Repro Strategy

Hearn Veterinary Services would like to take a moment of your time to explain the reproductive strategy to employ an intensive program that will achieve optimum pregnancy rates. Every mare is bred at the first opportunity during the breeding season, and if she does not become pregnant, she is returned as quickly as possible to the breeding shed.

Lighting Programs

Barren and maiden mares should be placed under lights in mid November. This starts them cycling with a transitional cycle approximately 60 days later (mid January), and insures that they are on their second cycle of the year by 90 days (mid February). It is worth starting a lighting program up until the beginning of February. After that, the normal lengthening of the day will do just as good a job (provided that they are turned out from dawn to dusk). Consult with your veterinarian when you are setting up your lighting program. Many mares are presented to breeding sheds in March, every year, that are supposed to have been on a “lighting program”, but are not cycling. These mares usually start irregular transitional cycles in March and do not have their first ovulation until April.

Exams to identify where a mare is in her cycle:

Mares should be teased every other day through the breeding season or until they are at least 30 days pregnant. Mares are examined by the veterinarian if they show estrus or if they have not shown estrus for at least 3 weeks (to identify mares that cycle but do not show estrus). Some mares do not show estrus well, so they must be examined more frequently to identify when they will return to estrus. Ultrasonography is a good tool to evaluate the reproductive tract, because it provides more information than manual palpation or vaginal speculum exams. This allows the vet to reduce the number of routine exams needed to monitor the progress of the mare.

Using Progesterone Levels to discover where a mare is in her estrus cycle

Progesterone (P4) levels are sometimes used together with an ultrasound or vaginal speculum exam to check where a mare is in her cycle. It can be very useful to determine whether a mare has started cycling yet this year.
Progesterone level <0.5 with no signs of estrus = Mare has not yet started cycling this year.
Progesterone level <0.5 with signs of estrus = Mare in Estrus
Progesterone level >3.0 = Mare in Diestrus, but has started cycling.

Breeding on Transitional Estrous cycles

Most mares do not cycle during the winter, and normally have their first ovulation in the first week of April. The first estrous cycle is called the transitional cycle, because it is often abnormally long. Mares bred on transitional estrous cycles are just as likely to become pregnant as mares bred on normal cycles later in the spring, but they are 3 times more likely to loose their pregnancy. It is hypothesized that while the ovaries have started to ovulate follicles again, the uterus is not yet ready to support a pregnancy in all mares. Barren mares have been found to be 2 times less likely to become pregnant, but 7 times more likely to loose their pregnancy than foaling mares. Therefore, we recommend that mares NOT be bred on their first estrus cycle, but on the following estrous cycle. To minimize the loss of time between the first cycle and the next, Progesterone and Prostaglandin (PG) can be used to return the mare to estrus about 1 week earlier than she would naturally.

Reproductive Ultrasonography during estrus

Ultrasonography provides detailed information on the structures on the ovaries and within the uterus. This allows the practicer to quantify and describe the level of uterine edema, uterine fluid, and uterine cysts. Post-breeding ultrasonography shows whether the mare has ovulated, and identifies whether a mare has had difficulty clearing her uterus after breeding. The presence of uterine fluid is indicative of a uterine clearance problem and can frequently indicate an early uterine infection before the mare even starts a vulvar discharge. This allows the vet to treat the infection with uterine lavages or uterine infusions of antibiotics, and significantly increases the chance that the mare will become pregnant on the same cycle. The use of ultrasonography increases the accuracy of rectal palpation. The increased information allows fewer exams, and improves the chance of the each normal healthy mare becoming pregnant by approximately 10% / estrous cycle. This means that if you use ultrasonography, you are 10% less likely to have to pay for another estrous cycle. The savings of board for another 3-4 weeks and the expense of veterinary care for another estrous cycle is significant.

Induction of Ovulation

To improve the accuracy of breeding, it is routine to use either hCG (human chorionic gonadotropin) or GnRH (Gonadotropin Releasing Hormone) to induce ovulation. There are several brands on the market. GnRH is more accurate than hCG but it is almost significantly more expensive.

Uterine Cultures and Uterine Cytology

A uterine culture is done on all mares prior to breeding, on every estrus bred, to protect the stallion from venereal diseases. Uterine infections are a problem for several reasons: they may prevent the mare from becoming pregnant, cause an abortion later in pregnancy, or infect a fetus in the uterus and produce a newborn foal with an infection. Not all inflammation is caused by an infection. Identifying the presence of inflammation as early as possible, and correcting the problem, provides the optimum chance of getting the mare pregnant.

Uterine Infections (Endometritis)

Following the identification of an infection on uterine culture, the lab will supply us with a list of drug sensitivities. A suitable antibiotic is then selected to treat the infection. Both intrauterine and systemic (antibiotics given in the muscle, vein or mouth) antibiotics have been shown to be effective against uterine infections, and veterinarians are now often using a combination of therapies for the best efficacy.

Post Breeding Examinations and Treatments

It has been noted in research during the last few years that mares that have uterine clearance problems react strongly to the stallion ejaculate, produce uterine fluid that is not cleared from the uterus, and often develop uterine infections after breeding. The problem is normally diagnosed with an ultrasound examination following breeding, at the time that the mare is being examined for ovulation. The diagnosis of uterine fluid following breeding (while it can still be treated) is one of the best reasons to use ultrasound exams during estrus. Mild and moderate problems can be managed with uterine antibiotic infusions, uterine lavages and oxytocin, and systemic antibiotics and carry a moderate chance that the mare will still become pregnant. Severe problems require extensive treatment and rarely become pregnant. Mares that are not treated are significantly less likely to become pregnant and significantly more likely to suffer a pregnancy loss if they do become pregnant.

15 Day Early Pregnancy Ultrasound Exams

We routinely do pregnancy ultrasounds at 15 days after the last breeding date: a) To identify mares that are not pregnant and return them to the breeding shed without losing any time. It is important to note that some non pregnant mares have persistent corpora lutea and do not return to estrus normally at 16-17 days after ovulation. b) To identify twins and crush one early before embryonic fixation. Early manual twin reduction significantly increases the chance of the mare retaining her pregnancy to foaling. c) To identify any signs of endometritis, such as severe uterine edema or uterine fluid. d) To provide early pregnancy data to update early pregnancy statistics on the stallion.


Thoroughbred mares have double ovulations and produce twins on approximately 20% of breedings. Twins may result from the splitting of a single ovulated ovum(rare = 5%) or from double ovulations (common = 95%). Mares that have double ovulations on one cycle are highly likely to do so on the next cycle. The embryo(s) are highly mobile within the uterus until 15-16 days post ovulation when uterine tone increases and the embryo(s) become fixed. With twins, both embryos will fix unilaterally (together) in the same horn about 70% of the time, and in separate horns (bilaterally) about 30% of the time. If the twins are fixed bilaterally in separate horns there is less than a 15% chance that one will reduce. If the twins are fixed unilaterally in the same horn and are of similar size (0-3 mm difference), 73% will lose one or both. If the twins are fixed unilaterally in the same horn and are of dissimilar sizes (>3 mm difference), about 85% will reduce on their own. The optimal time to crush a twin is prior to fixation at 16-17 days.

30 Day Pregnancy Ultrasound Exam

We routinely do a pregnancy ultrasound exam at 30 days after the last breeding date. Mares normally loose about 13% of their pregnancies over the 25 days between the 15 day exam and the 40 day exam. This exam is done to insure that the mare has not yet lost her pregnancy. If she has lost the pregnancy, she will probably have a persistent corpus luteum on the ovary which will prevent her from returning to estrus on her own. Therefore, she will probably need prostaglandin (PG) to assist her prompt return to estrus. The 30 day exam also allows a second chance to check for twins before the embryos implant into the uterus and it becomes extremely difficult to re-breed the mare.

40-45 Day Pregnancy Ultrasound Exam

Next to the early pregnancy examinations, this is the most important exam. The reasons to include 40 to 45 day pregnancy exams are: a) Mares identified as not in foal at 40 days can still be given prostaglandin(PG) and often may be bred again. It becomes significantly more difficult to return the mare to the breeding shed if she does lose her pregnancy after 40 days. The presence of endometrial cups (producing equine chorionic gonadotropin) creates multiple supplementary (secondary and accessory) corpora lutea, which are very difficult to remove with Prostaglandin. b) Once a pregnancy has carried past 40 days the probability of pregnancy loss drops dramatically. The probability of pregnancy loss between 15 and 40 days is approximately 13% over 25 days, but between 40 days and foaling is only 9% over 300 days. c) Pregnancy exams between 40 and 45 days are required by many insurance companies to insure fetuses, and are often required by sales companies to certify pregnancy and classify mares as pregnant at the end of the breeding season.

Progesterone Levels and Progesterone Supplementation

Low progesterone levels have been seen in mares about to loose their pregnancy. However, there is considerable controversy as to whether the corpus luteum is malfunctioning or if low concentrations of progesterone are caused by premature luteolysis as a result of uterine irritation. Progesterone supplementation (Regu-Mate) has been shown under experimental conditions to significantly reduce the probability of pregnancy loss in mares which have had a severe endotoxic insult or mares that have had their ovaries removed. Double dose Regu-Mate (20 cc / day) has been shown to maintain pregnancy in mares given 5 days of prostaglandin injections. It is therefore hypothesized that progesterone supplementation may prevent pregnancy loss from stress, colic, laminitis, or prolonged transport. If the mare is a likely to be stressed, or is a high stress individual, we recommend starting Regu-Mate 2-3 days after breeding and continuing it to at least 45 days. An alternate approach is to monitor progesterone levels and only supplement those that drop below 4 ng/ml. Unfortunately, this method frequently identifies a problem when it is already too late, therefore we rarely recommend this strategy. If you can afford it, double dose Regu-Mate is better than single dose in problem or high risk mares.


Hearn Veterinary Services would like to continue to offer state of the art reproductive services to our clients. Our strategy is aimed at producing the maximum number of live, healthy foals during the breeding season. Our statistics show that our batting average in the breeding shed is about 75% per cycle and over 90% per season. It is possible to sacrifice some pregnancies and breed the mares at less expensively. However, the extra cost involved in boarding the mare for another 3 weeks plus the veterinary care needed to breed her on another cycle exceeds the marginal increase in using the optimum veterinary technology on each estrus.

Dr. Patrick Hearn DVM, MSc.
Dr. Elizabeth Hearn BA., DVM.

713558 1st Line EHS,
Mono, Ontario, Canada, L9W 5V5
(Located NE of Orangeville in the Hockley Valley)

Office: 519-943-1153