TRANSFER IN THE MARE
Dr Mina C.G. Davies
Morel, University of Wales, Aberystwyth Ceredigion, SY23 3AL
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A general hormone routine that can be used to synchronise donor
and recipient mares for ET. In order for this regime to work
PGF2a must be administered between
6 and 13 days after the last ovulation. In practice several
variations on this general theme are used.
Ideally a number of recipient mares should be
prepared and synchronised, as synchronisation in the mare is
not that precise and so this allows the mare best matched to
the donor to be selected. Recipients should be five to ten years
of age with a proven breeding record and no history of uterine
infection. They need be of no particular genetic merit, as they
will in no way affect the genetic make up of the embryos transferred
to them. Ideally the recipient mare should be larger than the
donor, so providing a larger uterus and maximising foal birth
Both the donor and recipient should be teased, rectally palpated
or scanned to ensure that they have reacted to the synchronisation
programme and ovulated. The donor mare only should then be covered
either naturally or by AI.
Equine embryos in general are collected at between four
and eight days of pregnancy. They can be recovered at a relatively
late stage compared to other farm livestock, as equine concepti
remain spherical and free living for a prolonged period.
The age at which an embryo is to be recovered dictates the method
used. Younger embryos are collected surgically and older ones
Surgical collection is carried out under general anaesthetic
and is rarely used in practice for the routine collection and
immediate transfer of embryos. Additionally surgical recovery
only allows embryos within the fallopian tube and very top of
the uterine horn to be recovered, that is those younger than
five days. However, these young embryos are more robust and
so survive the processes of transfer, chilling and freezing
Non-surgical embryo recovery, is increasingly popular as general
anaesthetic is not required and hence this method carries lower
risks. For collection the mare is restrained in stocks with
her tail bandaged and perineal area washed. A three-way catheter
is introduced through the cervix of the mare and passed high
up into the uterine horn (Figure 1.). Once in position, the
cuff of the catheter is inflated via the air inlet tube so occluding
the base of the uterine horn and preventing any fluid escaping.
Fluid is flushed in through the entry catheter, up into the
top of the uterine horn, and then returns, along with any embryos
present, via an opening into the outlet tube for collection
in a collecting vessel (Figure 1). Non-surgical collection allows
only embryos within the uterus to be collected, that is those
older than 5 days, these embryos are less robust and so best
suited for immediate transfer rather than storage. However,
the technique runs a lower risk to the mare and allows for multiple
recoveries. As such non-surgical recovery is normally the commercial
method of choice.
A diagrammatic representation of the foley catheter illustrating
and outlet tubes plus the air inlet for inflating the cuff.
Once collected, embryos must be kept warm (35 - 38
° C). They are examined and
graded, one to five (one being excellent and five dead). Only
those graded three or better are selected for transfer or stored
for future use.
Recovered embryos are either transferred immediately, or
chilled or frozen for use at a later date. Equine embryos can
be stored for up to 24 hrs at 42 °
C. Alternatively they can be cooled and stored successfully
for 36 hrs at 4-5 ° C. Commercially,
embryos can be cooled, stored and transported in an Equitainer
(as used for equine semen) which acts like a cool box.
Finally embryos may be frozen. This is the only means of long-term
storage but as yet is relatively unsuccessful in horses, although
quite successful and commercially viable in other livestock.
The first successful birth of a foal from a frozen embryo was
not achieved until 1982. It is apparent that freezing young
embryos (those less than 5 days old) gives the best results,
this is one of the advantages of surgical collection. In order
to freeze embryos a cryoprotectant is required, unfortunately
as yet an effective, non toxic cryopreservation agent is yet
to be found.
The transfer of embryos can, as with collection, be done
either surgically or non-surgically. Surgical transfer, as with
embryo collection, requires a general anaesthetic, and as such
is not popular except in the transfer of young frozen embryos.
Non-surgical transfer is very similar to AI (Figure 2). The
mare is restrained and prepared as for non-surgical collection,
the transfer gun with attached syringe, containing the embryo,
is passed through the mare's cervix and into the uterine body.
Once in place, the embryo is expelled into the uterus by slowly
depressing the plunger of the syringe.
Non-surgical transfer of embryos into a recipient mare.
Non-surgical transfer is a relatively easy and
quick procedure. However, it does run the increased risk of
introducing low-grade infections into the reproductive tract
and only allows the deposition of embryos into the uterine body
or the lower part of the uterine horns. This technique is, therefore,
limited to embryos older than five days.
Embryo transfer in horses has significant potential for
development, especially in the area of cryopreservation, without
which its successful commercial application is limited. The
commercial application of embryo transfer in horses has, therefore,
a long way to go before it reaches the sophistication and success
of its application in cattle and sheep. One of the major constraints
on the development of embryo transfer in horses is the continued
reluctance of breed societies to register foals conceived in
this manner. The expansion of embryo transfer within the equine
industry is, therefore, dependent upon several factors including,
a change in breed registration restrictions; the value of horses;
the performance of embryo transfer foals; the cost of the procedures;
refinement of techniques; and the attitude of the equine industry
to its application. However, even within these constraints,
embryo transfer in horses is a valuable experimental tool. Embryo
transfer may find further application and commercial worth if
a reliable super ovulating agent can be found and embryo sexing