Horse
breeding is a time-consuming and expensive business. Therefore,
horse breeders must be able to assess the likelihood of a mare
being able to conceive, carry a fetus through a pregnancy, and
deliver a live foal. While unforeseen problems can always occur,
there are procedures that help you decide if a mare is suitable
for breeding. The suitability for breeding does not refer to the
quality of the mare or her temperament; rather it is an assessment
of the genital health of that particular mare. Collectively these
procedures are known as the breeding soundness examination.
For any mare, it is important that she
is properly prepared for the breeding season. This article will
focus on what owners should do before the breeding season to help
get their mares ready for a successful breeding and describe the
breeding soundness examination of the mare.
Understanding the Mare's Breeding Season
The mare is a seasonal polyestrous
breeder (her reproductive tract is active for multiple cycles
during a particular season of the year), and environmental and
other factors can exert a profound effect on reproductive
function, particularly during the transitional period between
winter anestrus (period of sexual inactivity) and the onset of
cyclical activity in the spring. During the breeding season, a
mare which is not pregnant will have recurring estrous or heat
cycles.
In nature, the mare does not usually
experience an estrous cycle during the winter and early spring
months (although some mares cycle all year round). Throughout that
time of year, her reproductive system goes into a rest period and
she is acyclical. This is known as winter anestrus. On rectal
palpation or transrectal ultrasound imaging, both ovaries will be
small (less than 3x2x2 cm) (see Figure 1 on page 58), and in some
mares there will be a number of small follicles. Plasma
progesterone (a hormone, more on this later) concentrations are
less than 1 ng/ml.
The mare doesn't suddenly begin to
cycle regularly in spring, but has what is known as a transitional
phase. This is the period between winter anestrus and normal
cycling. Once the mare is truly in the breeding season, her
estrous cycles last 22 days on average. Cycle length is very
variable and the cycle tends to be longer in the spring.
The regulation of cyclical activity is
under the control of the hypothalamus and pituitary gland (located
in the brain) and ovaries. The hypothalamus produces gonadotropin-releasing
hormone (GnRH) that stimulates the release of the gonadotropins
from the pituitary gland. The two gonadotropins from the pituitary
gland are follicle stimulating hormone (FSH) and luteinizing
hormone (LH); these hormones are responsible for follicle growth
and ovulation, respectively.
When nature has its way, the
production of the hormones that "switch on" the mare cycling is
controlled by the amount of daylight. During the short days of
winter, there's limited production of hormones; in the longer
spring and summer days, there is greater hormone production. The
horse is a long-day breeder and daylight has an important
influence on mare reproductive performance (see "How the Estrous
Cycle Works" on page 56 for a description of hormonal control of
the estrous cycle).
This effect of season on reproduction
effectively results in foals being born in the spring and early
summer. This is the time when environmental conditions should be
optimal for foal survival in the wild. Therefore, the natural
breeding season of the horse (in the Northern Hemisphere) is from
May until August. This is the time when highest pregnancy rates
are likely to occur.
Unfortunately, man has superimposed
his ideas of when mares should foal on this natural pattern. Since
the early 19th Century, when Jan. 1 was declared the official
birth date for Thoroughbred foals (irrespective of their actual
birth dates within that year), horse breeders have had problems
attempting to breed mares in the winter and early spring so they
would foal as early in the year as possible. For some types of
horse, notably the Thoroughbred, an operational breeding season
exists from Feb. 15 until the first week of July. This "man-made"
breeding season means that many breeders try to get mares in foal
from the middle of February onward.
It might not be clear to some readers
why so much trouble is taken in trying to breed mares before they
are in their most fertile period. The reason is that the two major
racing breeds (Thoroughbreds and Standardbreds) and the majority
of the breed societies use Jan. 1 as the official birth date. Some
breed societies use the actual birth date, but Jan. 1 is used for
show purposes. This means that a foal born on Feb. 1 and a foal
born on Aug. 1 would both be considered one year old on the
following Jan. 1. This is despite the fact that there is a
half-year difference in actual age.
Artificial Lighting
Fortunately, something as simple as
light can cause the mare to cycle earlier. Putting a mare under
lights is very common in horse breeding, and it involves providing
extra light at the end of the day.
Experience has shown that 16 hours of
light stimulus (artificial plus natural) is adequate. This means
providing light from 7 a.m. until 11 p.m. The extra artificial
light is best added at the end of the natural daylight period, and
works by suppressing the release of a hormone called melatonin. An
area of the brain known as the pineal gland produces this hormone,
which affects release of the gonadotropin hormones involved in
follicle production and ovulation.
Alternatively, a one-hour "flash" of
light from nine to 10 hours after nightfall can be used to achieve
the same effect. This is more difficult for most breeders to
achieve than simply providing light at the end of the day.
The light in the stall must be bright
enough. This means a 150-watt clear bulb in the middle of a 13
foot by 13 foot (4 meters by 4 meters) stall. If a strip light is
used, it should be 4.3 feet (1.3 meters) long (40 watts). Care
should be taken to eliminate shadows, as they can prevent a good
response. A practical method of checking that there is sufficient
light is to see if it is easy to read a newspaper wherever you
stand in the stall.
There are two important points to note
with putting a mare under lights:
- Extra lighting must be begun early.
Generally a minimum of eight to 10 weeks is needed, so mares
should be exposed to the lighting system by Dec. 1. This should
mean that they will begin to cycle normally by Feb. 15. It is
not good enough to decide to put a mare under lights and one
week later expect the mare to begin cycling, since this does not
happen immediately. Recently researchers in France found that
much lower light levels might be effective. They reported 20
watts was sufficient to photostimulate mares and inhibit
melatonin production. The exposure could be during late evening
or early morning, and might only be necessary for five weeks.
When the 20 watts was given as a "flash" in the middle of the
night, it was only effective in 50% of mares.
- Mares should be in good body
condition and well-fed when they begin to have extra lighting.
It is preferable to increase the mare's body condition, and this
might mean extra feed. It is also important that mares are
healthy, on an appropriate anthelmintic (deworming) program, and
have had regular dental checks.
In aged mares, delayed initiation of
normal cyclical ovarian activity might reduce the number of
estrous cycles during the breeding season. Therefore, it is
particularly important to prevent poor body condition from
occurring in such animals. Prolonged anestrus can be prevented by
good management.
Hormones for
Manipulating Estrus
Several hormones can be given to the
mare to induce ovulatory heats early in the breeding season in
barren and maiden mares. This helps ensure that foals are born
early and reduces pressure on the stallion by getting these mares
serviced before foaling mares come on line to be bred. (See
"Hormonal Control of Estrus" above for a description of the
various hormones' function in controlling estrus.)
Progesterone/progestogen-withdrawal
therapy has been used successfully in mares near the end of a
two-month light program. Progesterone (a hormone that keeps the
mare from coming into heat; it also maintains the uterine
environment for pregnancy) can be administered for 1-15 days as an
oil-based intramuscular injection, or orally as the synthetic
progestogen altrenogest (Regumate). However, such therapy is
effective only in anestrous mares which are already well into the
transitional phase to the resumption of cyclical ovarian activity.
Results are poor when used in mares with ovaries containing
follicles less than 0.8 inches (20 mm) in diameter.
Synthetic GnRH analog Deslorelin (Ovuplant)
implants have been used in anestrous mares, but only around 50% of
mares in anestrus will respond and several implants might be
needed. The mare might return to anestrus if she has an induced
ovulation, but fails to become pregnant.
Twice daily injections of synthetic
GnRH analog buserelin (Receptal) might induce ovulation in
anestrus mares, but results are much better with transitional
mares.
Transitional "Spring" Estrus
After a variable transitional period
of up to two months, larger follicles (greater than 1.4 inches, or
35 mm) will develop and ovulate, usually heralding the onset of
normal cyclical ovarian activity. During the transitional period,
behavior is variable, ranging from total rejection of the stallion
to interest but resistance to him mounting, to normal acceptance.
These behavioral signs can be consistent or inconsistent.
During this stage, ultrasonic
examination and rectal palpation reveal transitional follicles
reaching a size of greater than 1.2 inches (30 mm) (see Figure 2
on page 58). Visual identification of a corpus luteum (CL) or
progesterone levels above 4 ng/ml confirm that the first ovulation
has occurred and hence the onset of normal ovarian cyclical
activity.
Hormonal treatment of mares in the
transitional stage is based on progesterone or progestogens, with
or without the addition of estradiol esters, by one of several
routes of administration. Progesterone exerts a negative feedback
on gonadotropin secretion, which is followed by increased release
of FSH and LH. When the source of progesterone is withdrawn or its
effect wanes, there is follicular growth, maturation, and
ovulation.
Progesterone treatment is more
effective in mares in the late transitional stage, and is
ineffective in mares with minimal follicular activity,
particularly during deep anestrus. Currently, the most common
treatment is oral medication with the potent progestogen
altrenogest to mimic the luteal phase. This liquid should be added
to the food once per day for 10 to 15 consecutive days; estrus
should occur within six days and ovulation seven to 13 days after
the last treatment. Because of the possibility of ovulation
occurring during treatment, an injection of the luteolytic agent
PGF2-alpha (a prostaglandin) on the last day of in-feed
medication might be necessary to cause luteolysis (regression) of
any CL that is present. Oral progestogen therapy works when the
mare has estrogen competent follicles and an endometrial edema
pattern (see Figure 3 on page 58).
The use of intramuscular injections of
progesterone and estradiol-17 beta in oil for 10-15 days produces
a similar response to altrenogest, but the interval to estrus is
longer due to the suppression of follicular development by the
estradiol.
There has been much interest recently
in using GnRH or its analogs, administered by injection, infusion,
or subcutaneous implant, to hasten ovulation in the transitional
phase mare. I have successfully used buserelin given twice daily
by intramuscular injection. It is expensive, as treatment is
necessary for at least one to two weeks. The use of the short-term
implant of the GnRH analog deslorelin has been reported. Its value
in accelerating the first ovulation of the breeding season
following seasonal anestrus would appear to be a real benefit.
Regardless of the hormones used, mares
undergoing treatment early in the season need 16 hours of adequate
light and good nutrition to ensure success.
During the transitional period before
the year's first ovulation, mares demonstrate erratic estrous
behavior of varying intensity. The presence of multiple large
follicles, possibly as large as 1.2 inches (30 mm), makes
detection of ovulation difficult by palpation alone. Even outside
this transitional period, misinterpretation of ovulation--even by
experienced palpators--has been as high as 50% in some studies. It
is much easier to visualize the corpus hemorrhagicum/early CL with
ultrasound (see Figure 4 on page 58) when the anechoic
(fluid-filled) follicle is replaced by an intensely echoic (solid)
area--the early CL (see "Diagnostic Imaging in the Broodmare
Examination" on page 58 for more information on ultrasound use for
reproduction examination).
It is recommended that the interval
between matings be less than three days, although there have been
no critical studies on the survival time of sperm in the mares'
genital tract. It is important not to begin breeding too early, or
this will result in the mare being mated many times (which is less
efficient and increases the chances of normal post-mating
endometritis lowering fertility). The appearance of uterine edema
(fluid swelling) is an indication that the follicle should ovulate
within a few days.
A key factor in the emergence out of
vernal (spring) transition is the development of steroidogenic
competence (the ability of the follicle to produce hormones)
leading to an increase in circulating estrogen concentrations,
which cause the release of LH from the pituitary due to a positive
feedback mechanism. Estrogen is responsible for the appearance of
uterine edema (in the absence of progesterone), which might be why
the detection of uterine edema is important in signalling the
emergence of the mare from the transitional period.
Normal
Expectations of Fertility
Unlike many other domestic animals,
horses have never been selected for fertility. Popular horses to
breed from are those which can run fast, jump well, or look
pretty. Unfortunately, these are not necessarily the most fertile
horses. Additionally, how mares are managed has an enormous
influence on pregnancy rates, so therefore only approximate guides
are possible.
Of course, some mares lose their
pregnancies through abortion and the live foal rate (the number of
live foals born the following year) is lower than the pregnancy
rate. It is important to make sure that an owner knows these
figures so he/she is not under the impression of being guaranteed
a live foal if a stallion covers the mare.
Possibly the two most important (or at
least the two most-quoted) parameters in connection with
reproductive efficiency are end-of-season pregnancy rate and live
foal rate. From the veterinarian's point of view, pregnancy rate
per estrous cycle is a more up-to-date indicator of efficiency of
the breeding operation. A well-managed stud farm should typically
achieve pregnancy rates (diagnosed at 15 days of gestation) per
estrous cycle of 65%, an end-of-season pregnancy rate of 85%, and
a live foal rate in excess of 75%. The figures need to be reviewed
in light of the history and past breeding performance of each
mare.
Is Your Mare A
Healthy Breeder?
As well as using light to get your
mare to cycle early, it is important to make sure that her
reproductive tract is healthy. This is particularly important if
she failed to become pregnant last year or lost the foal at some
stage of pregnancy. Ideally such mares should be investigated in
the autumn rather than the spring to allow time for any
abnormalities to be treated and the reproductive tract time to
"settle down" before the breeding season. If you did not have your
mare checked out in the autumn, then at least have a thorough
examination performed in the spring. Even if your mare is a
first-time breeder, it is a good idea to have your veterinarian
perform a thorough gynecological examination known as a breeding
soundness examination. The aims are to detect any cyclical
abnormalities, physical defects, or signs of inflammation and to
assess the likelihood of a mare being able to conceive, carry a
fetus through a pregnancy, and deliver a healthy foal.
The breeding soundness exam involves
an external and internal examination of the mare. Using a
systematic approach, the ovaries, uterus, and cervix are checked.
Usually the veterinarian will use an ultrasound machine as well as
palpation. The vagina and cervix will also be checked by looking
at them through a speculum inserted in the vagina. A speculum is a
narrow tube through which you can shine a light and see the inside
of the vagina and the cervix.
Vulva--In the normal mare, the
vulva provides the first effective barrier to protect the uterus
from ascending infection. The vulval lips should be full, firm,
and meet evenly in the midline with 80% or more of the vulval
opening below the brim of the pelvis (see photo A on page 58). If
the vulval seal is high (more than 1.6 inches, or 4 cm, of length
dorsal to or above the pelvic floor) in relation to the pelvic
brim, the vestibular seal is incompetent and pneumovagina
(aspiration of air, bacteria, and contaminated material) can
occur. The initial vaginitis (vaginal inflammation) can lead to
cervicitis (cervical inflammation) and acute endometritis
(inflammation of the endometrium, or uterine lining), resulting in
subfertility.
Contamination of the caudal
reproductive tract with bacteria during pregnancy can result in
embryonic death, and in
late pregnancy contamination can cause placentitis and lead to
abortion. The more severe conformational abnormalities are more
likely to result in failure of the vulval seal and to increased
fecal contamination since the vulva forms a shelf on which feces
can collect (see photo B on page 58). The vulval lips might be
angled at 25 or even 50 degrees to the vertical in these cases.
Dr. E.A. Caslick first pointed out the
importance of this condition in relation to genital infection in
Thoroughbred mares in the 1930s. Interestingly, it is most
commonly found in Thoroughbreds, and, in the author's experience,
is almost unknown in Shires and native ponies.
Defective vulval conformation can be
congenital--which is very rare--or acquired. The latter can be due
to vulval stretching following repeated foalings, injury to
perineal tissue, or poor body condition (old, thin mares).
Older mares are more commonly affected
with pneumovagina. However, young mares which are in work and have
little body fat and/or poor vulval conformation can also develop
pneumovagina. In some mares, pneumovagina only occurs during
estrus when the perineal tissues are more relaxed. Some mares make
an obvious noise while walking, but in other mares the diagnosis
is more difficult.
Hyperemia (excessive blood) and a
frothy exudate in the anterior vagina on speculum examination are
pathognomic (characteristic of this problem). Rectal palpation of
a ballooned vagina or uterus from which air can be expelled
confirms the diagnosis. Real-time ultrasound examination of the
uterus might reveal the presence of air as hyperechoic (white)
foci (see Figure 5 on page 58). Cytological and histological
examination of the endometrium might show significant numbers of
neutrophils (pus cells) indicative of endometritis.
Treatment should be directed at
correcting the cause of pneumovagina and treating the resulting
endometritis. The former can be done surgically by Caslick's
operation, although in some cases increasing the physical
condition and fat status of the mare might be sufficient.
The aim of the operation is to reduce
the vulval aperture and thus prevent pneumovagina and fecal
contamination of the vagina. The time of suture removal is not
crucial; this is normally done approximately two weeks after
surgery. However, the vulva must be re-opened by performing an
episiotomy (a surgical incision into the perineum and vagina to
prevent traumatic tearing during delivery) before the next
foaling, otherwise major damage can result.
Mares which require natural mating
subsequently might need to have an episiotomy performed if
"tightly stitched." If not, the vulva can rip and/or the sutures
can injure the stallion's penis. The episiotomy wound should be
repaired soon after foaling or mating to prevent pneumovagina.
If repeated episiotomy followed by
closure is not done very carefully, there can be considerable loss
of vulval tissue, poor healing, and major problems for the mare.
Vagina--Urine pooling is the
retention of incompletely voided urine in the vagina due to an
exaggerated downward cranial slope of the vagina. Pneumovagina
from defective vulval conformation also predisposes mares to the
condition. Transient urine pooling, which is sometimes found in
postpartum mares, usually resolves after uterine involution
(return to normal sizes) has occurred.
Vaginal bleeding from varicose veins
in the remnants of the hymen at the dorsal vestibulovaginal
junction is occasionally seen in older mares, particularly during
estrus. Treatment is not usually necessary as the varicose veins
normally shrink spontaneously, although diathermy (surgical
destruction of these veins) can be used.
Cervix--The cervix forms the
important third (and last) protective physical barrier to protect
the uterus from the external environment. The cervix must relax
during estrus to allow intrauterine ejaculation or insemination of
semen and drainage of uterine fluid. An inflammation of the cervix
is usually associated with endometritis and/or vaginitis.
Anatomically, the cervix is a
thick-walled sphincter. Expansion and contraction are possible due
to the action of the longitudinal and circular smooth muscles,
which are rich in elastic fibres. Two distinctive features of the
equine cervix are its ability to dilate and the lack of rigid,
annular constricting rings seen in farm animals.
The cervix is a dynamic organ that
changes its size, consistency, and shape according to hormonal
changes during the estrous cycle. During anestrus, the cervix is
flaccid, dry, and might be partially open. During diestrus and
pregnancy, elevated concentrations of plasma progesterone cause
the cervical opening to close, although its lumen (cavity) can
still be entered. The cervix in this stage is pale, tonic, and
dry, and projects into the vagina. The cervix during estrus is
usually moist and sometimes hyperaemic (bloody). The cervical
opening is usually open, edematous, and rests upon the floor of
the vagina.
Often an older maiden mare has an
abnormally tight cervix due to fibrosis. In these mares, the
cervix fails to relax properly during estrus, thus fluid is unable
to drain and it accumulates in the uterine lumen. In many cases,
this fluid does not have bacterial growth or neutrophils. Once the
mare is bred, the fluid accumulation will be aggravated due to
poor lymphatic drainage and impaired myometrial contraction
compounded by the tight cervix. In many cases, these mares are
susceptible to post-breeding endometritis even though they have
never been bred before. To maximize the fertility of these mares,
it is vital that the veterinarian is aware of the possibility of
this type of cervical pathology.
Failure of the cervix to open during
estrus can lead to unwillingness of the stallion to complete
mating or intra-vaginal ejaculation. Artificial insemination has
been used successfully in mares with an abnormally narrow cervix.
Impaired cervical drainage of uterine fluid can predispose a mare
to persistent endometritis. The good news is that a mare with a
fibrosed cervix which becomes pregnant does not normally have any
difficulties at foaling.
Failure of the cervix to close during
diestrus can lead to persistent endometritis, failure to conceive,
or early embryonic death. Failure to maintain closure during
pregnancy can lead to gestational failure. Both surgery and
exogenous progesterone have been tried in an attempt to encourage
closure of the cervix. These treatment options can be successful
when the damage to the cervix is not severe.
Assessment of the cervix must be a
part of the routine pre-breeding assessment of a mare. The cervix
can be examined by either direct visualization using a speculum
per vagina (see photos on page 50)and/or by digital exploration.
Damage resulting in cervical
incompetence or fibrosis is usually traumatic in origin and most
often occurs during parturition. This is especially the case if a
fetotomy (removal of a dead foal in pieces) is performed by an
inexperienced clinician and without adequate instrumentation.
Damage to the cervix can also occur during vigorous mating by an
over-sized stallion, especially if the mare was not in full
physiological and behavioral estrus. Usually the damage is not too
severe in these cases. The cervix can also be damaged by irritant
chemicals such as povidone-iodine. Adhesions of the cervix can be
broken down manually, but this must be done daily for as long as
two weeks to prevent recurrence. If severe, adhesions can
contribute to the development of pyometra (pus accumulation in the
uterus).
Endometrial
Swabs
Infection is a risk in breeding
horses, and swabs (cultures) taken as health precautions are an
important part of ensuring your mare is ready for the breeding
season.
The terms "swabbing a mare" or "has
she been swabbed" are very commonly heard in the horse breeding
industry. There are two parts of the reproductive tract that are
commonly swabbed--the clitoris and the uterus. A swab is a small
piece of cotton wool twisted onto the end of a thin plastic rod,
like a long Q-tip that is rubbed on something to get cells for
analysis. A swab of the uterine lining will be taken to see if
there are any bacteria present or signs of an inflammation of the
uterus known as endometritis. Mares should be free from
endometritis before they are covered by a stallion or artificially
inseminated.
A diagnosis of endometritis can be
made by collection of concurrent endometrial swab and smear
samples during early estrus for bacteriological culture and
cytological examination, respectively. This allows time for
resolution prior to mating and maximizes the chances of pregnancy.
To reduce the risk of contamination, the use of guarded swabs is
advised.
Cultures should be examined at 24 and
48 hours. An air-dried smear is differentially stained and then
examined for the presence of inflammatory and endometrial cells;
the latter confirms contact of the swab with the endometrium (see
Figure 6).
A positive culture result, with no
evidence of inflammatory cells in the smear (usually neutrophils),
is likely to be due to contamination during collection. Diagnosis
of acute endometritis is based on the presence or absence of
significant numbers of neutrophils in the smear. Very rarely,
neutrophils can be detected, usually at the "foal heat" or the
first estrus of the breeding season in maiden mares, although
there is no endometritis.
Uterine Biopsy
In some cases, a small piece of the
uterine lining might be taken for examination under a microscope.
This procedure is termed a biopsy. The technique involves passing
a biopsy instrument through the cervix and into the uterus for
sampling.
If the uterus appears normal on
palpation, the sample should be taken from one of the areas of
embryo fixation, i.e. the uterine horn-body junction on either
side. Single samples are usually representative of the entire
endometrium. If the uterus is abnormal on palpation per rectum,
biopsy samples should be taken from both the affected area and a
normal area.
When a biopsy of the uterus is looked
at under a microscope, certain features allow the biopsy to be
categorized according to the likelihood of the mare becoming
pregnant and having a foal. This information is useful to decide
whether to try and get the mare pregnant. Mares with a lot of scar
tissue (fibrosis) in their uteri have a poor chance of getting in
foal.
Detecting Intraluminal Uterine Fluid with Ultrasound Imaging
Transrectal ultrasonography provides a
non-invasive method of assessing the uterus for the presence of
fluid. Significant accumulations of fluid might indicate
susceptibility to endometritis.
Careful examination of the mare well
in advance of scheduled breeding will help ensure that any
problems are detected and corrected in time for breeding. Manage
your mare accordingly, and you will have maximized your chances of
having a healthy, pregnant mare on time.
5 TIPS:
Broodmare Exam
- Getting a breeding soundness exam
performed on your mare early enough to treat any problems is
key.
- Manipulating hormone levels and the
number of hours a mare is exposed to sufficient light can induce
estrus during winter months.
- "Transitional" spring and fall
estrous cycles are erratic and fertility is variable.
- Pregnancy rates at any one heat
vary from 60-80% on a well-managed farm.
- The breeding soundness exam
involves an external and internal examination of the mare using
ultrasound, palpation, speculum examination, and uterine
culture/biopsy.
