Tick Tock The Breeding Clock

When it comes to horse breeding timing can be everything. During the breeding season, horse breeders are not only frazzled and irritated by the lack of sleep and perpetual worry and anxiety but also because we have to keep track of many key milestones and numbers.

Why is this so complicated you ask?

Because the vast majority of sport horse breeders use artificial insemination, hours of the breeding season are spent calculating the timing of everything from ultrasound evaluation to the injections of hormones,  insemination, and pregnancy checks. There is also the timing of the FedEx pick up and delivery, the meeting of veterinarian and the regular activities of the barn, foaling mares, training, overall management.

Are we having fun yet??

Horse breeder determining the best time to give prostaglandin so that mares comes into heat early in the week so she doesn’t need to be inseminated on a weekend or a Monday or a Holliday.

It seems that from early April to the end of the breeding season (whenever that might be: August?, September?…) we are constantly waiting for days to go by, counting hours and making decisions based on best knowledge and experience. Everything revolves around the clock of nature.

Have you ever seen an armillary sphere? No well, check this one out on the right: Those were complex models of the geocentric universe before it was accepted that the sun was at the center of the universe.

Thank you, Galileo for simplifying things!

Those complex cycles within cycles had nothing on what we breeders keep in our heads (whiteboards, apps, agendas, napkins…..) during the breeding season.

There should be a mare-centric one – just sayin’.


So what ARE we spending all that energy on? Let’s look at some critical time measurements of breeding horses.


Here are some numbers we keep in our heads:

Keep in mind this if for EACH mare. And no they don’t all synchronize… funny that.

Mare cycles: ovulation-to-ovulation: 21 days +/- 3 days. Can vary from mare to mare and from cycle to cycle and can be shorter at the solstice (no I am not kidding!).

Given at the optimum time (mature CL that is 5-ish days old) prostaglandin will trigger estrus within 3-5 days.

Cervix is open for 4-5 days

The peak of edema prior to ovulation: 36 to 24 hours

Timing for ovulation drugs (deslorelin): when given on a 35-mm+ follicle with uterine edema ovulation occurs 36-40 hours later.

Ovulation and evacuation of the ovum occurs in 5-15 minutes

Longevity of the ovum: 10-18 hours

Longevity of sperm: fresh (3-7 days), cooled and extended (3-5 days), frozen/thawed (12-48 hours).

Request for the stallion to be collected for fresh cooled semen: 24 hours is pretty standard. Most stallion owners need to get things lined up to make it happen although some are more accommodating if they collect at home.

Shipping time for fresh cooled: generally overnight but delays are common.

Oh and for most of us it is next to impossible to get a shipment on Sunday or Monday-that is usually when mares choose to ovulate mind you. And also at the peak of breeding season: May to August for us in Alberta: there are 3 blackout dates because of National Holidays, a few more if you are getting semen from the states. (I removed the calendars from the barn aisle as it was clear that the mares were figuring this out and timing their cycles to land on those days.)

Time to thaw frozen: generally 30 seconds, should be in the mare within 15 minutes.

How long for all the sperm to travel to the in the oviducts: 4 hours (so you can give oxytocin after that)

CL formation and detection: 6-12 hours

Effect of oxytocin: 3-4 hours

Pregnancy detection by ultrasound: soonest 11 post-ovulation days, most common: 14 days.

The ideal time to detect and pinch twins: 14 to 16 days post ovulation.

Gestation length in horses: 315 days to….?, well I think the longest was 425 days. (Horses have NO due date, only a “best after date”)

Foals should be up within 1 hour or 2.

Foals should get colostrum within a few hours of birth and after 24 hours of life, their gut is closed to new antibodies.

Foals should poop out their meconium within 2-6 hours or there is a risk of impaction.

Mare has to evacuate her placenta within 3-6 hours or risk complications.

Post foaling mare will be back in estrus 10-15 days later.

Did I forget anything.. probably!

Let me know! 😉

Owner and worrier in chief at Formosus Sporthorses in Alberta she's been breeding horses for over 18 years. Warmblood, warmblood crosses, and a few saddlebreds. She loves handling mare and foals and is passionate about giving foals the best start possible.

1 Comment

  1. […] 112 days of thinking about breeding, wondering and pondering, checking mares, ordering semen, and counting days. […]


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