Page 447 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

422 CHAPTER 2

VetBooks.ir Prostaglandin F alpha INDUCTION OF OVULATION
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PGF α causes lysis of the CL, resulting in a decline DURING OESTRUS
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in serum progesterone. A CL will only regress in
response to exogenous PGF α more than 5 days Human chorionic gonadotropin
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after ovulation, so it is usual to inject mares on hCG has an LH-like effect, helping to mature and
the 6th day after ovulation. The oestrous response ovulate a dominant follicle. hCG is given when the
starts at about 3–4 days after injection, with ovu- oestrous mare has a follicle of 35 mm or more. The
lation generally occurring between 5 and 12 days average dose rate is 1500–3000 IU. Under these cir-
after injection. Administration of PGF α to mares cumstances, 90% of mares ovulate within 48 hours
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on day 12–14 may not shorten the time to the next of injection. There have been reports of mares that
ovulation. Mares with follicles >35 mm at the time are repeatedly treated with hCG producing antibod-
of PGF α administration may ovulate very quickly, ies to injectable hCG, causing the mare to fail to
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or the follicles may slowly regress. Using a low dose respond to the drug.
of PGF α in these mares can be helpful in allow-
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ing them to come slowly into oestrus, for the uterus GnRH implant/injections
and cervix to develop the oestrous characteristics Ovuplant is a commercially available biodegrad-
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and for the mares to be bred before they ovulate. able, silastic implant containing deslorelin acetate.
Large follicles that slowly regress are usually fol- Injected into the mare’s neck or vulval tissue when
lowed by the development of a new dominant fol- the oestrous mare has a follicle >30 mm, it gives a
licle, which takes several days, and these mares response similar to hCG, but with no risk of stim-
often do not ovulate less than 12 days after PGF α ulating an antibody response. The treatment is
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administration. expensive compared with hCG. An injectable form
PGF α can also be given to aid synchrony of of deslorelin is available in some countries and is
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mares, as described below. used in a similar way. There is a reported increase in
the intraovulatory period when a deslorelin implant
SYNCHRONISATION OF OVULATION (Ovuplant ) is used. This can be avoided by insert-
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ing the implant into the vulval mucosa and removing
Combination of progestagens it when ovulation has occurred. Buserelin is mar-
and oestrogens keted in an injectable form called Receptal . This
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Oestrogen has a suppressive effect on FSH release. does not hasten ovulation unless given twice daily
The combination of progestagens and oestrogen during oestrus when a follicle >35 mm is present.
appears to have a greater negative feedback on
LH than progestagen alone. Intramuscular injec- CONTROL OF CYCLICITY IN
tion of 150 mg progesterone in oil and 10 mg THE POST-PARTUM MARE
oestradiol-17β in oil daily for 10 days, with PGF α
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given on the 10th day, can result in a tight syn- Foal heat is the first heat after parturition and gen-
chrony of oestrus and ovulation. Ninety percent of erally begins 5–12 days after foaling. As a general
mares treated in this way ovulate between 10 and rule, ovulation usually occurs 10–11 days after foal-
12 days after the last treatment. Unfortunately, ing, but this interval shortens later in the breeding
oestradiol-17β and progesterone in oil may not season. Studies have shown that pregnancy rates
be currently available commercially in some are higher if a mare ovulates >10 days after foaling
countries. Other oestrogens are available (oestra- compared with a mare that ovulates <10 days after
diol cypionate and oestradiol benzoate), but they foaling. This difference in fertility is partly due to
have different durations of activity and therefore the rate of endometrial recovery following foaling.
cannot be recommended for use with the above From a histological viewpoint the endometrium
regime. takes approximately 14 days to repair following
foaling and return to a normal ‘non-pregnant’ state.