ovulation induction to prevent the need for a second insemination, or simply
wait to see if the mare has ovulated before 9:00 two days later. In other cases,
AI is deliberately planned for 1.5 days before ovulation; for example, in
problem mares it allows more time to treat an anticipated post-breeding
endometritis before circulating progesterone concentrations start to climb.
There are also situations (e.g. a poorly fertile stallion) when it may be prudent
to plan to inseminate twice in a cycle (to improve the likelihood of pregnancy);
this can be achieved by inseminating on the day of ovulation induction and the
following day (unless the mare ovulates overnight); however, the owner of a
busy stallion may take some convincing of this plan of action. A more
palatable alternative for most stallion owners is when AI is performed one day
before and one day after hCG. On the occasions when a chilled semen dose
arrives that contains virtually no (motile) spermatozoa, the opportunity to
repeat the AI a day later can also be very welcome!
Within the Dutch system, a chilled semen dose for transport is supposed to
contain 600 million morphologically normal, motile spermatozoa (TNM), with
the idea that this will ensure that at least 300 TNM remain when the sample
reaches its destination. For many stallions, this is probably far in excess of the
sperm number actually required to offer an optimal chance of pregnancy;
however, in the absence of individual stallion data on optimal sperm number,
this system offers “quality assurance” to mare owners. Three hundred TNM,
or indeed 500 million progressively motile sperm, should be sufficient to offer
a good chance of pregnancy for all but a small minority of ‘normal’ stallions.
Frozen semen
The dogma for frozen semen has long been that the semen needs to be
introduced between 12 h before ovulation and approximately 6 h after
ovulation if pregnancy rates are not to be compromised. In fact, there is
almost certainly more room for manoeuvre. The major disadvantage of
attempting to inseminate all mares on a single occasion in the window of time
between 12 h before and 6 h after ovulation is that it is labour intensive;
indeed, the only truly reliable way of meeting this goal is to check mares at 6 h
intervals and AI post-ovulation. Even when ovulation induction is employed, it
is not possible to predict ovulation more than 6 h in advance with 100%
confidence; some mares will ovulate earlier than expected, others will ovulate
slightly later, and others will not ovulate at all and instead either develop an
anovulatory haemorrhagic follicle or undergo follicular atresia. For this reason,
when limited amounts of expensive semen are available, AI with frozenthawed semen is almost invariably delayed until ovulation has taken place or
is clearly in progress. Indeed, for a number of popular European stallions, the
regular frozen semen dose is now a single straw, and inseminating preovulation would thus risk ‘wasting’ a whole dose. On the other hand, checking
all mares at 6 h intervals over a number of days is neither practical nor
sensible. Moreover, there is sufficient evidence to suggest that the ‘window of
opportunity’ is probably greater than previously thought. In this respect, it
appears that when frozen-thawed semen is used, the oocyte can be fertilized
with equal success by an insemination performed 12 h after ovulation, without
a significant rise in the subsequent incidence of early embryonic death.
Indeed, Sieme et al. (2003) reported that equal pregnancy rates were
15-‐18
February
2016
East
London
Convention
Centre,
East