In general, most operators prefer to transfer a day 7 or 8 embryo into a
recipient that has ovulated 0-2 days after the donor; this is difficult if
donors and recipients are synchronized on a one-to-one basis, and is
more easily achieved by having access to a large herd of recipients or
beginning with a minimum of 2 recipients per donor. While a number of
hormonal regimes have been described for synchronizing the oestrous
cycle in the mare (for review see Allen 2005), the author prefers to limit
himself to a simple combination of frequent ultrasonographic examination
of the ovaries combined with tactical use of a PGF2a analogue and/or hCG.
PGF2a analogues can be usefully administered to a mare with a functional
corpus luteum (CL) at least 5 days old, where the relative timing of
administration to donor and recipient is based on the respective size of
their largest ovarian follicle. Once the mares are in oestrus, follicle
development should be monitored regularly and synchronization can be
fine-tined by using hCG to ensure ovulation of a >35mm follicle within 48
hours; this can be extremely useful in ensuring that a recipient does not
ovulate more than one day before the donor (i.e. treat the donor if the
recipient shows signs of imminent ovulation), but does ovulate within two
days after (i.e. treat the recipient as soon as ovulation is detected in the
donor).
An alternative approach described for ensuring adequate ‘hormonal
synchrony’ of a recipient, is to use an ovariectomised or seasonally
anestrous mare treated with progestagens from soon after the donor has
ovulated (Hinrichs et al. 1987); this approach may be particularly useful
early in the season when relatively few cycling recipients are available.
Although the numbers of reported transfers to such acyclic progestagentreated mares are low, pregnancy and pregnancy loss rates do not appear
to differ significantly to synchronized cycling mares (Carnevale et al. 2000;
Pessoa et al. 2002), as long as an adequate dose of progestagen is
administered until there is sufficient endogenous production of
progestagens from either eCG-induced secondary corpora lutea or the
placenta (McKinnon et al. 1988).
Other aspects of ‘recipient suitability’
Although synchrony of ovulation is the most important component of
recipient suitability, there are a number of other factors that affect the
likelihood of establishing and maintaining pregnancy. Recipients should be
in good general health and body condition, and they should be relatively
young (3-12 years old) since age is an important predisposing factor to
endometrial degeneration likely to compromise the ability to maintain
pregnancy (Ricketts and Alonso 1991: Morris et al. 2002); indeed,
recipients older than 10 years appear to be at greater risk of losing an ET
pregnancy (Carnevale et al. 2000). A recipient should also exhibit normal
cyclicity and show no obvious signs of reproductive tract pathology (e.g.
endometrial cysts, uterine fluid accumulation, pneumovagina). Of
paramount importance for a mare intended as a transcervical ET recipient,
is that the cervix is undamaged and not unduly tortuous; failure to
negotiate the cervix and enter the uterine lumen or the need to digitally
assist the passage of the pipette are probably common reasons for failure
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
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