Page 446 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.1 The female reproductive tr act 421

VetBooks.ir Synthetic GnRH Progestagens
These drugs mimic and therefore extend the luteal
Administration of GnRH to mares results in release
of endogenous LH and FSH and is the most suc-
cessful method of inducing ovulation early in the phase and produce two important effects on the
cycling mare: (1) they inhibit behavioural oestrus
transition period. In the literature, GnRH has been (mares will stop exhibiting oestrous behaviour after
administered by mini-pumps, implants and injec- 2–3 days of treatment); and (2) they diminish LH lev-
tions. Mini-pumps produce the best results. In the els and block final maturation of follicles and there-
period January to March (northern hemisphere) fore ovulation. This is not 100% effective in the mare.
(July to September, southern hemisphere), injec- On average a mare will show signs of oestrus for
tions given three times daily, followed by human 5 days (3–6 days) and will ovulate 10 days (8–15 days)
chorionic gonadotropin (hCG) administration, after cessation of progestagen treatment due to a
induces oestrus in mares within 12 days. The preg- rebound of LH levels that allows final follicle matu-
nancy rates following these oestrous periods are ration and ovulation.
around 50%. Important points to remember are:
In the UK, buserelin (2.5–3 ml i/m q8–12 h for up
to 10–14 days) has been used and the mare scanned • Progestagens do not interfere with the release
every 3–4 days to monitor follicular development. of PGF α from the endometrium. Therefore,
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When a follicle >30–35 mm is present, the mare can progestagen treatment should continue
be teased, covered and treated with hCG to encour- independently of the endogenous progestagens
age ovulation. Mares can, however, slip back into an for 14–15 days.
anovulatory state after an ovulation with GnRH. • Some mares may ovulate while on progestagens.
In some countries, GnRH is available as an implant The new CL formed will therefore be present
(Ovuplant ), which is implanted into the mare when the exogenous progestagens are stopped.
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every 48 hours until she ovulates (2–3 implants). These mares will not return to oestrus until the
The implants are more successful if a larger fol- new CL regresses. To avoid this, PGF α can be
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licle (30 mm) is already present, but there is a risk given at the end of the progestagen treatment
that excessive use can push the mare back into an period. If PGF α is given, the treatment period
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anovulatory state. Human goserelin acetate implants can be shortened to 8–12 days without any effect
(Zoladex , 1.8 mg) are used subcutaneously in a sim- on the synchrony of the mares. Ultrasound
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ilar way. The effect seems to be limited to 7–10 days examination prior to and on the last day of
and there is a need to monitor the mare every 24–48 exogenous progestagen treatment will allow
hours to ensure any ovulatory follicle is identified identification of any CL and, therefore, whether
and the mare bred. any PGF α is necessary.
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• Some mares develop large follicles towards
SYNCHRONISATION AND the end of the progestagen treatment period
MANIPULATION OF OESTRUS (FSH is not suppressed by progesterone). These
DURING THE BREEDING SEASON follicles may ovulate rapidly after cessation of
the treatment (2–4 days). Mares that ovulate at
The ability to manipulate the mare’s cycle is a this time may not show oestrus and therefore the
valuable skill to the veterinary surgeon. It can be opportunity to breed from them may be missed.
helpful to manipulate the oestrous cycle to make Scanning mares on the last day of treatment to
efficient use of the stud and veterinary staff, syn- identify those mares with follicles >35 mm in
chronise mares in embryo transfer programmes diameter may identify mares that could ovulate
and allow the use of chilled semen that may only early after synchrony.
be available on specific days. The basic approach • Synchrony of mares may be improved by
involves either extending or terminating the luteal administering hCG to mares that have follicles
phase of the cycle. >30 mm in diameter (see below).