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Artificial insemination (A.I.) is a technique used to transfer appropriately processed semen from a stallion into the uterus of a mare at the correct stage of her oestrus cycle resulting in a single pregnancy.
A.I. has become a very popular technique in stud medicine and in some cases it has significant advantages over natural service. It is not allowed in racing thoroughbreds under Jockey Clube rules.
Fresh semen - only used on studs and only lasts outside the horse for short periods of time
Chilled semen - is chilled to 4 C and stored for shipping in a special container or commonly polystyrene boxes with a chill pack. It should be inseminated within 48 hours of collection.
Frozen semen - Semen that is required to last longer than 48 hours is frozen in liquid nitrogen at a temperature of -196C. It is thawed and used when required
The success of the A.I. program is dependant on factors like the quality of the semen, the fertility status of the mare and the program used by the attending vet. Fertility with fresh and chilled semen is similar or slightly better than that using natural service with approximately 60% of mares becoming pregnant on the first cycle. Until recently conception rates with frozen semen were less favourable but as experience with these techniques has improved so have conception rates.
Prior to embarking on an A.I. program it is important the mare is assessed for factors that may reduce fertility, such as poor vulval conformation or any internal abnormalities that may reduce her fertility. This will include an ultrasound examination of the uterus looking for uterine cysts, a vaginal examination checking the cervix for any abnormalities and an assessment of vulval conformation. A uterine swab and smear may also taken during early oestrus to check for evidence of infection.
A successful insemination resulting in pregnancy depends on placing the semen in the uterus at the correct stage of the mares' cycle. This has to occur when the mare is in season and close to ovulation. Most mares have a 21-22 day cycle with oestrus lasting 3-6 days. Some mares cycle year round but most cycle between March and October. The oestrus cycles that occur early in the season and those towards the end of the season are less likely to result in pregnancy and are less consistent and so it is preferable to wait until the mare is cycling regularly before trying to inseminate your mare.
Some mares show clear signs of oestrus but in others it can be difficult to detect oestrus when there is no stallion present, for these mares an injection can be given that will bring the mare into season.
• Squatting and frequent squirts of urine
• Tail raising and adoption of a stance similar to urination. The tail is usually held to one side.
Other signs can be difficult to detect in the absence of a stallion, some mares with foals at foot may prove particularly difficult but the vet will be able to perform ultrasound examination to determine if your mare is in season.
To determine the best time to AI your mare, it can be helpful to keep a diary of her seasons prior to breeding. This allows us to predict when next she will come in season and prepare well in advance. We will normally scan the mare just as we are expecting her to come into season to check the size of the developing follicles and determine when we will need to order the semen and when to give the ovulation induction agent. If it is not possible to estimate when she will come in to season, we can give he an injection to bring her into season.
She will be inseminated just at the point of ovulation of the largest follicle and will normally be scanned the following day to check that the follicle has ovulated and that no fluid has gathered in the uterus.
Frozen semen survives in the mare for a shorter period of time (approximately 12 hours) and as such the timing of insemination is more critical as the mare’s egg is only available for conception for about 6 hours. This makes frozen semen programs more time consuming and labour intensive and therefore a more expensive option. The mare must be scanned every six hours to try to co-ordinate the time of insemination exactly with the time of ovulation
Rectal ultrasound examination between days 14 and 16 following ovulation is the best method of determining pregnancy. This is the most appropriate time to examine the mare as twin pregnancies may be detected and can be dealt with at this stage. After this time the pregnancy becomes attached to the lining of the uterus and so it is much more difficult to deal with without terminating the entire pregnancy.
A further scan should be performed around 28 days. This allows the vet to check again that there is not a twin pregnancy and to look for normal development (the heart beat should be visible by this stage).
It is advisable to check the pregnancy again around day 42 to ensure that the mare hasn't re-absorbed the pregnancy.
If the mare is not pregnant on the first scan then she should be back in season again in a few days to allow the program to be repeated hopefully with a more favourable outcome.