How To Get A Mare To Cycle
Generally speaking, the best time for mating of a mare and stallion is determined by several factors. These include the length of daylight (season), the daily temperature, the mare's general health, diet, the amount of rainfall, the climate, and the latitude.
Mares are seasonally polyestrus, meaning that they experience regular estrus cycles during late Spring, Summer and early Fall, and none during the Winter. These cycles are controlled by hormones, which respond to increases or decreases in daylight duration with the onset of the seasons. Mares typically cycle regularly between March and October, with each estrous period being an average length of twenty-one days.
The reproduction cycle of the mare is divided into two phases: the estrus phase, during which the mare is actively interested in and is receptive to the stallion; and the diestrus, which is a time of sexual disinterest that begins 24 to 48 hours after ovulation and lasts 14 to 16 days.
While fillies become sexually mature at around 18 months, they are still experiencing growth that may be hindered by pregnancy. The ideal age to begin foaling is around four years of age. Mares are capable of continuing to breed until late in life and do not usually suffer ill effects if nutrition and condition are maintained.1
|Reproductive Life of the Mare|
|Sexual maturity||Approx 18 months|
|Estrous cycle||22 days|
|Estrus (fertile) length||6 to 8 days|
|Diestrus (not fertile) length||14 to 16 days|
|Gestation period||340 days|
|Postpartum heat||7 days after parturition|
As increasing daylight stimulates the receptor centers in the brain to trigger reproductive hormones, these hormones begin the pattern of regular periods of estrus, also known as heat. The estrous cycle is the time period from one ovulation to the next. The average cycle is 22 days and this can vary by a few days especially at the beginning or ending of breeding season.
Manipulation and Management
A healthy mare will cycle naturally of course, so the question becomes one of how to get a mare to ovulate, or rather, to cycle so that she ovulates at an opportune time during her cycle. What constitutes an opportune time is wholly based upon the needs of the owner or manager (in the case of a breeding program).
The most common methods of manipulating the estrous cycle in equines are through manipulation of the photoperiod (length of light to which the mare is exposed) and hormone therapy.
Exposing the mare to a longer period of daylight, in addition to using artificial light, the onset of regular estrous cycles can be hastened, making breeding possible much earlier in the year. For example, if the breeding season is scheduled to begin February 15, artificial lighting needs to begin on December 1.
Use of artificial lights can be implemented abruptly, with mares being exposed to light for 16 hours a day, or the light can be increased gradually over a period of 60 days. With the increasing schedule, three hours of light are added during the evening of the first week, increasing by 30 minutes each additional week, until the goal of 16 hours of light is reached.1
With regard to the amount of artificial light that the mare receives (lumens, or the intensity of the light), there are a number of ways to measure light sources with precision, such as light meters and single reflex cameras. Many owners, managers and veterinarians maintain that an amount of light sufficient to comfortably read a newspaper should suffice, however.
Treatment of seasonally anestrus mares with GnRH has been shown to induce cyclicity, but is cost prohibitive and practicality outside a research environment is minimal.2 Altrenogest is frequently used with success to regulate follicular activity and ovulation in transitional phase mares. There is a higher success rate if there is uterine edema present and follicles present are 20mm or greater in diameter, or if there are more than 6 follicles of 10mm diameter present at the time of treatment. It should be noted the literature asserts that hormone therapy be used in conjunction with, not instead of, one of the available lighting regimes.
Treatment is typically carried out daily for 15 days after the mare has been subject to photostimulation for the requisite period.2 Some practitioners recommend following the final progesterone treatment with a dose of prostaglandin F2 alpha or one of its analogues suitable for equine use as some mares may ovulate while receiving the progestin therapy.
Estrus should appear in 3½ to five days [after treatment with altrenogest]. It has been observed in early estrus cycles, in mares that have undergone progesterone treatment, that follicles tend to be somewhat smaller, and estrogen and luteinizing hormone levels tend to be lower than those in mares ovulating naturally later in the season. Consequently, estrus in these early progesterone stimulated cycles is liable to be longer in duration, with ovulation occurring after more days of standing heat, than is seen later in the year.2
Effective estrus detection results in successful breeding management. Mares behave in distinct ways when they are responsive to a stallion. Mares in estrus raise their tail, squat, and may urinate in the presence of the stallion. This behavior is referred to as teasing and is a good indication that the mare is receptive and about to ovulate. The estrous cycle produces observable changes in the mare's reproductive tract. Rectal palpitation by an experienced veterinarian or handler can detect changes in the uterus, ovaries, vagina, and cervix of the mare.3
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