by Amanda Bradbery, Ph.D. Collaborator

Is My Mare Prepared for Breeding?

Is My Mare Prepared for Breeding?

Published: May 2020 | Updated: October 2022


  • Timing is everything: You must optimize your mare’s heat cycle.
  • Your breeding program should include considerations for your mare’s nutrition, supplements, and body condition.
  • Change is unavoidable, but we can prepare for it: Keep the mare’s gut microbiome stable with probiotic supplements during dietary shifts and stresses associated with breeding.

Whether you want your mare to foal as early in the year as possible or you simply want her to drop her foal after the last snow melts, breeding is all about careful timing. 

She must be cycling, her body must be in tiptop physical condition, and you must perfectly time either a live cover or an artificial insemination procedure with fertile semen. 

In the Thoroughbred racing world, these schedules are tighter than they are in most corners of the performance horse industry, though some breed shows and disciplines with futurities also favor a foal born early in the year. 

“Every Thoroughbred has its birthday on Jan. 1 in North America, or every Thoroughbred that’s registered with The Jockey Club,” says Dr. Keith Latson, an equine surgeon and co-founder of FullBucket Health. 

“So, if they’re born in June, they still turn 1 year old on Jan. 1. And that presents a distinct potential disadvantage. Say you want to run in the Kentucky Derby—well, at the time of the Kentucky Derby (which is typically the first Saturday in May), they’re not quite 3 years old if they were born in June, whereas a horse that was born in January is a full 3 years old.” 

That six-month difference can make all the difference in their ability to perform at that age and so precociously during the early parts of their career,” he adds. 

Veterinarians and horse owners in the Southern Hemisphere have the same time restrictions in breeding mares but in reverse seasons. 

Breeders not preparing for racing or futurities, on the other hand, have more time to accommodate perhaps the trickiest requirement for successful breeding: The mare must be cycling—something she doesn’t do year-round. 

Steering Mare Heat Cycles

Dr. Rob Franklin, an equine internal medicine specialist and co-founder of FullBucket, explains what happens to the mare’s cycle heading into fall each year: 

“The pineal gland inside the brain is triggered by light, so the length of the day, as it becomes shorter and shorter, that pineal gland receives less light signal, and it begins to secrete the hormone melatonin which changes the mare’s cycle.” 

“As the days get shorter, they come out of season and they quit going through their normal 21-day heat cycle. That’s important, because you can’t breed a mare when she’s not cycling, which is typically by the fall, when the days start getting shorter.”

On the other side of winter, as the days gradually lengthen, light stimulates the pineal gland causing less melatonin secretion which awakens the mare’s ovaries, kicking off one or two abnormal cycles that get things rolling again. 

Some mares are outliers, of course, and cycle year-round, such as mares that live at lower latitudes with longer days throughout the year. But for the most part, they don’t, which frustrates that late-winter breeding season timing. 

Veterinarians and breeding managers must use pharmacological or hormonal therapies or artificial light to coax them back into estrus by February, so they can be bred and foal 11 months later, early in the year—allowing  another late-winter/early spring breeding.

How Do You Prepare a Mare for Breeding?

Other important components of preparing the mare for breeding, no matter the time of year, include body condition and nutrition, vaccination, and deworming

Have your veterinarian out to assess your mare’s general health, including her body condition, which is a measurement of fat cover, to be sure she’s ready to breed and carry a foal. 

Your veterinarian or an equine nutritionist can guide you on your mare’s diet as you prepare for breeding. Also, talk to your vet about deworming and vaccinating pregnant mares. 

“The mare going to a breeding farm should be vaccinated against common communicable diseases and environmental diseases—just like a show horse,” says Franklin. “Breeding farms mix mares from different places, so it is a mixing ground for infections. We recommend deworming prior to sending and then every 2-3 months depending on the management at the farm.”

Supplements to Support Your Mare’s Microbiome

“Another way that we can prime our mares’ immune system is by feeding them prebiotics and probiotics,” adds Franklin. 

Studies show that providing prebiotics in combination with Saccharomyces boulardii equine probiotic supplements during the last trimester can result in a stronger immune response and richer antibody content in colostrum.  

This practice of feeding prebiotics and probiotics also favors the health of the barren or maiden mare entering a new environment. 

Mares being prepared for breeding experience stress from being handled and examined more frequently, being kept in the barn (so they can be under lights), and/or consuming a different diet. This stress can upend the ecosystem of microorganisms living in the hindgut—commonly referred to as the microbiome.

“One of the big problems we see in mares is colic,” says Franklin. 

These management changes are largely unavoidable, but by feeding FullBucket Equine Probiotic Pellets you can equip the mare’s hindgut with probiotics and prebiotics (live beneficial microorganisms and the foods that feed them) to stabilize her gut microbiome and prevent colic. 

This is crucial at a time where she needs to be ready for the breeding shed—again, often on a schedule. 

You value your mare, and you want her to be ready to conceive and carry that foal you so desire. Work with your veterinarian and use these tips to prepare her for the demands breeding and pregnancy will place on her body.

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