Should I Breed Her on Foal Heat?
Introduction
The mare has a relatively long gestational period and, therefore, for the mare to be
a yearly producer she must be back in foal, on average, 25 days from parturition.
Consequently 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).
This paper will discuss the changes which take place in the post-partum interval i.e. the interval from parturition to the first ovulation and how this period can be managed most effectively.
Characteristics of the first post-partum oestrus
The foal heat is characterised by normal follicular development and ovulation by day 20 post-partum (97%) of mares (Loy 1980). Other findings from this large study were that by day 9 post-partum, 43% of mares had ovulated and 93% by day 15. The average interval from parturition to first post-partum ovulation is 10 days. This interval, unlike the onset of foal heat, is affected by season: the interval to ovulation is shorter later in the breeding season.
The Post-Partum Interval
The Equine Placenta - classified as microcotyledonary or epitheliochorial
The microcotyledons are on the outer (towards the mare's uterus) surface of the
allantochorion, giving it a red, velvety appearance. As the equine placenta has six layers
of tissue separating the maternal and fetal circulations it can also be classified as
epitheliochorial.
The fetal membranes are usually, but not always, expelled with the allantoic surface of the allantochorion outermost as in this slide
The allantoic side (the side towards the fetus) is smooth and pink and has arteries and veins held on its surface by a thin layer of transparent membrane. The network of blood vessels converge at the umbilical cord attachment which is usually at the dorsal base of the pregnant horn. The star-shaped part of the allantochorion that lies over the internal os of the cervix is present on every placenta and is the normal site of rupture of the allantochorion during parturition. The whitish, pale colour of the cervical star is caused by the absence of chorionic villi in this area. A hippomane, soft putty-like aggregates of urinary calculus which form throughout pregnancy, can be seen, somewhat unusually, still attached to the allantochorion.
Uterine Involution: Reduction in Size
Uterine involution is characterised by many features, perhaps the most obvious of which is
reduction in size.
An enormous reduction in diameter of the uterus occurs which is largely due to an increase in uterine contractility. An early study using palpation to assess uterine horn diameter reported that the previously gravid uterine horn had returned to the pre-gravid state (4-6 cm) by day 32 (Gygax, Ganjam and Kenney 1979). Using ultrasound, it has been suggested that this may be earlier: the horns were similar diameter by 23 days (mean of 17 days) (McKinnon et al 1988) or even between days 15 to 21 (Sertich and Watson 1992). However, another ultrasound study suggested that the difference in size between the horns may extend beyond 35 days (Griffin and Ginther 1991).
Uterine Involution: Post-Partum Luminal Fluid (lochia)
As the uterus decreases in size, so post-partum luminal fluid (lochia) is discharged from
the uterine lumen.
This uterine discharge is normal and is often noticeable as a vaginal exudate (bloody, brown or muco-purulent) around days 3 and 4 after parturition. The colour 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 (Gygax et al 1979), 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.
Using ultrasound examination of the uterus, two studies have found that a decrease in detectable uterine fluid began at 5 days and only small collections (2 to 10 mm) were detectable at 9 days and no fluid was detectable by 15 days post-partum (McKinnon et al 1988; Griffin and Ginther 1991). Interestingly, these latter authors found that 5 of 10 mares had detectable uterine fluid collections the day after parturition, but 10 of 10 mares had detectable fluid at 3 days. They concluded that some of the fluid may represent an influx from the endometrial wall into the uterine lumen in association with the process of involution. It must also be remembered that thick exudates are difficult to detect on ultrasound examination.
McKinnon et al (1988) suggested that degree of echogenicity of the uterine fluid decreased as involution progressed. However, my experience would suggest that there is no effect of day on echogenicity of the fluid collections during the post-partum interval. This is supported by Griffin and Ginther (1991). The discrepancy may be due to the fact that McKinnon and co- workers claimed that degree of echogenicity is related to debris and inflammatory material in uterine fluid, but this is not always the case. On occasions, very thick exudates can appear anechoic. Ginther (1992) suggests that the fluid that enters the uterus after parturition remains anechoic as long as the involution process is uncomplicated.
Uterine Involution: Post-Partum Bacteriology
The bacterial species isolated from endometrial swabs taken at various times during the
post-partum period has been found to vary according to the number of days from parturition
(Koskinen and Katila 1987).
Early in the post-partum period (day 2), Escherichia coli was the most common organism (67% of positive samples), however, their numbers rapidly decreased to 29% on day 5 and no coliforms were found on day 9.
Not surprisingly, the proportion of -haemolytic streptococci was inversely related to E. coli with the proportion increasing from 13% on day 2 to 33% on day 5 and 67% on day 9. These workers also reported that growth of bacteria was maximal at 5 days post-partum. Saltiel et al (1987) reported an increase in bacterial flora from days 13 to 17 post-partum, following a decrease from days 2 to 9.
Uterine Involution: Post-Partum Cytology
There have, unfortunately, been few cytological studies of the post-partum uterus.
Saltiel et al (1987) found that the number of neutrophils declined so that there were only a few, scattered cells at the first post-partum oestrus.
Koskinen and Kotila (1987) on the other hand encountered more neutrophils on day 5 than on day 2 in endometrial smears.
Uterine Involution: Post-Partum Histological Changes
A detailed study on post-partum histological changes of the endometrium found that involution of the endometrium occurs very rapidly (Gygax et al 1979). Overlying luminal epithelium is intact by 4 to 7 days post-partum. Recently, Ginther (1992) has concluded that, apart from the area overlying the microcaruncles), loss of luminal epithelium has not been demonstrated as a post-partum phenomenon.
Endometrial gland distention is usually absent by 10 days post-partum and by day 7 the only remaining evidence of the microcaruncle is an area of condensed stroma containing siderophages covered by an intact epithelium (Sertich, Hinrichs and Kenney 1989). In conclusion, most workers agree that the endometrium usually has a normal pregravid histological appearance i.e. is essentially completely repaired by day 10 after parturition (Gygax et al 1979; Steven et al 1979; Sertich et al 1989; Sertich and Watson 1992). The fact that so little damage normally occurs to the endometrium is due to the fact that the mare has a this simple, epitheliochorial type of placentation.
Uterine Involution: Post-Partum Endocrinology
Gonadotrophin Hormones: LH and FSH
Levels of luteinising hormone (LH) have been demonstrated to rise slowly in both intact and ovariectomised mares for 10 days following parturition (Turner et al 1979). However, as ovulation approached in the intact mares, there was a rapid increase in LH, presumably due to a positive-feedback effect of oestradiol from the growing follicle.
There is a pronounced parturient surge of follicle stimulating hormone (FSH) which peaks on the day of parturition and this is probably responsible for the initiation of post-partum follicular development (Ginther 1992). FSH levels then gradually decline and the time interval from the parturient FSH surge to ovulation is similar to that seen from the dioestrus surge to ovulation in cycling mares.
Steroid Hormones: Oestrogen and Progesterone
Plasma oestrogen concentrations are similar to those seen in non-parturient mares i.e. an increase around days 5 to 7 after parturition associated with follicular recruitment and maturation (Lovell et al 1975). Progesterone concentrations remain low from parturition until after the first post-partum ovulation.
Prostaglandin F2 (PGF2 ) and Oxytocin
Both these hormones are raised during the first few days following parturition (Vandeplassche et al 1983). A more recent paper found that concentrations of 13,14-dihydro-15-keto-PGF2 (PGFM; the blood metabolite) were low by the day after parturition (Sertich and Watson 1992). Interestingly, these workers found that there was no significant correlation between uterine involution and PGFM concentrations.
The function of PGF2 and the other hormones discussed earlier in stimulating uterine contractions is poorly understood, although they are likely to have an important role. It is probable that uterine contactility plays an important role in rapidly reducing the large post- parturient uterus to its pregravid state, and the evacuation of uterine contents.
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 oestrus.
Assessment of Uterine Involution
Uterine Involution: Importance of Assessment
Important to be able to assess the rate of uterine involution in order to recognise if it is delayed. Delayed uterine involution invariably follows dystocia, abortion, placentitis and placental retention. Even if manual removal of a retained placenta is apparently successful, a delay in uterine involution is often a reproductive complication (Vandeplassche et al 1983). This may be due to retention of chorionic microvilli even though, grossly, the placenta appears to have been successfully removed.
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.
Delayed Uterine Involution: Clinical Signs
In the early post-partum stages (first 2 to 3 days) the mare may exhibit systemic signs such as dullness, inappetance and mild colic. In severe cases, usually subsequent to pla cental retention or dystocia, there may be evidence of septicaemia and laminitis.
Uterine Involution Assessment: Manual Palpation
Manual palpation per rectum 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 are generally size, attainment of a more tubular form and a consistency similar to that of the pre-gravid uterus.
However, a large field study conducted in Kentucky found that within wide limits, these physical characteristics were not very useful in predicting the outcome of mating at foal heat (Loy 1980).
Uterine Involution Assessment: Endometrial Bacteriology
The situation with using endometrial bacteriology is also not clear.
Most workers report a lack of correlation between recovery of bacteria from the uterus with measurements of uterine involution, including histological inflammation (Gygax et al 1979; Koskinen and Katila 1987; Blanchard et al 1991).
Some reports have even suggested that the recovery of bacteria from the uterus does not affect the fertility of mares which had a normal parturition and are mated early in the post-partum period (Koskinen and Katila 1987; Shideler et al 1987).
Earlier, Gygax et al (1979) had suggested that this lack of correlation was due to cervical patency allowing acute surface bacterial contamination.
Uterine Involution Assessment: Endometrial Cytology
The confusion continues when the significance of endometrial cytology is considered.
Finding large numbers of neutrophils is useful.
However, both Koskinen and Katila (1987) and Sertich and Watson (1992) suggested that the presence of low to moderate numbers of neutrophils had no effect on fertility of mares mated at the foal heat.
Uterine Involution Assessment: Ultrasound
Inevitably, ultrasound examination of the uterus was used to assess involution and suitability to mate at the foal heat.
The first report was by McKinnon et al (1988) and the purpose of their study was to see if uterine involution and fluid accumulation can be monitored by ultrasonographic examination of the uterus and if these parameters were useful to predict the fertility of mares mated at the foal heat.
They found that ultrasonically detected uterine fluid acumulations in the first post-partum oestrus period were associated with significantly decreased pregnancy rates.
Using ultrasound to detect intraluminal fluid would, therefore, seem to be superior to merely performing a vaginal examination for the presence of a discharge as Koskinen and Katila (1987) found that the presence of a vaginal discharge had no effect on pregnancy rate.
There would appear to be a lack of correlation between the presence of uterine fluid and neutrophil numbers (Blanchard et al 1989) which is not surprising as, in the normal cycling mare, the fluid which acumulates at oestrus is often sterile and contains no neutrophils (Pycock and Newcombe 1996).
Despite this lack of correlation, detection of fluid is important as even sterile accumulations of fluid at oestrus have been shown to reduce pregnancy rate (Pycock and Newcombe 1996).
Griffin and Ginther (1991) have also studied uterine morphology, as detected using ultra sound, in post-partum mares. They reported that endometrial oestral oedema persisted longer in post-partum mares than in non-parturient mares. This is in agreement with the author, who finds levels of uterine oedema at oestrus a useful guide in assisting the decision of when to mare the mare: oedema scores are generally maximal early in oestrus and decline 24-36 hours before ovulation. However, this is not the case at the foal heat, where oedema often persists until the day ovulation is detected. This increase in endometrial oedema is probably indicative of residual post-partum oedema.
The Decision To Use The First Post-Partum Oestrus: Difficult !
Don't be made to look foolish, get it right and head for the beach early!
Most commercial stud farms aim to produce as many healthy foals, as early as possible, from mares mated the previous year. Mare reproductive efficiency must be maximised and, as a large percentage of mares at stud will be post-parturient, it is particularly 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:
Foal Heat Breeding: Pregnancy Rate
Reduced
Most clinicians, and the literature, suggest that there is a lower pregnancy rate for mares mated at the first post-partum oestrus. 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.
BUT
My data would suggest that mating a mare on foal-heat has no subsequent detrimental effect on pregnancy rate in subsequent oestrous 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 workers do not find this reduction may be due to a routine post-mating treatment policy of mares mated at the foal heat.
In conclusion, as discussed below, management policy (selection of mares for mating, any post-mating treatment) is likely to significantly influence the pregnancy rates achieved by mating at this particular oestrus.
Foal Heat Mating: Early Embryonic Death
Also controversial is the question of incidence of pregnancy loss in mares conceiving to a mating at the foal-heat. Some studies suggest an increased incidence of early embryonic mortality (Meyers, Bonnett and McKee 1991) 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 (Loy 1980).
In any case, with the advent of ultrasound to detect early embryonic death more readily than before, mares which have lost an early pregnancy can be recognised more quickly and mated again.
Foal Heat Mating: 1993 Breeding Season (No Post-BreedingTreatment)
91 mares mated at foal heat of which 46% became pregnant. The variation in pregnancy rate between studfarms was 41% to 73% where 67% and 24% of mares respectively were mated. Three pregnancies failed.
Of the 49 mares which failed to become pregnant, 14 (29%) suffered luteal phase endometritis with premature luteal regression, only 2 had received post-mating antibiotics.
Foal Heat Mating: 1994 Breeding Season (Oxytocin Post-Breeding)
109 mares mated at foal heat of which 49% became pregnant. The variation in pregnancy rate between studfarms was 40% to 75% again largely dependent on degree of selection for suitability. Five pregnancies failed. Of the 56 mares which failed to conceive, 14 (25%) suffered luteal phase endometritis; all had been given oxytocin after mating, but none had received antibiotics.
Foal Heat Mating: Examination Procedure
It is useful to examine all mares as a matter of routine at day 7 afer 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 haemorrhage 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. Although uncommon, the uterus should also be examined for the presence of haematoma (Pycock 1994a).
Foal Heat Mating: Selection of Mares for Breeding
Presence of Inflammation: Any mare showing clinical evidence of endometritis should not be mated.
Intraluminal Uterine Fluid:Any mare with intraluminal uterine fluid should not be mated.
Events at Parturition: Any mare which had a dystocia, retained placenta etc should be carefully evaluated. Any vulval tears should be repaired as soon after foaling as possible to maximise the possibility of mating at the foal-heat.
Cervical and/or Vaginal Bruising: This can be difficult to assess and some studies have suggested that bruising per se is not detrimental to conception, it is only significant if accompanied by infection (Pascoe 1991).
Time of Ovulation: This is extremely important as the pregnancy rate at foal-heat matings increase significantly as the interval between foaling and mating increases. If mating can be delayed until after the tenth day, then, because of the 5½ day interval that the fertilised ovum remains in the oviduct following ovulation, the histological architecture of the endometrium has been returned to normal before embryo enters.
Minimum of eight days from parturition, preferably at least ten days Maximising Pregnancy Rates at Foal Heat
There are various suggestions to improve pregnancy rates from mating at the foal-heat which, from an understanding of the involution process discussed are fairly logical.
If there is any systemic involvement from the failure of the involution process, parenteral antibiotics are indicated. If there is any suggestion of laminitis, non-steroidal anti-inflammatory agents should be used.
Uterine lavage has been suggested as possible method of enhancing uterine involution, but a controlled study by Blanchard et al (1989) did not support this. These workers found that post-parturient uterine lavage did not significantly affect uterine involution. Neither has post- partum uterine lavage been found to increase foal-heat pregnancy rate (McCue and Hughes 1990). However, lavage 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 enourage uterine contractility and, in turn, enhance uterine involution (Blanchard et al 1991). They found that twice-daily administration of oxytocin or fluprostenol for 10 days post-partum did not increase the rate of uterine involution. However, an earlier study had found that administration of the prostaglandin analogue, prostalene, also given twice daily, did improve pregnancy rates from foal-heat matings (Ley, Purswell and Bowen 1988). Until the mechanisms responsible for ecbolic agent stimulation of uterine contraction during the post-parturient period are better understood, definite recommendations can not be made. 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 are also poorly understood. It has been suggested that the uteri of mares that have an early post-partum oestrus may involute more quickly than those that do not (Saltiel et al 1987). However, exogenous administration of progesterone alone (Loy et al 1975) or in combination with oestrogen (Sexton and Bristol 1986) does not increase the rate of histological endometrial repair.
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 oestradiol beginning 12 hours after parturition was effective in delaying the first post-partum ovulation (Loy et al 1982). Daily administration of altrenogest can also be used. 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 PGF2 or its analogues to reduce the interval to the second post-partum ovulation. The prostaglandin can either be given 5 or 6 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 mating on the naturally occurring second oestrus.
Foal Heat Mating: Possible Approach
The author's approach is to examine all mares routinely on day 7 or 8 after parturition and evaluate.
All suitable mares are then monitored closely for ovulation and mated at the foal heat unless more than 2 cm of fluid is present in the uterus
OR the mare ovulates before day 9 post-partum.
After mating, 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, post-mating treatment of mares with oxytocin and intrauterine antibiotics has been shown to increase the pregnancy rate attainable at the foal-heat with no detectable increase in early embryonic death rate (Pycock 1994b). 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 may also be useful.
The mare is re-examined on the next visit to the studfarm and ovulation confirmed and the uterus assessed and further treatment given as necessary.
Try to avoid a second breeding, perhaps by using hCG
Foal Heat Mating: Alternative Approach
In some cases the mare may not be suitable for breeding. Also certain stud farm owners/management are completely against foal-heat mating, certainly early in the breeding season.
On those farms, it may be best to use 20 day prostaglandin treatment
BUT an examination at 7 or 8 days should still 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 (Palmer and Driancourt 1983).
CONCLUSION
The results of recent research on post-partum reproduction in the mare and experience from the field are best summarised by quoting from the paper of Loy (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 15 years have not altered the validity of this statement.
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