Dr. David Scofield
Select Breeders Services

Oxytocin is one of the most utilized hormones in broodmare practice. With so many possible clinical applications, a review of the use of oxytocin in the mare highlights the benefits of oxytocin, as well as necessary precautions with its use. Oxytocin is a nine-amino acid neuropeptide that is produced in the hypothalamus and released by hypothalamic neurons that terminate in the posterior pituitary. It is released in a natural pulsatile manner and exerts its effects by coupling with oxytocin receptors on various tissues such as the endometrium, myometrium, heart, kidney, pancreas, and fat tissue. There are also local effects of oxytocin and receptor binding, notably in the utero-placental tissues that help to increase the effect and intensity of pituitary derived oxytocin pulses. Clinically, oxytocin is available as a sterile injection, 20 IU (international units) per milliter. It can be administered intravenously or intramuscularly.
Non-Pregnant MaresUse in the non-pregnant mare is commonly based upon increasing uterine contractility to facilitate clearance of fluid. The correct dose of oxytocin causes a pulse of progressive uterine contractility that helps move free fluid out of the uterus. However, too high of a dose of oxytocin (> 30IU) will cause a tonic contraction of the uterus and prevent the evacuation of fluid. In our clinic, we use 20 IU (1ml) of oxytocin in the muscle every 4 hours. Oxytocin has a half-life of only 6 minutes. That means every 6 minutes, 50% of a dose is degraded and no longer active. With such a short half-life, more frequent administrations of a lower dose can have a better effect in uterine clearance.
Breeding MaresI routinely use oxytocin therapy in the 48 hours following breeding a mare to help with normal fluid accumulation following breeding and to help remove any remnants of uterine therapy such as infusions of antibiotics or uterine lavages. Mares produce fluid in their uterus as a normal response to breeding. If this fluid persists after 24 hours following breeding, she may be termed “susceptible to Post Mating Induced Endometritis (PMIE).” A common therapy utilized is intramuscular oxytocin administration to help fluid clearance. I will also administer oxytocin both intramuscularly (and possibly in the uterus in lavage fluids) to help a mare clear abnormal fluid accumulation.
Post Foaling MaresI also routinely use oxytocin in post foaling mares to help clear any lochia or normal post-foaling debris, during retained fetal membrane therapy and following lavages preparing mares for foal heat breeding. Small and frequent doses of oxytocin are a first line therapy to help mares that have not passed their placenta within three hours of foaling. The smooth muscle contractions induced by the oxytocin facilitate the passing of the retained fetal membranes as well as clearing bacterial and inflammatory debris that can accompany retained membranes. Oxytocin also has some therapeutic actions in the maternal bond and developing mothering ability and can be used to augment a mare’s behavior towards her new foal. It should be noted that a post foaling mare has a high up-regulation of the oxytocin receptor. This effect means a smaller dose of oxytocin has a much greater effect. In post-foaling mares, I only use 5-10 IU of oxytocin at a maximum.
One place I see oxytocin utilized incorrectly in the post foaling mare is when some owners try to increase milk production by administering oxytocin to a mare with low milk production. Oxytocin aids in milk let-down from the mammary tissue into the teat cistern, to be available when the foal suckles. This milk letdown process is coordinated from sights, smells and nuzzling of the foal near the udder. A mare will have a release of oxytocin from the posterior pituitary and allows milk to leave the mammary gland and enter the teat cistern. Administering a dose of oxytocin mimics this normal event and will cause milk letdown. Oxytocin increases letdown only, not the production of milk. The only method to increase actual milk production is to use a dopamine antagonist such as Domperidone or Sulpride.
Caution with the Pregnant MareNow, there is a major caveat. Oxytocin, can have a profound effect on a pregnant mare. If administered at the correct timing, it will induce delivery (or premature delivery) of a foal. If inadvertently administered, a late term mare will be induced into parturition, having devastating results to both the mare and fetus.
Some Additional UsesThere are a few other uses of oxytocin which involve medical conditions related to esophageal choke and estrus suppression (described in our article Suppression of Stallion and Mare Behavior).
The use of oxytocin is an extremely valuable tool for reproductive medicine. Its use is common but adequate knowledge about the physiology, half-life, and contra-indications are important to understand the benefits as well as the potential downfalls of its use in the mare.

Natalie DiBerardinis, Hilltop Farm
First published in Elite Equestrian, January 2015
 
It’s a wonderful time of year for anyone considering breeding  a mare . While stallion shopping is perhaps the more glamorous, exciting part of planning your next breeding, I think this winter research time can be even more effectively spent if we first start by defining our breeding goals and critically evaluating our mares.  We need to put equal research into exploring our mares’ backgrounds at much greater depth to have the best chances of breeding our next champion.
 
Why do you want to breed a foal?  Will this be a future performance horse for yourself, a future broodmare for your growing program, or a sales prospect?  Each of these is a great reason to breed, but the goal will help define if your mare is the right one for the job.
 
If you’re breeding a performance horse for yourself, start with your mare’s own qualities as a riding horse.  Was she suited for the sport you compete in?  Did you enjoy riding her?  Did she have enough talent that even if you ended up with an exact replica of her you would be happy?  If the answer to any of those questions is no, then perhaps she is not the right breeding candidate for your goal.
 
If you are a long-term breeder and would like to produce a new filly for your broodmare herd, the depth of your mare’s pedigree is critical.  Look at the stallions represented, but even more importantly, look at the mares throughout her background.  What have they produced and done in sport?  Is there a consistency and strong tie from one generation of mares to the next?  Experienced breeders know its these strong producing mare families that they can rely on for the development and continuation of their own programs.
 
If you are breeding for sale, consider the commercial appeal of your dam and her pedigree.  Does she have a fashionable or well-recognized pedigree?  Did she have a strong sport career or have other siblings/offspring competed well?  Breeding is an expensive endeavor and key to financial success is being able to sell the offspring you breed.  Stallion names may catch buyers eye, but you can differentiate your offspring from the other offspring of the chosen stallion by the appeal of your mare.
 
So, you’ve established your breeding goals and believe in general your mare is a good candidate.  Now, we look towards more specifics to create a blueprint of our mare’s strengths and areas that we may need to improve upon with our stallion selection.  Temperament, character, and rideability are an excellent place to start when evaluating your mare.  Break these into different categories as you evaluate your mare.  Temperament includes sensitivity and reactiveness.  Think of an energy scale here -- is this a cooler, slower reacting sort of horse or one who may be considered hot or reactive to aids, environmental changes, etc.  Character, to me, is the evaluation of how a horse interacts with its people, how consistent it is from day-to-day, how honest it is in its responses.  Rideability includes not only willingness to work, but also evaluates if the horse gives a rider a good feeling naturally through the body, if it is willing to listen and wait for the rider’s aids, and how quickly it learns.  Where does your mare rate in each of these areas?
 
Conformation is the next area to consider in your evaluation of your mare.  What are her strengths and where are her weaker areas?  With a potential weakness, evaluate how severe it is - did it impact her success under saddle, did it contribute to an injury? No horse is perfect, but animals with a significant conformational defect are not good breeding candidates.  There are a lot of wonderful books available discussing sport horse conformation traits as well as seminars run by organizations such as USDF, USEA, and some breed registries.  If your mare has an injury that is leading you towards a broodmare career, it would be a good idea to discuss with your vet if he/she has any concerns related to her conformation or way of going that could have contributed to that injury and how inheritable that trait may be.
 
Your next step is to evaluate the riding strengths of your mare.  What made her a good mover, gave her good technique over fences, etc.  What was easiest for her in work and what areas did she struggle with?   Your trainer’s input could be very valuable here.  Again, as we identify areas that we would like to improve in the resulting foal, we need to be aware that breeding isn’t an exact science.  I always cycle back to one question. “If I got an exact replica of my mare in the foal would I still be satisfied?’  If the answer to that question isn’t yes, I don’t feel she’s a  strong choice as a broodmare.
 
Now we get to pedigree evaluations.  This can take a lot of time, but can also be one of the most fun processes in research.  Find out everything you can about the stallions, and dams, in your mare’s pedigree.  Does she embody the characteristics those lines are known for?  What are the most successful horses from these lines and what bloodline crosses do you consistently see with this pedigree.  That can give you a guide a potential nicks to  follow in your breeding plans as well.

After all this evaluation, I’d suggest picking two or three things that you most want to improve on your mare.  Use those specific traits to guide your stallion selection.  Enjoy your winter research and evaluation of your mare.  Best of luck in your breeding decisions this coming season!

Julie Skaife
Select Breeders Services

The horse is a seasonal breeder meaning that natural mating occurs during certain times of the year to ensure that the timing of birth is optimal for survival with regard to ambient temperature, food and water availability, and even changes in the predation behaviors of other species. Alternative breeding behaviors used by animals include opportunistic breeders that mate whenever the conditions of their environment become favorable and continuous breeders that mate year-round. Seasonal breeders are controlled by the length of daylight, i.e. the photoperiod, and can be divided into long day breeders, e.g. the horse, that start cycling when the days get longer (spring) and short day breeders, e.g. deer, that start cycling when the days get shorter (fall). The length of photoperiod can be manipulated in order to hasten the onset of the breeding season. This is a popular management technique used in Thoroughbred breeding, so foals can be born as close to the standardized January 1st birth date as possible. It is also common with show or halter horses, with the goal being to maximize maturity for age determined competition.


Putting Your Mare Under Lights
Breeding season in the Northern hemisphere mare typically runs from late spring (April) through early fall (September). The mare, being seasonally polyestrous, has multiple heat cycles during the reproductive season. During the winter months (mid-Nov to mid-Feb) the mare is considered to be anestrous, demonstrating no behavioral signs of sexual receptivity and a failure to develop follicles that ovulate. Although it is estimated that 75-85% of mares are truly anestrous, it is possible that the remaining mares may show some estrous activity, albeit erratic and usually anovulatory. Either side of anestrous the mare is in a transitional period that is also characterized by inconsistent heat cycles with inconsistent ovulations. Year round estrous behavior can be seen in mares closer to the equator.

Females are typically more “seasonal” than their male counterparts. Thus, unlike mares that enter an anestrous period, stallions remain fertile year-round, usually showing only a mild decline in sexual behavior and sperm production out of season. Consequently manipulation of the photoperiod is predominantly targeted to the mare and is accomplished by “putting the mare under lights”, which is (almost) as simple as it sounds. Supplementing the length of “daylight” with artificial lighting is referred to as phototropic stimulation. The mare should begin to cycle 50 to 60 days after the beginning of a lighting program. So, beginning a lighting program on December, 1st will allow for normal estrus cycles and breeding during February. The results of this lighting program usually allow for a foal to be born in January of the following year.


Lighting Programs
There are a number of different lighting programs that can be used to accomplish the same goal. The first and most preferred method is to provide from 14-1/2 to 16 hours of continuous light each day. If the mare is turned out during the day she must be brought in and put under artificial lights before the natural light outside fades. If you want to coordinate the breeding cycles of a group of mares, this can be accomplished by lighting their shared paddock.
Lighting can be started abruptly on December 1st or can be increased gradually over a period of 60 days. For example, under the constant regime the lights are turned on from 4.30pm to 10 or 11pm for 10 weeks prior to the desired breeding date. With the stepwise regime 3 hours of light are added the first week and then another 1/2 hour of light each additional week until a 15-16 hour “day” is achieved.

Another lighting plan involves turning on the indoor lights, for a short period, starting nine hours after sunset, and continuing that program until the nine hours coincides with sunrise. However, since the time of the sunset is constantly changing, this program requires routine management to make sure you change the timing of the lighting to coincide with the changing sunset. Success has also been achieved with turning on the lights for as little as two hours when programmed to occur 9 hours after sunset. Whatever lighting program you decide to utilize, consider using an automatic timer to save both time and money.


How much light is required?
A critical factor is the amount of light that must be provided to simulate daylight. A minimum of 10 footcandles of light (107 lux) is required, either incandescent or fluorescent. A good rule of thumb is to provide enough light to comfortably read the small print in a newspaper seated anywhere in the stall. A 100 watt light bulb (or two 40 watt bulbs) provides enough light for the average sized stall.


Additional Considerations
The lighting program is usually discontinued once the mare is cycling and ovulating regularly. Research suggests that a period of short days is required prior to entering a photostimulation period, so there is no advantage to starting your lighting program earlier than November and a year round lighting plan may actually be disadvantageous. In this respect, if the lights go out and the mare is subjected once again to short days (even for as few as 3 days), this may reset the system, so care must be taken to ensure the lighting regime is maintained. Research also suggests that ambient temperature may play a secondary role in seasonality, so mares in colder climates that are exposed to lengthening daylight may start cycling later than mares similarly exposed in warmer climates. Remember, that not only does the lighting regime hasten the onset of the breeding season; it will also cause early shedding of the winter coat, so don’t forget to bear this in mind for those mares that spend all or part of their time outside and provide blanketing in colder climates if required.


Foaling Mares
Lighting programs also have an effect on your foaling mares. Research has shown that foaling mares, exposed to 16 hours of daylight during the last portion of their gestation, will foal on average 7-10 days prior to mares who are not exposed to lights. More importantly, if you plan on breeding the mare again following her foaling, a lighting program will helps prevent her from entering a post-foaling anestrous period that would coincide with a natural anestrous period. If foaling mares are not exposed to a lighting program, a proportion may not cycle until March or April when the natural breeding season begins.


How it Works
The hypothalamus of the brain is considered to be the central endocrine or hormonal control for reproductive activity. Hence, factors that determine when a seasonal breeder will be ready for mating affect this portion of the brain. In humans and other mammals, the light signals necessary to set circadian rhythms are sent from the eye through the retinohypothalamic system to the suprachiasmatic nuclei and superior cervical ganglion to the pineal gland. The pineal gland produces melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonality. The production of melatonin, also know as the “hormone of darkness”, by the pineal gland is stimulated by darkness and inhibited by light. Melatonin has an inhibitory effect on the production of gonadotropin releasing hormone (GnRH) by the hypothalamus in long day breeders. The decrease in melatonin production due to longer days in the spring, results in a reduction in the suppression of the hypothalamus and thereby an increase in GnRH pulses. GnRH subsequently promotes an increase in the release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are called gonadotropins because they stimulate the gonads - in males, the testes, and in females, the ovaries. In mares, these hormones, LH and FSH, control the cyclic nature of estrous periods by promoting follicular development and ovulation.


Hormonal Therapy
Lighting programs for mares are the most common method for advancing the breeding season. There is, however, extensive research for alternative methods that would not require 45-60 days of artificial light. Hormonal therapies using various forms of FSH, GnRH analogs, prolactin, or dopamine antagonists have been proposed and shown various degrees of success in stimulating a fertile estrus period during winter anestrus. Speak with your veterinarian or call your local SBS-affiliate for more information regarding these historical therapies and some potential new therapies that are promising.
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A lighting program is a cheap and effective way to manage your mare’s seasonality, but remember that it doesn’t shorten the transitional period of the mare’s reproductive cycle, it just advances it forward in the calendar, so be sure to start the lighting regime 50-60 days before the date you would like to get your mare bred. Also, a good practice in reproductive management is to perform a pre-breeding exam on the first heat cycle of the year. If your aim is for a January foaling date, then lighting should commence in December. The efficiency of any lighting regime is subject to a number of factors, e.g. lack of sufficient lighting, interruption of lighting, significantly low ambient temperatures etc., and therefore must be well managed to be successful and may benefit from combination with hormonal therapy as well.