Starting with the right mare
Starting with the right mare is one of the keys to successful breeding. At the start of the breeding season, mares should be in good body condition but not overweight, and should be up to date with their vaccinations, deworming, dentistry, and farrier work. Mare fertility decreases with age, and as mares reach their mid to late teens they may become less fertile than younger mares. Some conformational or medical conditions such as obesity, equine metabolic syndrome, laminitis, and pituitary pars intermedia dysfunction (PPID) can have an effect on fertility or a mare’s suitability for breeding. Mares should be examined by a veterinarian for breeding soundness before the breeding season. This involves checking the mare’s general condition and health, assessing conformation, as well as ultrasound examination of the reproductive tract.
Breeding the mare
Natural mating with fertile mares typically results in a conception rate of around 50-60% per cycle; therefore, it usually takes 1-2 cycles for a fertile mare to conceive. With natural mating, mares are sometimes mated every second day while they are heavily in season. Ultrasound scanning of the mare’s ovaries can help to optimise the timing of mating, therefore improving the conception rate. Artificial insemination (AI) is when semen previously collected from the stallion is introduced through the mare’s cervix into the uterus via a small pipette. AI holds several advantages over natural mating. The most important of these being increased safety for the mare and stallion. In addition, there is the added benefit of being able to ship the semen over large distances from the stallion to the mare. Due to the limited life span of semen, artificial insemination requires accurate timing in relation to ovulation. Mares that show visible signs of being in season are usually scanned as soon as they are seen to be in season. Mares that don’t show obvious signs of being in season are often administered a synchronising injection of prostaglandin (PG) by a veterinarian and are then scanned 2-3 days later. Mares which have foaled are usually scanned at 8-9 days post foaling or are administered PG 7 days after their foal heat then scanned 2-3 days later. Ultrasound scans are then carried out every 1-3 days until a large enough follicle and an “in season uterus” are reached. Usually a hormone injection is administered prior to, or at the time of, artificial insemination to further ensure and optimise the timing of ovulation.
Artificial insemination with freshly chilled semen typically has a similar or slightly higher conception rate than natural mating. When stored correctly, fresh chilled semen usually remains viable for 24-30 hours during transport, then a further 24-48 hours once inseminated. Frozen semen on the other hand has a poorer and more variable conception rate, which can range anywhere between 0 to 70% (average around 40% per cycle). Because of this it is often recommended that frozen semen only be used in younger mares or mares which have been proven to have good fertility. Frozen semen has a limited lifespan of 6-12 hours once inseminated and must be inseminated as close to ovulation as possible (within 6 hours). Because of this, mares will require much more intensive scanning leading up to ovulation (sometimes every 3-6 hours).
Following breeding or artificial insemination
Mares will sometimes have an ultrasound scan performed 6-24 hours following insemination, to ensure ovulation has occurred, and to check for inflammation or infection. If inflammation or infection is present this can be treated before the fertilised embryo enters the uterus from the fallopian tube at approximately 5-6 days after ovulation. If embryo transfer (ET) is being carried out, Embryo collection is done at 7-8 days following ovulation. The collected embryo(s) are then transferred directly to the recipient mare(s) or transported to a facility where the embryo can be frozen.
A minimum of three pregnancy scans should be carried out; these are timed to monitor the development of the pregnancy and to minimise the risk of a twin pregnancy being missed. The first pregnancy scan should be done at 15 days following ovulation, as this is the earliest time to reliably detect a pregnancy, and to diagnose and address twins (Figure 1) if present. If the mare is not pregnant at this time a follicle for the next “cycle” may be present. The next scans are usually done at approximately 25-28 days (Figure 2), and 45 days. Sometimes an additional scan is performed between the first and second scans if the mare previously had two large follicles and is at risk of having a twin pregnancy.