The major disadvantage of frozen semen
is that, on average, pregnancy rates from
frozen semen are approximately 50% of the
same stallion’s pregnancy rates with fresh/
chilled semen. That is 35-40% of mares are
expected to become pregnant from each
breeding cycle. Some stallions will do better
than this, and others will do worse, and
not all stallions are candidates for freezing
semen. It is important to select a mare that
has a good chance of going into foal, rather
than a mare with reproductive problem s, as
failure to conceive may be due to the mare
rather than the frozen semen quality. There
is usually an increased cost associated with
using frozen semen. Assuming that the se-
men is of high quality and freezes well, when
thawed it still has a shorter survival time in
the mare’s uterus than good quality fresh or
chilled semen. This makes it important to
inseminate the mare very close to her time
of ovulation which requires multiple ultra-
sound examinations to accurately detect the
time of her ovulation.
There is no strictly right or wrong decision
when selecting semen type for your mare
but as a guide:
- A young fertile mare is a candidate for
fresh, chilled or frozen semen.
- An older maiden mare may have better
pregnancy results using fresh or chilled
semen
- A mare susceptible to post-breeding
endometritis (inflammation, or fluid in the
uterus) will potentially have better preg
nancy rates with a single AI of chilled or
fresh extended semen.
The success, cost and convenience of the
breeding are factors that will also play a
major role in the decision of which method
to utilise.
Embryo Transfer
Embryo transfer is the process by which
a very young embryo (6.5-8 days old) is
flushed from the uterus of a valuable donor
mare and recovered via filtration. The
embryo is then transferred into the uterus
of a recipient mare to complete develop-
ment. The resultant foal will be the genetic
offspring of the donor mare and stallion of
your choice. The recipient mare will give
birth to and raise the foal.
Advantages
• Allows a valuable mare (donor) to
continue a performance career while the
recipient mare carries the foal
• More than one pregnancy per year can
be achieved from outstanding mares
• Foals can be obtained from mares that
cannot carry embryos beyond eight days
of pregnancy
• Younger mares of 2 & 3 years of age can
produce offspring
• A foal can be obtained from a mare
foaling late in the breeding season thus
enabling them to conceive early the
following year.
The Embryo Transfer
Procedure
1. The first step of the program is to
synchronize the reproductive cycles of the
donor and recipient mares. It is important
that the recipient mare is at a similar stage
in her cycle as the donor mare to ensure
that the uterus is ready to accept the
embryo. The more recipient mares that are
available for this process the easier (and
quicker) it is to find an appropriate can-
didate. We routinely maintain a recipient
mare herd of between 30 and 40 mares
for this process. These mares are younger
than 10 years old, generally Standarbreds
and have no history of reproductive
issues. It is ideal to have at least three
recipient mares synchronised with the
donor mare to allow for further selection
at the time of the donor mare flush.
2. The donor mare (your mare) is bred
using semen of your choice. This can
be fresh, chilled or frozen semen. It is
necessary for the cycle of the mare to be
followed closely by ultrasound examina-
tion and the breeding timed according
to these findings. It is important that the
time of ovulation is known.
3. At 6.5 to eight days after ovulation is
detected, a sterile catheter is placed in the
uterus of the donor mare and the embryo
removed by flushing a sterile solution into
the uterus. The recovered fluid then passes
through a filter cup allowing the
microscopic embryo to be retrieved.
4. The small amount of fluid in the filter
cup is than searched under a dissecting
microscope in order to locate the embryo.
Up to this point it is not known whether
the donor mare has become pregnant and
produced an embryo.
5. Once the embryo is found it is then
further washed in a sterile media contain-
ing antibiotics and graded according to its
morphological appearance. A grade 1 or 2
embryo is of greater quality than a grade 3
or 4 embryo which is less likely to achieve
a pregnancy.
6. The embryo is then placed in a catheter
and carefully transferred into the uterus
of a previously chosen recipient mare.
This process must be done as aseptically
and gently as possible so as to minimise
the risk of introducing contamination and
trauma into the recipient mare’s uterus.
7. A pregnancy scan is performed on
the recipient mare 6-7 days following
the transfer of the embryo to see if the
embryo has survived and developed
accordingly.
method of preservation. In young, healthy
mares with no history of reproductive
problems inseminated with fresh semen
from fertile stallions, typical embryo re-
covery rates near 80%. A reduced embryo
recovery rate occurs when aged mares
(greater than 14 years), or those with a
history of subfertility are used as donors, or
when chilled, or in particular, frozen semen
is used.
The time at which an embryo is flushed from
the uterus also affects recovery rates. An
embryo usually does not enter the uterus
until day 6.5 after ovulation. Increased age
of mare, breeding early in the season and
using frozen semen can delay when the
embryo enters the uterus. A lower embryo
recovery rate may be because the embryo
has not exited the oviduct and entered the
uterus. Recovery rates of embryos can be
improved in mares that ovulate multiple
follicles per cycle. Typically, a mare will only
ovulate a single follicle however, some mares
will double or triple ovulate spontaneously.
While there has been much research into
techniques for superovulating mares, they
have generally been unsuccessful and there
is no current reliable and affordable option
on the market in Australia.
The likelihood of establishing a pregnancy
following transfer depends upon many
factors including embryo quality, trans-
fer technique, donor-recipient synchrony
and other aspects of recipient suitability.
Typically a survival rate of 75-80% of good
quality embryos recovered from a donor
mare should be expected. Lower grades of
embryos, as might be expected from mares
with unresolved post breeding inflamma-
tion or advanced maternal age, have higher
pregnancy failures. In summary, a young
fertile donor mare inseminated with fresh
fertile semen, will achieve a pregnancy rate
of 60-65% per cycle. This indicates that it
will take an average of two breeding cycles to
produce a pregnancy via embryo transfer in
an ideal situation.
This leads to an obvious disadvantage of
cost of an embryo transfer program. On
average a fee of $2500 - $4000 per pregnancy
will be charged. The major cost associated
with an ET program is maintenance of a
large number of recipient mares, which are
often kept for a few years and need to be
maintained in excellent body condition and
health to sustain reproductive soundness.
Many embryo transfer centres offer packages
that are designed to offer the best opportu-
nity to secure a pregnancy at an affordable
price however, starting with a suitable donor
mare and stallion offers the best outcome for
all involved.
Success Rates
The most important factors that affect the
recovery rate of embryos are the timing of
the insemination and the fertility of the do-
nor mare and stallion. Stallion fertility is fur-
ther influenced by semen dose, quality, and
July • August • 2018 • The Australian Quarter Horse Magazine • 25