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 (although it is not allowed in racing thoroughbreds) in some cases with significant advantages over natural service.
Prior to embarking on an A.I. program, it is important to weight up the pros and cons, breeding your mare can be expensive and does not always result in a live foal the following year.
With all breeding programs, it should be remembered that there is a risk to the mare both when carrying the foal and particularly at the time of foaling, but also during mating or when performing rectal examinations. For this reason, ideally, rectal examinations are performed at premises with stocks to restrain the mare. If these are not available it is possible the vet will sedate your mare or at least twitch her during the examination for the protection of themselves but also the mare.
Artificial insemination involves the use of:
- Fresh semen
- Chilled semen
- Frozen semen
Fresh semen is usually used on studs and only lasts outside the horse for short periods of time. Chilled semen should be inseminated within 48 hours of collection and is chilled to 4 Cº and stored for shipping in a special container or commonly polystyrene boxes with a chill pack. Semen that is required to last longer than 48 hours is frozen in liquid nitrogen at a temperature of -196 Cº.
Poor quality semen can be an expensive and frustrating problem for all involved so it is worth ensuring that a vet or experienced stud technician is involved in collection of the semen and evaluating the semen prior to transport.
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. 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 is also taken during early oestrus to check for evidence of infection.
Timing the insemination
A successful insemination resulting in pregnancy depends on placing the semen in the uterus at the correct stage of the mare’s 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 (the receptive period) lasting three to six days. Some mares cycle year round, but most cycle between March and October with the cycles at the beginning and the end of the season being somewhat erratic. 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.
Signs of Oestrus in the Mare
- Squatting and frequent squirts of urine
- Tail raising and adoption of a stance similar to urination but maintained for longer periods and without evidence of straining (the tail is usually held to one side)
Other signs are 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 and vaginal examination to determine if your mare is in season.
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.
Chilled semen A.I.
It can be difficult to determine if your mare is in season in the absence of a stallion. We usually examine mares on a Friday, often giving them an injection to bring them into season. The mare will than be re-examined on the Monday to check she has come into season, to assess the size of the follicles on the ovaries and to perform her pre-breeding check including a swab and smear from the endometrial lining of the uterus to check for evidence of infection. The size of the follicles on the ovaries determines when the mare will require another examination, when we need to order semen and when we are likely to inseminate her.
Frozen semen survives in the mare for a shorter period (approximately 12 hours) of time and as such the timing of insemination is more critical as the mare’s egg is only available for conception for about six hours. This makes frozen semen programs more time consuming and labour intensive and therefore a more expensive option. The conventional approach was therefore to scan the mare daily until she had a 3.5 cm follicle and then following injection of a drug to control the time of ovulation the mare was scanned a minimum of every six hours. This is just not possible outside a hospital setting and so more recently fixed time A.I. programs have been developed and has been reported to have similar conception rates to those, where mares have been inseminated with chilled semen.
Transrectal 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 multiple pregnancies may be detected and 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 twins.
A further scan should be performed around 28 days before endometrial cup formation. 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.
If the mare is not pregnant on the first scan, then she should be back in season again imminently allowing the program to be repeated hopefully with a more favourable outcome.
Please contact the practice to discus the needs of your mare and for further information on A.I.