Preparing Mares for the Breeding Season

Scott Madill BVSc DVSc DACT
University of Minnesota

As a broodmare owner you would like a reasonable return on investment for yearlings you are planning to sell or a competitive individual for those you are keeping. While the greatest influence on the price you obtain for yearlings at the sales is the sire stud fee; other variables also play a role and important among these is the month the foal is born, with January and February foals having a distinct advantage. Similarly, since you are paying maintenance costs on the mare and economically she is depreciating annually, you would like her to produce a foal each year; however studies suggest that only 20% of mares do so while the average mare only foals five years out of every seven. Key to keeping mares foaling annually is having early foals (before April) and avoiding having her foaling date “drift” later every year. With the limited breeding season mares that drift later or mares that are subfertile and require breeding on multiple cycles have later and less valuable foals and eventually have to skip a season. For mares presenting early but with problems that reduce their fertility, each additional cycle she takes to conceive costs approximately $450 and that is without considering costs for infertility treatments. For these reasons it is vital that mares are presented to the stud early in the year and with their chances of conceiving optimized.

Nutrition and Body Condition

To ensure mares are cycling well at the start of the season they need to be in good body condition and have been exposed to artificial lights for about 60 days. Ideally mares enter the breeding season in moderate fleshy condition, at a body condition score of 5-6 on the usual 1 (emaciated) to 9 (obese) scale. These mares have better fertility and seem to respond better to the lighting schedules. Mares that are lighter (condition score 3-4) can also have good fertility provided they are gaining weight.

For maiden mares coming off the track there are a couple of options. If she has been off since early fall then she has had a good chance to let down and become accustomed to life away from the track and a change to a diet based more around forage, similar to what she will get at stud. She has also likely been mixed with older mares and adjusted to life in a social setting where she isn’t top of the heap. Maidens in this category will tend to do reasonably well at stud given the period they have had for adjustment. Those coming straight from the track where they have been used to hard feed and individual housing may lose considerable weight and not cycle well if sent straight out into the broodmare band at the start of the season. For optimal fertility in these “no let-down” mares it may be worthwhile paying to keep them stalled with turnout and a gradual feed change while they are being bred, completing their let-down during early pregnancy.

Lighting Schedules

Mares are naturally designed to breed in the late spring and summer. Several studies have shown that under natural light conditions the average mare has her first ovulation for the year between mid March and mid May, with an average for later April. In order to have her ready to go when the breeding shed opens in February she needs to go under artificial lighting, fooling her body into an early transition to cyclicity. This is generally achieved by giving her a total of 16 hours of light exposure (natural and artificial) per day for 60-70 days prior to the start of the breeding season. This means starting the lighting regimen December 1st and a lot of people like to use the day after Thanksgiving as their marker. The extra hours of artificial light seem to work best when added in the evening, so if it gets light at 7 AM then the lights are generally turned on before dark in the evening (at about 4:30 PM for the depths of a Minnesota winter) and left on till 11 PM. Automatic timers are simple to install and operate and remove the “oops” factor for busy or forgetful people. The rule of thumb for how bright the light needs to be is that you should be able to read newsprint anywhere the mare can get her head. This generally has been interpreted as a single 100-200 watt incandescent bulb or two 40 watt fluorescent tubes in a typical stall. If the mare likes to hang her head outside the stall then the aisle needs to be lit as well. Mares in paddocks can be lit by bringing them into a stall or an illuminated loafing barn for the evening, though particular care must be taken with the latter to ensure there are no dark corners or areas around hay feeders.

It should be remembered that while “the average mare” responds with ovulation in 60-70 days after starting lights this means that half the mares will take longer, and it can be 90 days before most mares in a group will have ovulated. Mares being bred as 2- and 3-year olds may take this extra month to start cycling as can those in their late teens and twenties. With these lighting regimens, in order to respond more predictably, the mare should not have been exposed to long days in the early fall (i.e. she should have had at least a month of ambient “short-days” before the increased light starts). This can be an issue for mares housed in busy barns where the lights come on early and stay on late fairly continuously, and such mares tend to cycle unpredictably. Teasing mares that are towards the end of the 2-month light treatment seems to improve response rates and there are some drugs such as sulpiride that can help augment the onset of cyclicity in mares that are slower to respond, though like lights themselves these are not a quick-fix and treatment periods of several weeks to a month can be needed. Depending on the stallion’s workload early in the season, open mares may also be placed on progesterone or progesterone and estradiol combinations for 10 days to help their movement through the last stages of transition and synchronize them for breeding.

Pregnant mares that are due to foal before mid April should also go under lights to ensure they cycle well after giving birth. Some caution should be exercised when lighting mares due to foal in early January as the 60 days of light exposure may shorten pregnancy by up to a week and we don’t want them to go before January 1st.

Reproductive Health

Even for maiden mares, most studs will require a negative uterine culture (swab) before she presents for breeding. You should check the wording of your contract for the timing and requirements for these, most need to be performed within 30-60 days before her 1st service. Swabs will occasionally grow contaminant organisms and the interpretation of “negative” often allows for a few of these, especially when no inflammatory cells are present (a negative cytology). Ideally the swab is taken when the mare is in heat but this is not always possible. If the swab is obtained when the mare is out of heat she is often short-cycled to bring her in afterwards to ensure she can deal with any introduced contaminating organisms (mares are more able to eliminate infections when in estrus). For maidens the time of swabbing is also a good time to check her reproductive tract to ensure there are no abnormalities and assess how well she is cycling before she goes to the breeding farm. This may include examination of her external conformation to determine if she needs a Caslick, palpation and ultrasound of her uterus and ovaries and a speculum exam of her vagina and cervix. Some contracts also require that maidens be “jumped” by a tease stallion (without actual breeding) before being presented to the farm to ensure she will stand reasonably well or give notice of potential behavioral issues.

Barren mares also need a negative swab in a similar time-frame before going to the breeding shed. Ideally these mares that failed to conceive or slipped a pregnancy were fully worked up to determine the cause of their infertility following the last breeding season. Leaving such work-ups till the start of the coming season only means wasted time needed for diagnosis and treatment when she could have already been bred. It also does her future fertility no favors to have let her maintain an infection in the off-season prior to treatment. Getting the treatments completed and the mare “clean” several months before breeding also allows residual inflammation in the uterus to settle down, returning her uterine environment to optimum before the season commences. Examination of these mares includes techniques used for maidens such as palpation and ultrasound, vaginoscopy and swabbing, but may also involve vaginal and cervical palpation, uterine lavage, biopsy and an endoscopic exam of the uterus. Older mares with signs of Cushing’s disease or equine metabolic syndrome should be tested and necessary treatments or dietary modifications commenced at least 2 months prior to breeding as it seems to take this long to return these mares to optimal cyclicity.

Health Testing and Health Maintenance

Mares moving out of state will need a health certificate and a current negative Equine Infectious Anemia (“Coggins”) test. Most in-state breeders also require the negative Coggins. Mares should be current on standard vaccines and these should have been given at least three weeks before she goes to the breeding farm so she has time to respond. Given the number of horses on many breeding operations and their frequent movements, some farms have requirements for respiratory vaccines, especially Equine Herpes Virus which can be associated with abortion. These are spelled out in your contract but generally require a vaccine to have been given between 10 and 90 days prior to the mare being covered. It is also good practice to have the mare up to date on dental care and hoof trimming before sending her off. Providing the farm with the mare’s past breeding history, especially as it relates to any reproductive problems or idiosyncrasies, is invaluable in getting her to conceive in a timely manner.

If your mare is going to a stallion that is a known to carry viral arteritis (“EVA shedder”) she will need vaccination against this disease at least 3 weeks prior to breeding, or have antibodies to the disease in a serum sample. She will also be isolated for 21 days following her first breeding.


Consistent, successful breeding of mares takes planning and attention to detail. While fate and unavoidable complications can derail the process; ensuring mares are presented to the stallion early in the season with their chances of conception optimized is the best way to produce early, high-value foals on a yearly basis.

Care of the Broodmare

Once your Broodmare has been covered and confirmed in foal, you are now caring for two horses in one package!  The University of Minnesota’s article below will provide you with great information about caring for the broodmare and the future racehorse she’s carrying!

 Care of the Broodmare