practitioners will therefore use 3000 IU in the spring. Certainly, a proportion of
mares will fail to respond as expected to hCG; this seems to be more common
in older mares, mares treated repeatedly within a single breeding season, and
mares that are treated too early in oestrus. In this latter respect, it is probably
not sensible to stick slavishly to a follicle size of 35mm as the sole criterion for
when to administer hCG. In a mare that normally ovulates a 50mm follicle and
develops obvious endometrial oedema, it is preferable to wait until the
oedema pattern is clear and the follicle is somewhat larger (e.g. 40-45mm)
before administering the hCG. Conversely, it is not sensible to wait for a
follicle to reach 35mm so that hCG can be administered before ordering
chilled semen for a mare that has previously ovulated 30mm follicles.
In mares that have previously failed to respond to hCG, older mares, or when
ovulation needs to be induced earlier (e.g. 30mm follicle) than considered
ideal (e.g. because chilled semen is only available on Saturday or the
following Tuesday), a long-acting GnRH analogue is preferred as the
ovulation induction agent. The GnRH analogue of choice in Europe is
currently the deslorelin implant, Ovuplant™, primarily because it is the only
such product officially registered; ovulation is anticipated 41 ± 3 h after
subcutaneous introduction of the implant (McKinnon and McCue, 2012). The
major disadvantage described when using Ovuplant as an ovulation inducing
agent, is suppression of pituitary gonadotrophin secretion and a resulting
inhibition of follicle development; fortunately, this suppression seems to be
fairly transient (approx. 10 days: Johnson et al., 2002) and is therefore only
really an issue in embryo donor mares. Moreover, the risk of ovarian
suppression can be averted by placing the implant in the vaginal muscosa,
where it can be easily located and removed once ovulation has been
confirmed. While deslorelin is often considered to be more reliable and to yield
a more predictable time of ovulation than hCG, there are still mares that fail to
ovulate at the expected time (i.e. within 48h), or that develop an anovulatory
haemorrhagic follicle.
While they are not commonly used, short-acting GnRH preparations (e.g.
buserelin) are also effective for inducing ovulation in mares, particularly if
administered repeatedly (e.g. at 12 h intervals: Barrier-Battut et al., 2001). The
author quite commonly uses twice daily injections of 20 mg buserelin to
promote ovulation in mares that have failed to ovulate within 48 h of hCG
administration.
Chilled semen
Within the Dutch AI network, if semen is ordered before 09:00 it will generally
be delivered on the same day, and within 12 h after collection. For the majority
of stallions, we aim to perform AI with chilled semen 15-40 h prior to ovulation
(AI is performed in the afternoon, and the ovulation checks with respect to
potential reordering in the morning); for some stallions, however, previous
experience may result in a strong preference to ensure AI no more than 15 h
prior to ovulation. In many cases, therefore, the AI protocol simply involves
inducing ovulation and ordering semen for the following afternoon, which is
fine if the mare is presented early in oestrus. However, mares are often
presented late in oestrus (i.e. with a large soft follicle, little uterine oedema),
and it is then prudent to order semen for the same day and either use
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
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