Equine Breeding


The universal birthday of Jan 1st for horses, has resulted in a breeding season that is different than the mare's natural breeding season. Horse owners desire foals to be born close to Jan 1st so they are bigger within their age group when young. This creates breeding problems by forcing breeding into the non-physiologic breeding season.




Detection of heat is essential to breed efficiently and teasing is helpful to detect heat. You must use a stallion to tease efficiently. Shetland studs work well because they are small and typically sexually aggressive. The stallion will give a typical flehmen response when exposed to an estrual mare. Individual teasing is done one mare at a time and is very time consuming, but is the best way to accurately detect estrus. Shy mares may not show estrus very well in some types of teasing systems. Without stallions available we often resort to multiple ultrasound exams of the mare's ovaries or synchronization with hormones.


Natural Service


Natural service is required by some breed associations (such as the Jockey Club) Breeding should be done ideally at ovulation, but this requires frequent palpation during estrus. A common practice is to start breeding on the second day of heat and breed every other day. Natural breeding is more dangerous to all involved, including the stallion, mare and handlers. Natural breeding also allows contamination of the mare and stallion and spread of venereal disease.


Artificial Insemination (AI)


With AI there is less chance of venereal disease spread, you can spread out the stallion to more mares, you can collect semen using a dummy mount, which won't kick the stallion, you can evaluate the semen for each breeding. You can use a Minimum Contamination Technique (MCT), which decreases the introduction of contaminants into the mare. With this technique you put antibiotics in the semen extender, or if breeding Thoroughbreds, you place extender into the uterus of the mare that is being bred naturally.




Tease the mares into heat, or use hormone therapy. Palpate and/or ultrasound for a 35 mm follicle. Collect semen from the stallion and inseminate a minimum of 500 million normal, motile cells into the uterus. The volume of the insemination is not important ( at least if it less than about 100 ml), only that there are enough motile sperm cells. You can use fresh or extended semen. Up to 20 inseminations/ ejaculate are possible from some stallions.


Chilled semen


Many breeds allow the use of chilled, extended, transported semen. To inseminate mares using chilled semen they are usually palpated/ultrasounded until a 30-35 mm follicle is present, the semen is ordered and hCG is administered after the semen is in the mare. Inseminate a minimum of 500 million normal motile cells. Expect 10% lower fertility with chilled semen


Frozen semen


Frozen semen requires insemination more closely timed to ovulation because of the reduced longevity of the sperm. Mares are typically palpated every 6 hours and bred as soon as ovulation is detected. Breeding is done after ovulation using frozen semen because the sperm cells undergo capacitation during the freeze/thaw process. This limits their longevity within the mare. If semen supplies are plentiful, bracketing ovulation, i.e. breeding before anticipated ovulation and again after, may be practiced. For example, give hCG when a follicle > 35 mm is present, AI 24 and 40 hr later. Expect about a 20% reduction in fertility with frozen semen


Mare Management




Mares reach puberty at 12-18 months of age. A mare is maiden until she is bred for the first time. Abortion rates are high in mares under 3 years of age. If the mares are less than 5 years old, you can expect about a 75 % foaling rate.


Foaling mares


This is the group with the highest fertility, because they have proven themselves by having a foal at their side. Foal heat is generally around at 9 days postpartum, but may be as early as 5-6 or as late as 12. Foal heat breeding is good in that you can easily detect the estrus, whereas subsequent heats may be harder to detect. Foal heat breeding also keeps the mare foaling earlier in the year, if that is desired. The pregnancy rates on foal heat breeding are dependent on when a mare ovulates postpartum. This is because the mares that ovulate early do not have uterine involution that is as complete, so conception rate is lower.


If foal heat ovulation is on day 5-6 postpartum the conception rate is 0%, if it is on day 7-8 postpartum it is about 32%, if it is on days 9-12 postpartum conception rate is about 60%, and if it is greater than 12 days postpartum conception rates run greater than 75%. Conception rates are definitely lower if fluid is observed in uterus on ultrasound examination. Do not breed if fluid is observed.


Barren mares


Barren mares are those mares that were open the previous year. Whether or not the mares were bred is an important question. A mare that foaled late and was held back and simply not bred, is not as big a problem as a mare that was bred multiple times and did not get pregnant. Barren mares tend to be harder to get pregnant. Mares are funny in that the more they are pregnant the easier they are to get pregnant. If they are not pregnant for a year or two, they become more difficult to get pregnant.

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