Tori Doyle BVSC (Hons) MANZCVS (Equine Medicine)
Westvets Animal Hospital
Breeding season is approaching and owners
of mares are faced with a myriad of options
regarding the choice of a stallion. While
pedigree, performance and conformation
of the sire should be paramount to the de-
cision, selection of a stallion may be limited
by the fertility of the mare, the fertility of the
stallion and the type of semen that is offered.
A stallion may be available to live cover a
mare, or alternatively, artificial insemina-
tion using chilled or frozen semen may be a
viable option. Fresh, chilled or frozen semen
has its advantages and disadvantages and
each should be considered when deciding on
a stallion for your mare.
Fresh Semen
Natural cover can allow a mare in season to
be left at a stallion facility and mated over
several days. This process should not take
longer than 5-7 days and can eliminate the
need for ultrasound examinations of the
mare’s reproductive tract. Some stallions
have semen that is unable to survive the
chilling or freezing process, making it nec-
essary for live cover or immediate insem-
ination of collected semen into the mare.
Natural service of a mare may be the only
way available and some breed societies, such
as the Australian Stud Book, will not allow
registration of Thoroughbreds that are born
by AI or embryo transfer.
Natural service is not without its disadvan-
tages. It usually means travelling the mare,
possibly with a foal at foot, to the stallion
with associated expense. There is also
the chance of physical injury and disease
transmission during the cover. Some mares,
particularly older ones, will develop an
inflammatory reaction to semen within the
uterus which can be difficult to clear and
greatly reduce their chance of conceiving.
These mares benefit from a single insemi-
nation with low volumes of semen such as
is achievable with artificial insemination
(AI). It is a long held and false assumption
that a mare is more likely to conceive when
covered by a stallion rather than undergoing
AI. Overall average conception rates (the
percentage of mares that become pregnant
over the stud season) are very similar, at
75% for AI, compared to 80% for natural
service. There is a slight difference between
conception rates per cycle between natural
cover and AI. Pregnancy per cycle occurred
in 65% of mares covered naturally, com-
pared to conception rates of 55-70% for AI
with chilled semen and 35-50% for AI using
frozen semen. Although overall pregnancy
rates are similar it may take more attempts
to get a mare in foal when using AI versus
live cover.
Chilled Semen
This process involves the collection, eval-
uation and extending of the semen. The
extender is a liquid diluent that protects the
sperm against possible damage by toxic sem-
inal plasma, as well as providing nutrients,
cooling buffers, and commonly antibiotics,
which reduce disease transmission. Follow-
ing collection the semen is cooled slowly
within a transport container. The cooling
process allows shipment of the semen
to most areas within Australia and New
Zealand, increasing the access to stallions
nationwide. An advantage of chilled semen
24 • The Australian Quarter Horse Magazine • July • August • 2018
is that insemination can be performed either
at home or at a veterinary practice nearby,
reducing transport costs and decreasing the
chance of injury to the mare or her foal at
foot. Ultrasound examination of a mare’s
reproductive tract has the benefit of
detecting abnormalities in the cycle. It also
allows accurate timing for a single insemi-
nation of chilled semen. This is particularly
beneficial in mares susceptible to post
breeding inflammation and fluid accumu-
lation, as the reduced volume of semen and
number of inseminations decreases this
reaction in susceptible mares. Increased
costs associated with chilled semen include
stallion collection fees, semen transport fees
and scanning fees required to coordinate
insemination with the time of ovulation.
It is important to note that not all stallions
provide semen that is suitable for chilling.
Evaluation of a stallion’s chilled semen
should be performed prior to the start of
the breeding season to ensure only quality
semen is available when requested by a mare
owner.
Frozen Semen
Frozen semen is created by mixing the
semen with a liquid diluent which allows a
quantity of the sperm to survive the freezing
and thawing process. The semen can then
be stored indefinitely in liquid nitrogen at
a temperature of -196 degrees celsius. The
semen is only thawed when it is needed to
inseminate a mare immediately upon her
ovulating. The major advantage of this
semen is its use allows access to
international stallions and those stallions
that are no longer surviving.