaluations: 25 to 30 days of gestation to assess the fetal heartbeat; 45 and 60 days to assess health of the pregnancy, and periodically in mid to late gestation (gestation is on average 342 days). The highest rate of pregnancy loss is before 60 days of gestation. Determining the foal’s sex may be done between 60 and 70 days of gestation. Blood drawn from pregnant mares can be analyzed for serum progesterone levels, the hormone that maintains pregnancy. Mares with low progesterone levels can be supplemented orally.

“Working closely with a veterinarian ensures a mare has the best chance of becoming pregnant, and will not be bred needlessly if she has any problems that may prevent her from becoming pregnant,” Dr. Pearson said. “Planning ahead ensures that when breeding season arrives, owners have a good idea of what to expect both for their mare and from their veterinarian.”

For more information regarding mare breeding, including concerns about high risk pregnancies, contact Dr. Ahmed Tibary, head of WSU’s large animal theriogenology service, at 509-335-1963 or To schedule a breeding soundness evaluation, contact WSU’s Veterinary Teaching Hospital at 509-335-0711.

The Equine Group at Washington State University’s College of Veterinary Medicine is a team of professionals devoted to providing exceptional veterinary care for horses in the Pacific Northwest. Particularly well recognized for their advances in lameness diagnosis, techniques for surgical intervention, assessment of neurologic conditions, and understanding of infectious diseases, the Equine Group’s combined efforts in clinical service, teaching, and research have made them national and international leaders in equine medicine and surgery. For more information, visit

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