Breeding basics

Selecting a broodmare

For the professional horse owner selecting a broodmare is a decision mainly based on performance, offspring, pedigree and confirmation. 

If you’re considering breeding a pleasure horse or a nice hack you might want to consider the following:

  • Suitability for breeding: temperament is highly heritable, you might reconsider breeding from a mare because she is difficult to ride.
  • As in other species unfortunately mares are less fertile when they get older. A mare that is too old to ride is not the perfect candidate to breed from. Please contact the clinic to discuss the possibilities for breeding from older mares.
  • Available facilities: in order to perform safe reproductive exams a crush is needed to confine the mare. With multiple crushes in our clinic we can offer you a full service if you don’t have the facilities on your farm. Alternatively we can give you advise on building one yourself.
  • Think ahead: where is the mare going to foal? Have you got a stable available if the foal needs extra care?
  • Can you afford breeding a foal? Other than the service fee there are some extra costs to cover when breeding a foal. Veterinary management costs of the mare can vary, feel free to let us know if you’re on a budget so we can discuss the possibilities. And keep in mind that not all foals are born healthy.


Reproductive physiology

The mare is a seasonal breeder with an anestrus period during the winter (when she doesn’t cycle). After and before winter anestrus the mare goes through a transitional period before she has regular cycles. Various factors can influence the mares reproductive cycle and especially the transitional period. Environment, condition, light and temperature are the most important.

The reproductive cycle of a mare is approximately 21 days, of which she will be in estrus (season) for 5-7 days. Ovulation happens at the end of the estrus, and we aim to breed the mare (live cover or AI) close to ovulation.
After foaling a mare comes back into heat after 7-12 days. Depending on the mare and the stallion you’re using she can be bred on her foal heat or shortly thereafter.

In order to determine at which point the mares is in her cycle, monitor follicular growth, and predict the timing of ovulation, repetitive ultrasound scans of both ovaries and uterus will be performed. If needed, we can control the estrus cycle with various hormones to optimize your breeding program.


As mentioned previously the transitional period can be influenced by multiple variables. The main factor that “switches mares on” in the spring is daylight. We aim to breed our Thoroughbred mares as close to the first of September as possible, given the climate in the Waikato not all mares have been exposed to enough daylight in this period of the year. Unwanted long transitional periods can result in:

  • No estrus behavior
  • Inconsistent or confusing signs of estrus behavior
  • Estrus behavior but no dominant follicle and no ovulation and regressing follicles
  • Unexpected long periods of estrus.

This transitional period can be influenced by putting the mares under artificial light, either paddock/stable lights or light masks starting at the first of July or hormonal treatments.

Live cover, chilled AI or frozen AI

Various techniques are available to breed a mare. The Thoroughbred studbook only allows live covering. For all other types of horses the mare can be bred by either live cover or AI. Not all stallions will live cover mares, obtain information from the stud or stallion owner about the possibilities.

When a mare gets live covered, healthy semen can last up to 4 days, therefore the mare doesn’t have to be bred very close to ovulation. Availability of the stallion can be an issue, which means early bookings can be essential.

Chilled AI is commonly used when live covering is not possible for various reasons. There is a wide variety of the life span of chilled and extended semen, as a guide line we use 48 hours as a maximum, but often the stallion owner will give us a recommendation when the mare has to be bred again if she hasn’t ovulated.

Frozen AI is used when chilled semen is not available, e.g. the stallion is not in New Zealand, dead or not available at the time. AI with frozen semen has to be timed well since the semen has a shorter life span and AI on ovulation is recommended. Ovulation inducing hormones will be used and multiple ultrasound examinations will be performed, therefore we keep mares for frozen AI in our clinic around the day of ovulation. Please inform us at the beginning of the season if you wish or might need to use frozen AI so we can optimize your mares management for the best result.


Embryo transfer

Embryo transfer is one of many assisted reproductive techniques and is not allowed by the  Thoroughbred studbook. 

When performing embryo transfer the embryo will be flushed out of the mare and placed in a recipient mare. Using this technique a mare can produce multiple foals a year or offspring of the mare can be produced while the mare is still competing. It can also be a solution for mares that are not able to carry a foal up to full term themselves.


Diagnosis and monitoring of pregnancy

The embryonic vesicle can first be seen on ultrasound on day 10. For accurate pregnancy and possible twin diagnosis we advise to scan the mare 14-16 days after ovulation. If needed a twin reduction can be performed during this period with minimal risk.
A second scan between day 25-30 gives the opportunity to asses fetal viability and development, if a non-viable fetus is found interference is possible with good chances of the mare starting to cycle again as she hasn´t formed endometrium cups yet. A final scan after day 45 is often performed to confirm pregnancy. This scan is also often required for fetal insurance. 

In certain cases, e.g. mares that slipped foals in previous years, monitoring of the placenta and fetus is desired so illness can be detected at an early stage and possible therapy can be started to prevent loss of the pregnancy. Please let us know if you´re mare has a bad or troublesome history.

The average gestational length of the mare is 340 days but a wide variance of 320-360 days exists. Therefore critical assessment of the foal is necessary to diagnose if a foal is born full term or is premature.


Mare management during pregnancy

High quality foot care, dentistry and nutrition are all very important for your broodmare. Around 60% of the total growth of the foal occurs in month 9-11 of the pregnancy, so changing to a special broodmare mix is advised in this period of the pregnancy.

Depending on the density of horses on the farm and the farm history broodmares can be vaccinated against various diseases. When mares get vaccinated 8-6 weeks prior to foaling the mare will pass antibodies on to the foal via the colostrum. This way the foal will be protected for the first 3 months of its life. Routine vaccination of the foal can be started when the foal is 3 months old.

Tetanus: all broodmares 6 weeks prior to foaling

Strangles: all broodmares 6 weeks prior to foaling

Salmonella: broodmares on high risk farms 6 weeks prior to foaling

We also advise that all mares are wormed 2 weeks prior to foaling.

About Us

Cambridge Equine Hospital is the largest equine practice in the country, with fifteen veterinarians solely dedicated to equine work. Situated in Cambridge, we have been providing top quality equine veterinary services to clients in the Waikato area for over 30 years. We cater for all types of horses, from ponies through to elite performance and breeding horses.



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