Breeding on Foal Heat

The mare has a long gestational period and to be a yearly producer she must be back in foal, on average, 25 days from giving birth. It is important to understand all the processes that take place in the period from parturition to the restoration of a state supporting the initiation and maintenance of another pregnancy (this period is often termed the puerperium). Among domestic species, the mare is unusual in that there is a return to a fertile estrus within two weeks of birth, and a new pregnancy can be established very early in the post-partum period. The decision on whether or not to use this first estrus, known as the "foal heat," remains a vexing question among breeders and veterinarians alike. There is no simple answer, but this article will discuss the changes that take place in the post-partum interval--the interval from parturition to the first ovulation--and how this period can be managed most effectively.

The foal heat is characterized by normal follicular development and ovulation by Day 20 post-partum in almost 100% of mares. Most mares return to estrus approximately five to eight days after parturition, with the average interval from birth to first post-partum ovulation being ten days. Often the foal will develop diarrhea at this time, which can make the detection of estrus in the mare easier. For a long time, it was thought that the diarrhea was due to hormonal changes occurring in the mother. However, the diarrhea has been seen in orphan foals, and this would suggest that the relationship is coincidental. The diarrhea is now thought most likely to be a normal physiological feature of the foal's early development.

Occasionally, a mare might not appear to have a foal heat. This can occur when the mare gives birth early in the year and the mare effectively enters a seasonal anestrus. It is important to remember that some mares are reluctant to show behavioral signs of estrus when they have a young foal due to their normal protective, maternal instincts.

An enormous reduction in the diameter of the uterus (uterine involution) occurs very rapidly after foaling, which is largely due to an increase in uterine contractility. This uterine involution is characterized by many features, perhaps the most obvious of which is reduction in size of the uterus. Recognition of delayed involution is a key issue as it forms an influential part of the decision to mate the mare on the foal heat.

As the uterus decreases in size, the post-partum luminal fluid (lochia) is discharged from the uterine lumen. This uterine discharge is normal and is often noticeable as a vaginal exudate around Days 3 and 4 after parturition. The color generally becomes paler by Day 5. Since the cervix does not close until after the foal heat, when progesterone production from the corpus luteum begins, this fluid is discharged via the cervix. As the endometrial epithelium of the mare remains almost intact after passage of the placenta, there is less post-parturient uterine discharge in the mare compared with species such as the bovine.

The remainder of this paper will consider the diagnosis of delayed involution and the larger subject of management considerations for mating at the first post-partum estrus.

Assessing Uterine Involution

It is important to be able to assess the rate of uterine involution in order to recognize if it is delayed. Delayed uterine involution invariably follows dystocia, abortion, placentitis, or placental retention. Even if manual removal of a retained placenta is apparently successful, a delay in uterine involution is often a reproductive complication. This might be due to retention of chorionic microvilli (vascular part of the placenta that attaches to the endometrium and allows for transfer of nutrients) even though, grossly, the placenta appears to have been successfully removed.

Assessment of uterine involution is best achieved by a combination of rectal palpation and ultrasound. Manual palpation has been used for many years to assess uterine involution. Observations on involution are recorded by general subjective categories such as poor, fair, good, and excellent.

The criteria assessed generally are size, attainment of a more tubular form, and a consistency similar to that of the pre-gravid uterus. However, these physical characteristics might not be very useful in predicting the outcome of breeding at foal heat.

Using rectal palpation alone, it can be difficult to determine when involution is complete. More recently, the veterinarian has been able to use ultrasound examination of the uterus to assess involution and suitability to breed at the foal heat.

Uterine dimensions and fluid accumulation can be accurately monitored by ultrasound. Uterine fluid accumulations in the first post-partum estrus period are associated with significantly decreased pregnancy rates.

Post-partum histological studies of the lining of the uterus have found that involution occurs very rapidly, and there is very little damage to the endometrium at parturition. The uterine horn that carried the pregnancy remains larger than the other uterine horn for a long time after birth, and the mare usually will establish the new pregnancy in the smaller uterine horn, i.e., the previously non-gravid uterine horn.

Endometrial Cytology

Finding large numbers of inflammatory cells (neutrophils) on an endometrial smear is useful and indicates that the mare should not be bred at foal heat. Finding low numbers of neutrophils appears to have minimal effect on the fertility of mares bred at the foal heat.

Endometrial Bacteriology

Bacteria will enter the uterus of the mare after foaling. The number can be reduced by careful attention to hygiene at foaling and ensuring that, if the mare has a poor vulval shape, this is attended to as soon after foaling as possible.

Whilst bacteria detected on a uterine swab can indicate infection, there is a lack of correlation between recovery of bacteria from the uterus with measurements of uterine involution, including histological inflammation, and the recovery of bacteria from the uterus.

The aim of many breeders is to produce healthy foals, as early as possible, from mares bred the previous year. Mare reproductive efficiency must be maximized and, as a large percentage of mares to be bred will be post-parturient, it is important to manage mares effectively during the foal heat.

Against the obvious time-saving advantage of breeding at the foal heat, there are two negative issues to examine: 1) Pregnancy rate at foal heat; and 2) Subsequent foaling rate of mares diagnosed pregnant from a foal-heat mating.

There is a lower pregnancy rate for mares bred at the first post-partum estrus. This is not surprising and is probably due to several factors, such as incomplete uterine involution, fluid in the uterine lumen, presence of infection, inflammation, events around foaling, etc.

My experience would suggest that breeding a mare on foal heat has no subsequent detrimental effect on pregnancy rate in subsequent estrous periods.

This is a key issue, and some clinicians do find a reduction in subsequent fertility, probably due to the mares becoming infected at the foal-heat mating. The reason other researchers do not find this reduction might be due to a routine post-breeding treatment policy of mares at the foal heat.

Some studies suggest an increased incidence of early embryonic mortality in mares conceiving to a breeding at the foal heat, whereas others suggest that management differences and year-to-year variation account for any observed differences and that there is no real increased incidence of pregnancy loss.


With the advent of ultrasound, mares which have lost an early pregnancy can be recognized more quickly and bred again. All mares should be examined as a matter of routine at Day 7 after foaling, even if the foal heat is not to be used. Problems can be detected and a treatment protocol begun.

The examination should include:

1) Vaginal Speculum Examination--This allows detection of any urine pooling, which is often seen temporarily in older mares. Also any cervical damage (lacerations, bruising) can be assessed.

2) Rectal Examination--Specifically, the presence of any evidence of uterine arterial hemorrhage should be noted. An assessment of uterine involution should be made, although the questionable significance of this should be remembered.

3) Ultrasonographic Examination--The uterus should be examined for the presence of fluid and its depth and character noted. Mares with clinical evidence of endometritis should not be bred. Any mare with intraluminal uterine fluid should not be bred.

Any mare which had a dystocia, retained placenta, etc., should be carefully evaluated. It is probably better she not be bred at the foal heat. Obviously, any mare with a severe trauma to the birth canal is not a candidate for breeding at the foal heat. Any vulval tears should be repaired as soon after foaling as possible to maximize the possibility of mating at the foal heat.

Cervical and/or vaginal bruising can be difficult to assess, and some studies have suggested that bruising itself is not detrimental to conception, it is only significant if accompanied by infection.

Pregnancy rates at foal heat increase as the interval between foaling and breeding increases. If breeding can be delayed until after the 10th day, then, because of the five-day interval that the fertilized ovum remains in the oviduct, the histological architecture of the endometrium has been returned to normal before the embryo enters.

Improving Pregnancy Rates

There are various suggestions to improve pregnancy rates from breeding at the foal heat. Uterine lavage has been suggested as a possible method of enhancing uterine involution, but not everyone is in agreement about this. Post-partum uterine lavage is not a guarantee of increasing foal heat pregnancy rate, but it is undoubtedly beneficial in individual cases which are complicated by factors such as retained placenta and endometritis.

Oxytocin and prostaglandins have been used in an attempt to encourage uterine contractility and, in turn, enhance uterine involution.

Subjectively, it would appear that exercise is beneficial in aiding elimination of uterine fluid and promoting uterine involution.

The steroid hormonal aspects of uterine involution also are poorly understood. It has been suggested that the uteri of mares which have an early post-partum estrus might involute more quickly than those which do not.

As an alternative therapy, attempts have been made to either delay the first post-partum ovulation or induce a second ovulation after a shortened interovulatory interval.

A daily treatment regimen of progesterone and estradiol beginning 12 hours after parturition was effective in delaying the first post-partum ovulation, according to Robert G. Loy, MS, PhD, and his co-workers and reported in 1982. However, the first post-partum ovulation should be delayed for more than five days to get the best results. This is the main disadvantage to steroid hormonal therapy, i.e., that to obtain the increase in pregnancy rate, the onset of the foal heat must be so delayed that foaling intervals are not significantly reduced.

The alternative is to use the luteolytic action of prostaglandin F2 or its analogs to reduce the interval to the second post-partum ovulation. The prostaglandin can either be given five or six days after ovulation, when the exact date is known, or as a routine on Day 20 after parturition, when most mares should respond. However, it must be appreciated that, in many cases, only one week will be saved over breeding on the naturally occurring second estrus.

All mares should be examined routinely on Day 7 or 8 after parturition and evaluated. Suitable mares are monitored closely for ovulation and bred at the foal heat unless more than one inch of fluid is present in the uterus.

After breeding, all mares are treated with a single treatment of intrauterine broad-spectrum antibiotics and two injections of oxytocin. One injection of oxytocin is given as an intravenous bolus 30 minutes before administration of intrauterine antibiotics, and the second is given 12 hours later.

In a controlled clinical study conducted by the author, post-breeding treatment of mares with oxytocin and intrauterine antibiotics was shown to increase the pregnancy rate attainable at the foal heat with no detectable increase in early embryonic death rate. The pregnancy rate in the group of mares treated with antibiotics and oxytocin was 68%, compared with the untreated control group, which had a pregnancy rate of 46%. Sometimes uterine lavage is also useful.

The mare is re-examined on the next visit to the breeding farm and ovulation confirmed. The uterus is assessed and further treatment given as necessary. Try to avoid a second breeding, perhaps by using hormones such as chorionic gonadotropin to try and ensure ovulation.

In some cases, the mare might not be suitable for breeding. Also, certain stud farm owners/managers are completely against foal heat breeding, certainly early in the season. On those farms, it might be best to use the 20-day prostaglandin treatment.

However, an examination at Day 7 or 8 still should be performed to treat endometritis or fluid retention and to estimate/establish the time of ovulation. It should be remembered that some mares revert to ovarian inactivity following the first post-partum ovulation. This is more noticeable when foaling occurs early in the breeding season and is probably due to a seasonal influence rather than a lactational effect.

The results of recent research on post-partum reproduction in the mare and experience from the field are best summarized by quoting from the work of Loy reported in 1980:

"The information now available, or likely to become available in the near future, does not justify a sweeping statement that breeding mares at foal heat should be universally practiced or condemned. It is equally certain that this is an area requiring a high degree of management skill and technical expertise. As research provides new information, its result will be to facilitate decision making--not to eliminate decision-making responsibility."

The intervening 17 years have not altered the validity of this statement. By careful management on the part of breeder and veterinarian, mares with a young foal at foot can become pregnant at the foal heat and deliver a healthy, normal foal within a year of her last giving birth.

About the Author

Jonathan F. Pycock, B. Vet. Med, PhD, DESM, MRCVS

Jonathan F. Pycock, BVetMed, PhD, Dipl. ESM, MRCVS, operates Equine Reproductive Services, a first opinion and referral private equine practice based in Yorkshire, England. He has published many papers and book chapters on a variety of equine reproductive topics, and edited the book Equine Reproduction and Stud Medicine. His main interests include ultrasonography, breeding the problem mare, and artificial insemination. Currently, he is evaluating the use of oxytocin and depot oxytocin as a post-breeding treatment for mares.

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