By Roger Dixon BVM&S MRCVS Ashbrook Equine Hospital.
This article follows on from last month’s introduction to Artificial Insemination, which discussed the advantages of A.I. and the differences between fresh, chilled and frozen semen.
Once the initial Breeding Soundness Examination has been performed the mare is assessed to establish whether she is in oestrus (‘season’) or in dioestrus (the 14 day period between seasons). It is possible to tease some mares at home if a stallion is at hand; indeed, some mares will show oestrus signs when presented to a gelding. In many cases however, it is not possible to detect oestrus on behavioural signs alone. For these mares, veterinary examination and ultrasound is the only reliable method of oestrus detection. Your vet will examine three structures to determine your mare’s stage of oestrus; the cervix, the uterus and the ovaries.
The Cervix
Many mares object to a vaginal examination and it is usually much safer for your vet (and the mare) if the exam is performed either in a set of stocks or under sedation.
Before examining the cervix, it is very important that strict attention is paid to cleanliness. The mare should have a tail bandage applied and the vulva and perineal region washed with clean water before being thoroughly dried. A disposable speculum is gently inserted into the vagina and the cervix visually assessed using a small light.
If the mare is not in season the cervix is pale, dry and lies high up in the vagina. If palpated it feels very toned and firm. If the mare is in season the cervix lies on the floor of the vagina and is soft, relaxed and may be partially open.
The Uterus
The uterus and ovaries are examined via a rectal exam. Again, many mares will need to be sedated for both the vet and the mare’s safety, unless a set of stocks is available. It is strictly illegal for anyone other than a qualified veterinary surgeon to perform a rectal examination on a mare.
The innermost layer of the uterus is called the endometrium. When the mare is in season the endometrium develops oedema, which is fluid within the tissue. This oedema has a characteristic pattern on ultrasound which resembles a ‘cartwheel’ or ‘cut orange.’ It may be graded on a scale of 0 to 4.
When the mare is not in season the uterus feels more toned when palpated and it appears smaller on ultrasound. A cross section scan through the uterine horn reveals a distinct circle that contains no oedema (Uterine Oedema Score = 0)
As the mare progresses through her season the uterine oedema increases. It may decrease markedly in the 12 hours just before she releases her egg (ovulation) and has usually disappeared by 2-3 days after ovulation.
At this stage the uterus is also examined for evidence of :
- Unexpected pregnancy
- Uterine fluid, which may indicate infection (endometritis)
- Uterine cysts – These do not usually affect fertility but may resemble a 14-17 day pregnancy and should be recorded at this stage to avoid confusion later on.
The Ovaries
When the mare is in dioestrus, there are waves of follicle growth on the ovaries. Each follicle contains one oocyte (‘egg’). At any time during dioestrus, ultrasound exam may reveal several follicles of varying sizes and at different stages of development. Follicles appear as black circles on ultrasound. There will also be a corpus luteum present on one or both ovaries; this forms from the follicle which released an egg/ovulated at the end of the previous oestrus period. The corpus luteum produces a hormone called progesterone that prevents the mare from coming back into season. About 14 days after ovulation, if the mare is not pregnant the uterus releases prostaglandin which destroys the corpus luteum and allows the mare to come back into oestrus.
When the mare comes into oestrus, one or two follicles become ‘dominant’ and grow much larger than the others. The dominant follicle grows at a rate of 3-5mm per day and ovulates once it reaches a size of 35-50 mm diameter. Some mares may ovulate smaller follicles than other mares. Also, earlier in the breeding season follicles tend to ovulate when they reach a larger size, whereas later in the year the same mare may ovulate a smaller follicle.
As the follicle approaches ovulation, it softens and one side may become flattened as the ultrasound probe is applied. Immediately before ovulation the follicle wall thickens and becomes ‘crinkly.’
From this thorough examination it is usually possible to establish whether the mare is in season or not. If she is in dioestrus, it may be possible to work out whether she has only recently come out of season or if she may be coming into season in the near future. However, it is not usually possibly to tell exactly when the mare is next due in season on the basis of a single examination. Repeat scans are often necessary, particularly in mares that due not show behavioural signs of oestrus and for which there is no information available about when she was last in season.
Short –Cycling with Prostaglandin
When a mare is in dioestrus, it is very common to administer an injection of Prostaglandin to bring her into season. This works by destroying the corpus luteum on her ovary (which is producing progesterone therefore preventing her from coming into oestrus). Most mares come into season 3-5 days after giving prostaglandin and ovulate 5-7 days after injection. However, it is not an exact science and the time to ovulation may range from 2 –10 days. It is crucial to note that a mare must be out of season for at least 5 days for the injection to work. When the mare does not come into season after injection, it is usually because the injection was unknowingly given within 5 days of ovulation. In this situation, a repeat injection 7 days later should bring her into season.
Procedure For A.I.
In many cases the mare is admitted to the A.I. Centre or Veterinary Hospital once she comes into oestrus and this has been confirmed by ultrasound. The uterus and ovaries are monitored daily or every other day for uterine oedema and follicle size respectively. Once a follicle of diameter 35 mm is detected in the presence of uterine oedema, the mare will be given an injection of an ovulating agent; either Chorulon or Ovuplant. An ovulating agent is a hormone that causes the follicle to ovulate within 36 – 42 hours; this enables your vet to accurately predict the time of ovulation and inseminate the mare at the optimum time to maximise the chances of conception. The exact regime depends on whether the semen is fresh, chilled or frozen.
Fresh Semen A.I.
The mare is inseminated 12 – 72 hours before ovulation, within 3 hours of semen collection. As a typical example:
- The mare is examined at 8am Monday morning and has a 38mm diameter follicle and uterine oedema
- The stud / stallion owner is contacted to ensure semen can be collected on Tuesday
- The mare is given an ovulating agent (Chorulon or Ovuplant) Monday morning
- The semen arrives on Tuesday and the mare is inseminated
- The mare is scanned Wednesday morning and should have ovulated
Chilled Semen A.I.
The mare is ideally inseminated about 12 hours before ovulation, approximately 24 hours after semen collection. As a typical example:
- The mare is examined at 8am Monday morning and has a 38mm diameter follicle and uterine oedema
- The stud / stallion owner is contacted asap and the semen is ordered to arrive the following day.
- The mare is given an ovulating agent Monday morning as soon as the semen order has been confirmed
- The semen arrives Tuesday and the mare is inseminated
- The mare is scanned Wednesday morning and should have ovulated
Frozen Semen A.I.
The mare must be inseminated within 6 hours of ovulation (see previous article). As a typical example:
- The mare is examined 8am Monday morning and has a 38mm follicle and uterine oedema
- An ovulating agent is given 6pm Monday evening
- The mare may be scanned Tuesday to ensure she has not ovulated prematurely
- Ovulation should occur between 6am and 12 noon Wednesday
- The mare is scanned every 2-6 hours from 8am Wednesday morning and is inseminated as soon as ovulation has been detected
- Some A.I. Centres may have a policy of ‘fixed time A.I.’ whereby the mare is inseminated at a set time (e.g. 36-42 hours) after the ovulating agent has been given. This reduces the number of ultrasound scans the mare is subjected to and is a good policy in very busy A.I. Centres. However, in the small number of mares which fail to ovulate within the expected time frame, it means a dose of expensive frozen semen has been wasted.
Post- Insemination Treatments
The mare is usually scanned 12-24 hours after insemination. In the case of fresh and chilled semen this is to confirm that ovulation has occurred as expected. In all mares, the uterus is carefully assessed for the presence of fluid.
When a mare is bred, either by A.I. or by natural cover, a mixture of live and dead sperm, seminal plasma and semen extender (in the case of A.I.) enter the uterus. This is all foreign material and causes a transient inflammatory reaction; a normal post-breeding endometritis. A normal fertile mare will clear this material by a combination of uterine contractions to expel dead sperm and fluid through the open cervix and increased lymphatic drainage through the uterine wall. Sub-fertile mares are not as effective as clearing the fluid from the uterus. If the fluid is not cleared by 5-6 days after ovulation when the embryo descends from the fallopian tubes into the uterus, the pregnancy will not be maintained. There are several treatments routinely carried out to aid fluid clearance and reduce the post-breeding endometritis
Uterine Lavage
This is the infusion of warmed sterile saline into the uterus to physically flush out dead sperm and fluid. The fluid is recovered by gravity flow and the lavage is repeated until the recovered fluid comes back clear instead of cloudy. It is routinely performed 12-24 hours after insemination, but sometimes as early as 4 hours in very susceptible mares. Within 4 hours of breeding the mare, the sperm are in the oviducts; safely away from the lavage fluid.
Oxytocin Injection
This is an injection which causes the uterus to contract. It is usually given in combination with uterine lavage to aid expulsion of fluid from the uterus. It may be given by intravenous or intramuscular injection every 6 hours.
Intra-Uterine Antibiotics
Oral or injectable antibiotics do not penetrate into the uterus very well to treat endometritis. However, directly instilling antibiotics into the uterus via the cervix is very effective at reducing bacterial contamination. This is sometimes performed following a uterine lavage.
Caslick Procedure
Mares that have poor perineal conformation and a backwards-sloping vulva are more likely to suck air and faecal material into the vagina. This leads to uterine infection (endometritis) which reduces fertility. A common procedure in veterinary stud practice is to perform a Caslick procedure under local anaesthetic. It involves removing a narrow strip of the skin/mucosal junction either side of the top part of the vulva and suturing the vulval lips together. The bottom of the vulva is left open to allow the mare to urinate normally. A temporary seal is created which helps prevent endometritis during pregnancy. It is crucial that the Caslick is removed before foaling, otherwise vulval tears may result.
