Sara K. Lyle, DVM, MS, Diplomate American College of Theriogenology
Clinical Instructor, Theriogenology
One of the greatest problems limiting the production of foals is man. Despite what we know about the anatomy and reproductive physiology of horses, many breed organizations have arbitrarily assigned January 1 as the birth date of foals. Therefore, breeders strive to get their mares to foal close to the first of the year in order to give the foal the maximum number of days to grow before becoming a year of age the following January 1. This yields the largest weanlings for the halter ring, the largest yearlings for the sales, and the most mature two-year-olds at the racetrack.
Mares are photosensitive breeders. Their season is initiated by increasing amounts of daylight, and thus we call them long day breeders. In addition to light, mares are also somewhat sensitive to ambient temperature. Adequate nutrition, especially energy, is necessary to overcome the stress of winter and the low temperatures of early spring. Mares need to be in good condition or a positive energy balance before they will begin to cycle properly.
Mares may have one of three cycle patterns during the year. Seasonally polyestrous mares have a definite physiological or ovulatory breeding season and periods of anestrus where they have no ovarian activity (follicular development and ovulation). This type of mare enters and exits the breeding season with irregular cycles. These irregular periods are termed “transition periods.” During these transition periods, mares can show variable cyclic irregularities. The average first ovulation was April 7 on a large broodmare farm in north central Florida. Latitudes north of this region will experience ovulations later in the year, whereas latitudes south of this region will experience ovulations earlier in the year. This is the most common pattern of cyclicity with approximately 80% or mares displaying this pattern.
Polyestrous mares cycle regularly throughout the year and seem to be less responsive to ambient light. The closer that one gets to the equator, the greater the number of mares that are polyestrous because day length is less variable throughout the year. These mares will have an occasional period of anestrus that doesn’t seem to have any relationship to the time of year. Irregularly polyestrous mares cycle regularly during the breeding season similar to the seasonal mare, and they have irregular cycles during the transition season. But unlike the seasonal mare, these mares never really enter a true anestrus period. They cycle year-round like the true polyestrous mare, but the cycles during the winter months of these mares are very irregular, thus the name irregularly polyestrous. Although the cycle pattern may be confusing, mares tend to fit one pattern throughout most of their lives and only occasionally deviate from their usual pattern. Thus a reproductive history of the previous season(s) can be quite helpful in managing the mare during subsequent breeding seasons.
The next major problem in equine reproduction arising from people is confusion about what constitutes estrous behavior. Owners frequently confuse behavioral problems or urinary tract problems for heat. Unlike cows, homosexual riding is not seen in normal mares. Since mares do not ride each other, we are forced to resort to observing the mare’s response to a “teaser” male. Teasing is extremely beneficial for efficient breeding and allows more accurate identification of mares that are truly exhibiting estrous behavior and thus receptive to breeding.
The estrous cycle of the mare averages about 20-21 days. She usually remains in heat (estrus) approximately four to seven days, but this varies with the mare and the season. The period of estrus is often longer during the transition periods, and early or late during the physiologic breeding season. Signs of heat are typically not displayed during diestrus, which lasts 14-16 days. Thus, the cycle length (estrus + diestrus) averages approximately 20-21 days in most mares. Occasionally mares ovulate during diestrus which lengthens the number of days that she is out of heat (“prolonged diestrus”). In the normal cycling mare, variations in the length of the cycle are usually due to variations in the length of estrus. Regardless of the length of estrus, most mares ovulate 24 to 48 hours before the end of heat and most mares are often bred on the second and fourth days of a five-day heat.
Determining the stage of the mare’s reproductive cycle based on clinical signs and response to a teaser male are useful in breeding management of the mare. Estrus: The mare in heat raises her tail, squats, urinates frequently in the male’s presence, winks (everts the clitoris), and does not kick or strike at the male. Most mares wink a few times after urination but the estrous mare shows exaggerated winking even before urinating. Some mares may be resentful initially when teased (especially maiden and dominant mares, or those recently introduced to the herd), but then they “break down” and become receptive to the stallion with continued teasing. Therefore, the handler and observer must be patient with these mares. Diestrus: Mares not in heat squeal, pin their ears, and strike and/or kick when the male approaches. Passive: The passive mare shows little or no positive or negative signs. A mare will usually not be passive the entire estrous cycle, but will either display negative signs during diestrus and be passive during estrus, or, display positive signs during estrus and be passive during diestrus. Thus, close observation and obtaining an accurate history regarding past cycles is important in managing these mares.
Understanding an individual mare’s behavioral pattern is valuable in assessing her response to the male, which helps determine the stage of her cycle. A good record keeping system records the reproductive tract findings and the teasing results, and can be useful to predict the onset of the next heat.
Now that we understand the different patterns of the mare’s reproductive cycle and how to detect estrus, we need to compare events in the natural breeding season with events in the arbitrarily defined stud season. The percentage of mares showing estrus in any month doesn’t reach 70% until the middle of March. Some of the mare’s early displays of heat are associated with follicular development that does not result in ovulation and formation of a corpus luteum (CL); this typically occurs during the transitional period. The percentage of mares that have ovulated as assumed by the presence of a CL on the ovary may not reach 70% until mid- to-late April, and the peak of ovulation activity occurs from the middle of July to August. On farms where nutrition is suboptimal, these activities are usually delayed – the majority of mares will not be cycling until mid-May. Thus, normal reproductive physiology is working against us in getting conception in February, March and April.
Comparing the most fertile periods of the mare and the stallion, with the traditional breeding season for a breed having a January 1 birth date, it is evident that the peak of equine fertility occurs at the end of the stud season. In fact, fertility of the male and female is much reduced in the first half of the season. The normal gestational (pregnancy) length of the mare is approximately 11 months with a range of 320-360 days. So if we add a month to the foaling date we can determine in which month most mares conceived the previous year. Even though most breeders attempt to get January, February and March foals, most foals are born in April and May; therefore, conception occurred in May and June, respectively. What can we do to overcome or manipulate nature to make the mare’s reproductive physiology capable of producing earlier foals?
When scientists discuss the role of light in initiating reproductive cycles they divide the day into the photophase (light) and the scotophase (dark). They have also been able to define an inducible period when light exposure can initiate the hormonal events that cause a mare to begin to cycle. Extending light into the inducible period over time causes initiation of cycles. In the mare, the inducible period occurs 8-10 hours after the onset of darkness (or the scotoperiod). The amount of light needed is 10 foot candles (1 lux) at the level of the mare’s eye. This is usually equivalent to a 100 watt bulb for a 12 x 12 box stall or two 40 watt fluorescent bulbs per box stall. The light level can be tested by using a 35mm camera with the film speed set to 400 and the shutter speed at ¼ second. Place a styrofoam cup diffuser over the lens and set the aperture for the correct exposure. The f stop should be about 10 or greater to ensure sufficient light.
Artificial light schemes have been used to induce cycling in mares for years. The length of the transition period is unchanged, but the anestrus period is either eliminated or greatly shortened. Three basic light methods have been used. They all begin about 60 to 75 days prior to the desired date to begin breeding. Abrupt Method: This method involves extending the duration of light to 16 hours with lights beginning about 60 to 75 days prior to the first day of the breeding season. Although this scheme is not as physiological as other methods, it is effective, but may result in higher electricity bills than some of the other methods. Gradual Method: This method begins 60 to 75 days prior to the onset of breeding, but in this case the day length is increased by one-half hour per day on a weekly basis. Add two to two and half hours of light after sunset to achieve this goal. By February 1, mares are receiving 16 hours of light per day. Intermittent Method: This method may be the most physiological and is also the most conservative of electricity, but it requires the purchase of a timer switch for the lights and is the most complicated to regulate. In this case, day length is lengthened to 10 hours, followed by eight hours of darkness, then two hours of light, and another 4 hours of darkness. The two hours of light in the middle of the night occurs during the inducible period. Alternatively, one hour of light 9½-10½ hours after the natural onset of darkness is also effective.
Once mares are on a lighting program they should be kept under the lights until the natural daylight reaches 16 hours. Otherwise they may react as if it were autumn and have irregular cycles as if they were in the fall transition period. Another consequence of a lighting program is that these mares shed their hair coats early and therefore may need to be protected from the cold in colder climates. Mares under lights will still have transitional cyclic irregularities, but these are moved up to earlier in the year, prior to the onset of the breeding season. The physiologic ovulatory season is moved up into February, and thus we get better breedings, earlier conceptions, and mares foaling closer to January 1.
