8 Steps for Breeding Your Mare

Take a systematic approach to managing your broodmare to have the best chance at a successful pregnancy

Sperm, meet ovum. If breeding horses were as simple as that, we wouldn’t have an entire branch of the veterinary profession devoted to equine reproduction. Even in-heat mares and virile stallions don’t always a foal make. And if your broodmare herd includes a collection of maiden, older, and/or subfertile mares, you have even more to think about and keep track of to ensure a successful breeding season. Taking a systematic, step-by-step approach to managing each mare, in partnership with your veterinary team, can help cultivate success next spring.

Step 1: Consider the mare’s overall health

Margo Macpherson, DVM, MS, Dipl. ACT, a professor of Large Animal Clinical Sciences at the University of Florida’s College of Veterinary Medicine, in Gainesville, says there’s no one recipe for broodmare management; veterinarians must assess each mare individually.

Before focusing on a mare’s reproductive health, owners must first note her overall wellness. Does she appear healthy? Are her hooves in good shape? What vaccinations are due? Does she need a fecal egg count to check for parasites? Is she carrying too much or too little weight? Does she need a dental exam? 

Karen Wolfsdorf, DVM, Dipl. ACT, a field veterinarian and reproductive specialist at Hagyard Equine Medical Institute’s McGee Fertility Center, in Lexington, Kentucky, says one of the most important initial observations she makes is the mare’s Henneke body condition score (BCS). Ideally, broodmares should score between a 5 and 7 on the 1-to-10 scale prior to breeding.

“I like to be able to feel ribs but not see ribs,” says Wolfsdorf.

She says she also looks for protruding fat pads, a cresty neck, or other abnormalities that might suggest the mare has pituitary pars intermedia dysfunction (PPID, or equine Cushing’s disease), insulin dysfunction, or some other systemic illness that could affect the reproductive system.

She also takes into account the mare’s breeding history, including issues getting pregnant, endometritis (inflammation of the uterine lining), or abortion.

If the breeding will be via live cover, it’s important to know the chosen breeding shed’s stipulations. For example, Wolfsdorf says some breeding sheds require certain vaccinations, such as for rhinopneumonitis (herpesvirus-1), which veterinarians usually administer three weeks to three months prior to breeding to prevent the stallion from potentially being exposed to the virus. Wolfsdorf says owners should know the mare’s vaccination history and keep her current on her vaccines against diseases in that region, including during pregnancy.

To reduce the risk of spontaneous abortion, the American Association of Equine Practitioners recommends vaccinating mares against herpesvirus-1 at five, seven, and nine months of gestation. On some high-traffic farms, veterinarians might administer the herpesvirus vaccine every two months, but this is off-label use.

8 Steps to Breeding Your Mare

Step 2: Schedule a breeding soundness examination and address any problems

Getting a mare pregnant is a team effort between the broodmare owner and the veterinarian, says Wolfsdorf. Veterinarians typically perform a breeding soundness exam—a reproductive exam for broodmares—to identify and manage possible problems before breeding.

When discussing these exams, Macpherson says she’s mindful of a client’s budget and lists all the options to equip him or her to make an educated decision on which evaluations to perform. Often, a mare might only need a breeding soundness exam if she does not conceive after the first two or three attempts.

“The breeding soundness evaluation is far more important for the mare with difficulty in getting pregnant or the aged or middle-aged maiden mare,” says Macpherson. “I really think that a breeding soundness evaluation is meant to problem-solve or to provide prognostic information, so that the owner can make informed decisions.”

For most broodmares that have not had any breeding-related issues, Wolfsdorf starts by examining the mare’s reproductive anatomy, including perineal conformation—which involves the vulvar lips, the vestibular-vaginal fold and/or the hymen, and the cervix. If a mare has poor perineal conformation, she is at risk for reproductive tract contamination with feces, air, and microbes. To prevent this and possible resulting infection, veterinarians can suture the vulvar lips together (a procedure called a Caslicks), later removing the stitches for breeding and foaling.

An exam will also include transrectal palpation and an ultrasound examination. These tools help determine the stage of the mare’s estrous cycle. They can also allow the vet to verify the size and functionality of her ovaries and identify potential abnormalities within the uterus and vagina, such as excessive fluid, which can be a sign of inflammation or poor uterine clearance.

Macpherson says she performs an ultrasound exam every time she evaluates a mare prior to breeding. “I think the tools of palpation are extraordinarily important, but there’s so much that we can see with an ultrasound that makes an impact on breeding management,” she says.

Palpation and ultrasound examination results often determine the next steps in abnormal cases, says Wolfsdorf. “What we see in her uterus will dictate what type of culture and cytology we may want to do,” to confirm and identify pathogens, she says. 

She says she most commonly performs this in mares that have a “baggy, saggy” uterus, such as older broodmares, those that have had multiple foals, or ones with a history of endometritis, to look for inflammation and infection. These mares might also be more prone to breeding-­induced endometritis because of impaired uterine clearance.

An endometrial biopsy adds another piece of information to the evaluation. With this procedure the veterinarian evaluates a piece of the endometrium microscopically to look for abnormalities, such as inflammation, scar tissue around glands and vessels, and dilated lymphatics. Evaluating endometrial tissue can help the vet predict the probability of a mare becoming pregnant and maintaining the pregnancy to term.

If a veterinarian needs further information on a mare, he or she can perform a hysterocopic exam, which involves inserting an endoscope into the uterus to look for abnormalities such as foreign bodies, adhesions, or fungal/bacterial plaques.

The information gathered during the breeding soundness exam can guide your veterinarian to recommend certain management techniques or treatments before breeding.

Step 3: Get the mare cycling.

Now that your veterinarian has diagnosed and treated any possible reproductive issues, you must make sure your mare is cycling normally before breeding. Mares are seasonally polyestrous, meaning they cycle and come into heat during periods of long day length, such as during the spring and summer. However, some mares cycle year-round.

Many breeders, especially Thoroughbred breeders, try to get their mares pregnant in late winter or in spring so they foal early in the year. Therefore, you might need to get the mare cycling in winter, when she’d normally be in anestrus (a time of no estrus). Breeders can stimulate mares to cycle using methods such as artificial lights and/or hormone therapy. (For more on getting a mare to cycle, see TheHorse.com/166159.)

As nights become shorter and daylight increases (or as artificial lighting or hormone therapy takes effect), says Wolfsdorf, melatonin levels in the body lower, prompting gonadotropin-releasing hormone (GnRH) secretion.

“GnRH will then go to the anterior pituitary gland, which functions to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH),” she explains. “And those influence the ovaries, stimulating follicular development.”

The mare builds and regresses follicles until a dominant follicle forms that produces enough estrogen to cause a strong first estrus, also known as heat.

“Then luteinizing hormone induces ovulation (in the dominant follicle),” Wolfsdorf says. “The mare goes from anestrus, where she’s not cycling, through a transition period that usually lasts about 60 days prior to (this) first ovulation.”

8 Steps for Breeding Your Mare

Step 4: Track the mare’s estrous cycle to know when she’s ovulating

A mare’s estrous cycle lasts 21 days, but she will only come into heat and be receptive to breeding for five to seven days at the beginning of the cycle. Ovulation occurs during the last 24 to 36 hours of behavioral estrus. 

“Breeding as close to ovulation should always be the goal,” says Macpherson.

Knowing when a mare is in heat is important to determine when the veterinarian should start examining the mare for follicles, which can help determine ovulation and time of breeding, whether via natural cover or artificial insemination (AI).

Macpherson uses a teasing stallion to identify signs of heat in the mare, such as interest in the stallion (especially with head-to-head contact), vulvar “winking,” squatting into a breeding position (receptivity), tail-raising, and frequent urination. Signs a mare is unreceptive to a stallion include pinned ears, tail-clamping, aggression and striking toward the stallion, and disinterest. For more on teasing, visit TheHorse.com/14717.

Step 5: Determine when to breed the mare

Once a mare has started cycling, the owner must decide whether to breed her on the first heat—the less common approach, says Wolfsdorf—or wait for subsequent heats, which have higher pregnancy rates. If you’re breeding for a potential Thoroughbred racehorse, however, you want that foal to be born as close to Jan. 1—the breed’s official birth date—as possible.

Breeding method—live cover or AI—also dictates breeding timing. Breeding via AI is partially based on whether you’re using fresh, cooled, or frozen semen. Fresh semen has the highest fertility rates because it’s not processed, but it can’t be transported and must be used immediately. Cooled semen viability is stallion-dependent but is usually good for 36 to 48 hours from collection to insemination. With these methods the veterinarian should prepare the mare to ovulate within 24 to 40 hours of insemination (see step 6). Frozen semen can be even trickier, because insemination must occur within six hours pre- or post-ovulation and, therefore, produces the lowest success rates. 

“With cooled semen, you need to determine what days the stallions are collected and shipped and whether they’re shipped counter to counter (by commercial airplane) or by FedEx overnight,” says Wolfsdorf. “This will help you … time your insemination using ovulatory-inducing agents for when you get the semen.”

Step 6: Use veterinary technology to time breeding with ovulation

One way to get a mare pregnant sooner and/or better track her cycle is to have a veterinarian “short-cycle” her off the first heat post-foaling (aka foal heat), if you’re breeding her back. The veterinarian will do this by administering exogenous (not derived from the mare) prostaglandins to bring her back into heat sooner. This shortens the typical 14- to 17-day diestrus (between estrus) period. When the mare comes back into heat depends on follicle size at the time she receives prostaglandin (usually six days post-ovulation).

“Let’s say she has a 25-millimeter follicle on her ovary when you give her prostaglandin; it’s probably going to take four or five days for her to come into heat and another three to four for her to ovulate,” says Wolfsdorf. “However, if she has a big 35-millimeter follicle on her ovary when you give her prostaglandin, if you get rid of the corpus luteum (CL, the progesterone-producing structure formed after the follicle releases the egg), that mare comes into heat in two to three days and ovulates in (another) two to three days.”

An ovulatory-inducing agent, such as a GnRH analogue, can also help mare owners predict when to breed. “To me, using an ovulatory-inducing agent is just an insurance policy to make sure that they are, in fact, going to ovulate when you want them to,” says Wolfsdorf.

Macpherson then recommends examining a mare daily once she’s in estrus. “I’ve learned that I do a much better job if I get to look at the mare a lot,” she says. “A mare who is not in heat versus a mare that is in heat or a mare that is in early heat versus a mare that’s in her standing heat (when she’s showing full behavioral signs of estrus) can be very different,” she says. “With a frozen semen mare, we’re sometimes looking at her three times a day, and we learn a lot every time we look at her.”

Once you’ve pinpointed a time to breed, have your veterinarian oversee the live cover or artificial insemination.

Step 7: Encourage uterine clearance, especially in problem mares

Every time a mare is bred, her body has a natural inflammatory response, usually within the first 12 hours after breeding, to rid the uterus of dead sperm, inflammatory cells, debris, and contaminants. “Under natural breeding conditions, mares are bred many, many times, and her reproductive tract has the ability to clear and clean up after those multiple breedings,” says Macpherson.

Breeding in a man-made situation can affect the clearing process, resulting in persistent post-breeding-induced ­endometritis.

“If they have fluid in their uterus, I see it as an indication that there’s something physiologically impaired,” says Macpherson, such as an issue with uterine contractility, uterine drainage, or cervical relaxation.

Some mares, especially those that have had several foals and a stretched uterus, might have difficulty with this natural clearing process because gravity is not on their side. A young mare’s uterus is above the pelvis and can clear contaminants better through the cervix, vagina, and vulva. In a mare that has had multiple foals, the uterus can hang below the pelvic level, impairing uterine clearance as the fluid has to move “uphill.”

To help clear the uterus, Wolfsdorf recommends broodmare managers turn recently bred mares out so they can move around.

Oxytocin administration, uterine lavage, and acupuncture can also help the uterus contract. In addition, medications such as misoprostol or Buscopan can encourage the cervix to relax, which can help older maiden mares with tight cervices that prevent drainage.         

Step 8: Check for pregnancy

Once your mare has been bred, the final crucial step is to ensure she is pregnant. Your veterinarian can confirm pregnancy via ultrasound 14 to 15 days post-ovulation. If the mare is not pregnant, your veterinarian can explore possible causes and address them before trying to breed her again.

For more information on how to manage a pregnant mare, talk with your veterinarian and visit TheHorse.com/topics/breeding-and-reproduction/mare-care-and-problems/mare-care/.