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Many stallion sheds will open their doors in mid February and the current breeding season will be off and running. Although you may think there is plenty of time to get your mare in foal, many stallion managers close their sheds in July only 135 (from February 15th to June 30th). If you’re breeding a sport horse mare the stallion sheds may stay open until July or August. If you are one of the breeders who wants an early foal and you haven’t put your mare under lights you are a little late and you need to get into action now. If you are a breeder who wants to wait for the mare’s natural breeding season, spring and summer, it’s still not too early to start. If your mare had difficulty settling last year, one factor is clear: the earlier you get started, the more cycles you have to achieve a successful breeding season this year.

Preparing your mare for the breeding season involves having her healthy – both in terms of general conditioning and reproductively, completely vaccinated, and cycling before she is to be bred. Because under the best circumstances horses are not the most fertile animals (conception rates for the species are in the range of 60%), optimal health is essential to achieve the maximum reproductive efficiency.

General Health
It is best to have your mare in good physical condition before breeding time. You don’t want a mare requiring a weight loss or gain diet when you’re trying to breed her. Neither scenarios are conducive to regular cycling and conception and add unwanted and avoidable complicating factors. Optimal health is achieved by good body condition which in turn is directly affected by high quality feed, healthy teeth, and low parasite load. Have your horse’s teeth examined and floated before you take her to be bred. Good dental health and proper deworming every 6 to 8 weeks are the best ways to ensure the optimum feed efficiency (amount of nutrition absorbed per pound of feed consumed).

Vaccination requirements for your area should be met before you breed. Although vaccine reactions are rare, the remote risk of such a reaction to interfere with breeding is eliminated if vaccinations occur before the season. A comment should be said about West Nile Virus vaccine. Rumors about this vaccine’s supposedly detrimental effects on reproduction are rampant. None of them, that I am aware, are corroborated by veterinarians involved in the breeding business. Reproductive vets want only one thing, pregnant mares. If any vaccine was thought to have a detrimental effect on this outcome we would not use it; more than 95% of the mares I successfully bred last year were vaccinated for this disease. If one uses the same line of logic proposed by critics of this vaccine (horse vaccinated = problems followed) the vaccine seems to be a fertility enhancer in my hands (horses vaccinated = pregnancy followed). In reality the vaccine doesn’t enhance nor decrease fertility. The threat of this disease is real and has been seen across the continent (see USDA site To date the concerns for not using the vaccine unfounded and appear to have reached urban myth proportions.

Artificial Lights, Mare Transition, and Early Cycling
Having the mare cycle before you are trying to breed is one of the most important aspects of preparing for the breeding season. Mares for the most part are seasonal breeders. They are receptive to the stallion, ovulate, and are able to become pregnant during spring and summer- when the days are getting longer. The time between non breeding and breeding season is known as the transitional phase or transition. During transition mares don’t show heat consistently, don’t ovulate in a predictable manner, and generally make breeding inefficient, frustrating, and needlessly expensive. Transition ends when the mare ovulates her first follicle. After this first ovulation, mares will cycle regularly and ovulate every 21 days until fall when they again enter the non breeding time of the year. It is to the advantage of every breeder to have their mare ovulate at least once before attempting to breed. One of the main goals of preparing for the breeding season is to get them through transition as early in the season as possible.

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The easiest way to get your mare cycling early is to put her under lights. We can shorten transition and bring about normal heat cycles by pushing their physiological 'clocks' a little. Exposing mares to 16 hours of continuous light per day during the winter makes for a "long day' and fools the brain into thinking it is springtime. Following 70 days of artificial light 50% of mares will ovulate, after 85 days 95% of mares will have ovulated. The protocol for lights is simple. Bring your mare inside at dusk and leave a light on until 11:00 p.m. The lights need to be bright enough so that you can easily read newsprint from any corner in the stall. Also make sure that she can’t place her head into a dark space (like out a window or dutch door into the darkness).

Although lights are very effective some mares will not respond as expected. One reason is due to the way some horses are stabled. Many performance and show mares are housed in a barn where lights are left on late in the evening all year long. Getting these mares to cycle normally can be frustrating. The problem arises because mares exposed to long day length (via artificial lights) all year long will still continue to be seasonal breeders. That is, although we can fool them for a little while with lights, if they are kept under lights all year their breeding and non breeding seasons can have no correlation to the calendar and they may go into their non breeding season in late winter or spring!

One way to correct this problem is to take these mares outside and allow them to experience darkness and cold weather (use a blanket if your mare isn't used to the cold). After as little as 30 days of short daylight and winter conditions, these mares often respond and start cycling when you put them under lights

Shortening the Transition Period With Light and Pharmaceuticals.
Breeders have long known that putting the mare under lights in November will usually result in a mare cycling when the breeding sheds open in February. Recent research has shown that using 2 drugs domperidone and sulpiride along with light can shorten this time by up to 2 weeks. Also if you were late in putting your mares under lights these drugs may help. Seventy five percent of mares put under lights and treated with sulpiride beginning January 1st will ovulate within 40 days. Mares that were put under lights without sulpiride ovulated 2 weeks later. Sulpiride and domperidone treatment can be valuable to the breeder wanting an early foal by shortening the transitional phase.

Because of the limited time available during the season it is particularly important to prepare infertile mares (required 3 or more cycles to get in foal) for the breeding season. Placing these mares under lights is very helpful because the sooner you have this type of mare cycling the more chances you have for pregnancy. Uterine cytology and culture are commonly taken and additional examinations via ultrasound, biopsy, and endoscopy are all valuable diagnostics in infertility.

Maiden Mares
Preparing maiden mares (mares never bred) can be useful. Maiden mares under 6 years of age are more fertile than older maidens. When mares reach 12 years of age many will start to have lower fertility and by the time they are 16 most will have declining fertility. Like infertile mares, older maiden mares need the benefit of lights and early cycling.

Pregnant Mares
The last type of mare we'll discuss are pregnant mares. How can we prepare these mares for the next breeding season? Once again, the answer is lights. If your mare is due this winter you may want to put her under lights now. Doing so will increase the likelihood that she cycles after she foals. If she is due in late spring or summer, lights aren't usually necessary, as almost all mares will cycle after foaling in the long day season. If you do put her under lights now, her pregnancy won't be altered and it will help ensure that she will start cycling after foaling. One note of caution on these mares: if you use the lights before foaling, do not stop using the lights after she foals, turning the lights off may inadvertently encourage the mare to go into the non breeding mode.

Breeding on Foal Heat
Foal heat usually begins 5-12 days post foaling, with most mares ovulating by day 21. Through the years many conflicting things have been said about foal heat breeding and run the whole gamut from "always breed on foal heat" to "never breed on foal heat". Historically mares bred at foal heat had pregnancy rates 20% lower than mares bred later. New research shows that if mares are looked at as individuals they usually fall into a few basic categories upon which sound breeding decisions can be made.
Foal heat usually begins 5-12 days post foaling, with most mares ovulating by day 21. Through the years many conflicting things have been said about foal heat breeding and run the whole gamut from "always breed on foal heat" to "never breed on foal heat". Historically mares bred at foal heat had pregnancy rates 20% lower than mares bred later. New research shows that if mares are looked at as individuals they usually fall into a few basic categories upon which sound breeding decisions can be made.

General guidelines which increase breeding efficiency in foal heat mares.
  • Each mare is an individual, what works for one will not necessarily work for another.
  • Have the mare examined daily starting on day 6 or 8 after foaling.
  • Don’t breed the mare earlier than 10 days post foaling.
  • If the mare has fluid in the uterus when bred it will lower pregnancy rates.
  • Breed mares one time during foal heat (obviously close to ovulation).
  • Post breeding treatments are valuable in foal heat mares; they include uterine lavage and oxytocin for uterine evacuation, and uterine antibiotics.
  • Mares who had dystocia or retained placenta are not good candidates for foal heat.
  • Check foaling mares early in the season. If it’s possible don't wait until late in the breeding season (May) to breed. Time is running short.

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The first breeding is often the best chance; repeated attempts frequently result in residual inflammation. When looked at as a “group” mares bred on foal heat have about 20% lower pregnancy rates than mares bred at later cycles. Recent research involving hundreds of mares bred artificially showed that if mares were bred properly on foal heat their pregnancy rates were comparable to maiden, and mares without foal at side. Other researchers have shown similar research and what is becoming quite evident is that following foaling mares can not be "grouped" into one large category. In other words, the decision to breed on foal heat should not be based on your previous experiences with foal heat breeding, "common knowledge", or even the mares previous history. Instead the decision to breed post foaling should be based on the individual mare and her specific condition following foaling. Several factors are critical in the decision to breed on foal heat or not. They all revolve around the health of the uterus and to a physiological process known as uterine involution. Uterine involution is the process of the uterus shrinking in size, becoming more capable of muscular contraction, and evacuating all the fluid associated with pregnancy. Normal mares will contract their uterus from the large size capable of holding a foal to one that is often less than 6 inches in diameter. Involution and muscular contracting ability of the uterus are key in the ability of establishing the next pregnancy. One very interesting new finding is that mares that are exercised have a dramatically better involution with better evacuation of pregnancy associated fluids than stall confined mares. Take home message: make sure those post foaling mares get lots of exercise!

Some veterinarians recommend uterine treatments to improve pregnancy rates in foal heat mares. These treatments include antibiotics, hormones (oxytocin and prostaglandins - to improve uterine contraction), and lavage (to physically evacuate uterine fluid). These treatments have been advised either on the day of or shortly following foaling. Unfortunately the benefit of these treatments post foaling is unreliable and inconsistent when scrutinized with controlled research.

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