By Roger Dixon BVM&S MRCVS Ashbrook Equine Hospital.
Artificial insemination (AI) is a well-established assisted-breeding technique. It involves a veterinary surgeon or qualified AI technician depositing semen into the uterus of the mare at the optimum time to allow fertilisation of her egg. It is widely used in show-jumpers, dressage, eventing and pleasure horses. However, AI is unable to be used in thoroughbreds because, by convention, the offspring are not eligible to be registered for racing. Natural cover therefore widely predominates in the thoroughbred industry.
Natural cover; systems and conception rates
There are several different management systems for natural cover. In the least intensive and least expensive system the stallion ‘runs’ with a number of mares in the field and covers them as they come into season. This mirrors the situation in the ‘wild’ and favours those mares of superior fertility. It may take several oestrus cycles to achieve conception; indeed, the average ‘per-cycle’ conception rate in this situation is a surprisingly low ~40%. A variation on this system involves visually monitoring the mares and ‘hand-covering’ them when they are receptive to the stallion; this at least ensures that mating occurs. At the other end of the spectrum are the intensively managed thoroughbred studs. The stallions may be in very demand with just one opportunity per cycle for mating; close veterinary management of the mares is required to ensure the mare is presented to the station at exactly the correct time to achieve conception. Although veterinary costs will be higher, a well-managed thoroughbred stud can expect to achieve per-cycle conception rates of up to 70%.
Why Choose AI?
For the mare owner considering breeding from a mare, the decision of whether to proceed with natural cover vs. A.I. will depend on several factors. These may include:
➢ The availability of the stallion:
– Many sports horse stallions will only be available by A.I. due to competition commitments or concerns about the stallion suffering injury during covering.
➢ The location of the stallion:
– A mare may go to a stud close to home and the semen can be brought in from elsewhere in the UK or abroad. This removes the need to subject the mare to long-distance travel.
➢ The age and breeding history of the mare:
– AI is particularly suitable for sub-fertile mares e.g. older mares or mares with poor vulval conformation who are prone to uterine infection (endometritis).
Advantages of AI:
➢ Mares may be bred to a stallion many miles away or abroad, without the need to transport them vast distances.
➢ AI allows quality control of the semen. Semen quality can vary significantly between ejaculates, even from a fertile stallion. The ejaculate should be examined prior to shipping/insemination and if it is noted to be sub-standard, the collection may be repeated.
➢ By using frozen semen, mares may be bred to stallions who are currently competing.
➢ Mares may be bred with frozen semen from a stallion who has since died or been castrated.
➢ It is considerably safer for the mare, stallion and the handlers.
Considerations of A.I.
➢ Veterinary costs are higher due to the increased number of ultrasound scans required to inseminate the mare at the optimum time.
➢ Shipment costs of semen, particularly if importing from abroad.
➢ There may be a temptation to ‘cut corners’ in screening for venereal transmissible diseases e.g. contagious equine metritis (CEM) and equine viral arteritis (EVA). Contrary to popular belief, these causal agents of these diseases survive in transported semen and may be inadvertently transmitted from stallions to mares. Reputable stallion owners and AI centres will adhere to the HBLB Codes of Practice, which advises that stallions and mares are tested to ensure they are free of venereal disease before breeding commences.
➢ Good communication between the mare owner, the stallion owner and the vet is essential. Regularly updating the stallion owner of the mare’s progress can aid planning to ensure a collection at the optimum time. It can be intensely frustrating and expensive if the mare is ready for insemination but no semen is available due to a communication breakdown.
➢ If ordering semen from abroad, many mare owners choose to order semen through an experienced semen importation agent. Importing semen can be logistically difficult; communicating with the stud, tracking couriers and ensuring the semen is accompanied by original Health Papers (a legal requirement) can be stressful! An agent will deal with these challenges and ensure that the process runs smoothly. The veterinary surgeon can then order semen directly through the agent at the appropriate time.
Choice of Semen; Fresh, Chilled or Frozen
A.I. may be performed using fresh, chilled or frozen semen. The choice is usually governed by the availability and location of the stallion. Traditionally, pregnancy rates have been highest with fresh semen, followed by chilled then frozen semen.
Fresh Semen A.I.
The semen is collected from the stallion into an artificial vagina before being immediately (within 1 hour) inseminated into the mare. Semen extender is added to the semen first; this contains nutrients for the sperm and helps to prolong their lifespan and improve their motility. The typical volume of fresh semen may be 10-60 ml, depending if the ejaculate is divided between several mares. The total number of good quality progressively motile sperm (sperm moving in a definite direction) per insemination dose should be at least 600 million.
Fresh semen A.I. is commonly used when the stallion is standing at the same stud as the mare or a short drive away. It is very useful for sub-fertile mares that have had problems conceiving in the past e.g. due to mating-induced endometritis (uterine inflammation). Fresh semen from a fertile stallion often survives in the reproductive tract of the mare for 72 hours. This is in contrast to the mare’s egg (ovum) which only survives for about 6 hours after ovulation (sometimes longer).
Within 4 hours of insemination, the sperm are transported from the uterus to the oviducts/fallopian tubes; this is where fertilisation takes place. The aim of fresh semen A.I. is to deposit the semen in the uterus about 48-12 hours before ovulation so that the sperm are waiting in the oviducts and are ready to fertilise the egg as soon as it has been released. The earlier the semen is inseminated, the more time your vet has to perform post-breeding treatments e.g.uterine lavage to clean up the uterus before the cervix closes 24-48 hours after ovulation. This is especially useful in mares prone to post-breeding endometritis, who pool large amounts of fluid in the uterus after insemination and who require intensive treatment if they are to get in foal.
Chilled Semen A.I.
The semen is collected at stud, extended and transported, usually overnight, to the mare in a specialised container. This method is suitable for stallions located throughout the British Isles, Eire and some Continental European countries, so the collection to insemination time is no more than 24-36 hours. As with fresh semen A.I., the aim is to deposit the semen into the uterus before ovulation so the sperm are waiting in the oviducts for the egg. However, the window for successful A.I. is narrower than when using fresh semen because by the time the semen is deposited in the mare, it is already 24 hours old. Insemination ideally takes place 6-12 hours before ovulation.
Frozen Semen A.I.
The semen is collected at stud and frozen in liquid nitrogen where it lasts indefinitely. This method is suitable for stallions located in Europe, the USA or Australia. Many competition stallions are only available by frozen semen, because semen may only be collected for a limited period between competitions.
Immediately before insemination the frozen semen is thawed in a water bath at 37 oC for 30 seconds. It is then inseminated into the mare. The freeze-thaw process damages many of the sperm and reduces their lifespan. There is a huge variation between stallions with regard to the fertility of their frozen-thawed semen. Some stallions are not available by frozen semen because even with modern freezing techniques, the motility of the sperm after thawing is not good enough to result in pregnancies. The fertility and freezability of a stallion’s semen may also vary from one ejaculate to the next, so it is essential that the semen is frozen at a reliable centre with excellent quality control measures.
The sperm usually survive for at least 6 hours after thawing. The mare’s egg, as mentioned, is assumed to survive for only 6 hours after ovulation. Therefore timing of insemination using frozen semen is absolutely critical; the semen must be deposited in the uterus within 6 hours of ovulation to maximise conception rates. Accurate prediction of ovulation is therefore of utmost importance with frozen semen A.I.
With improvements in ovulation prediction and insemination techniques, the average conception rates have dramatically increased from 40-50% to 70-80% over recent years.
Part 2 will focus on how we can use targeted treatments both before and after insemination to improve conception rates in sub-fertile mares.