very expensive. However, while minimizing the number of inseminations per
cycle is probably advisable for mares likely to have difficulty resolving the
post-mating inflammatory reaction, i.e. as part of a ‘minimum contamination
breeding protocol’, a number of surveys have reported that pregnancy rates
are higher for mares inseminated more than once per oestrus (e.g. Sieme et
al, 2003; Vidament, 2005;). While it is not entirely clear why extra
inseminations should help, there may be an advantage of having previously
established an oviductal sperm reservoir, or it may simply be that the
additional inseminations increase the likelihood that a mare is inseminated in
the optimal period with respect to ovulation or ensures that sufficient fertile
sperm are introduced in a given cycle. Indeed, in the case of poorly fertile
stallions or when a dose of semen contains very low numbers of (motile)
sperm, the author prefers to inseminate twice at a 15-24 h interval, if possible.
As will be detailed below, planning for a double insemination can also make
the monitoring process considerably less labour intensive.
Predicting ovulation / induction of ovulation
The key issue when planning the timing of AI is the predicted time of
ovulation. The general indicators of impending ovulation include follicle size
(preferably compared to maximum pre-ovulatory size in previous cycles),
follicle tone and shape (regularity) and follicle wall appearance (thickening,
anechoic line), along with changes in the degree of uterine oedema, cervical
relaxation and teasing behaviour. However, none of these parameters are
entirely reliable predictors of exactly when, and indeed whether, a mare will
ovulate. This is not critical if the semen is readily available and there is no
good reason to avoid multiple AI; however, the combination of a long oestrus
and difficulty in accurately predicting the time of ovulation can make planning
a challenge if the semen is in short supply, is very expensive, is only available
on certain days of the week, or has to be ordered more than a day in advance.
Induction of ovulation is therefore an invaluable aid in commercial equine
practice, notwithstanding the fact that no inducing agent is 100% reliable and
that there is still significant variation in response between mares. Moreover, in
practice it is quite common for mares to be presented so close to ovulation
that they are unlikely to ‘wait’ until the chosen ovulation inducing agent has
had its effects. Nevertheless, even when ovulation appears imminent at first
presentation, an ovulation inducing agent can be a useful failsafe for
minimizing the period of intensive management.
The major ovulation inducing agents in (equine) clinical practice are human
chorionic gonadotrophin (hCG) and analogues of GnRH; recombinant equine
LH is not yet available commercially and, while some PGF2a analogues
increase the likelihood of ovulation within 48 h, they are not considered
reliable enough for routine use. Currently, hCG is probably still the most
commonly used inducer of ovulation in equine AI, primarily because it is
inexpensive. It is typically administered at a dose of 1500 IU when a mare is
clearly in heat (e.g. obvious uterine oedema pattern) with a follicle exceeding
35mm in diameter. More than 80% of treated mares will ovulate within 48h,
with the majority expected to ovulate at 36 ± 4 h after administration
(McKinnon and McCue, 2012). There are anecdotal reports that response
rates are lower, and time to ovulation longer, early in the season and some
15-‐18
February
2016
East
London
Convention
Centre,
East
London,
South
Africa
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