Reproductive Seasonality

II. REPRODUCTIVE SEASONALITY

OVERVIEW
1. The mare is a long day breeder; cyclic activity peaks during mid-summer and is minimal during late winter. Although many environmental changes occur with the seasons, daylength is the important cue controlling the physiologic breeding season. Months of the breeding season are reversed for the southern hemisphere.
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2. Because of the competitive advantage of early foals, an "operational breeding season" begins February 15 and extends to the first week in July. Breeds vary as to how closely they adhere to the operational breeding season. Thoroughbreds follow it most closely.
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PHASES OF THE EQUINE REPRODUCTIVE CYCLE
1. Anestrus- December, January to mid February. Absence of large follicles. Ovaries are essentially inactive.
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2. Vernal transition- March and April. Follicles grow and regress, no ovulations. Diameter of largest follicle gradually increases as season continues. Spring transition ends with the first ovulation of the year.
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3. Ovulatory season- April, May, June, July, August, September. Cyclic follicular activity accompanied by ovulation and formation of corpora lutea.
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4. Fall transition- October, November, December. Follicles develop but fail to ovulate. Diameter of largest follicle gradually decreases as fall transition continues.
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PHYSIOLOGY OF SEASONALITY
1. Melatonin and day length-melatonin levels rise as daylength decreases after the fall equinox and peak in early January. Melatonin levels then decrease during the remainder of January and February as day length increases.
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2. Actions of melatonin-melatonin inhibits release of GnRH, preventing synthesis of LH and release of LH from the pituitary. Lack of GnRH also inhibits FSH release.
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SEASONAL CHANGES
1. Estrous behavior-Horses vary greatly from each other in the intensity duration and frequency of estrous behavior, but in a given horse the pattern is relatively consistent. Horses in seasonal anestrus may display occasional estrous behavior, although passive acceptance or refusal of the stallion is more likely. In vernal transition horses may display prolonged heat or heat every few days. During the ovulatory season, estrus occurs at regular intervals, with periods of estrus shortest at midsummer (4-5 days) and longest at the beginning and end of the breeding season (about 7 days). Diestrus is consistently 14-15 days throughout the ovulatory season.
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2. Hair coat-Seasonal change in hair coat tends to follow reproductive function in the mare, and is a rough indicator of ovarian activity. Long in the winter, shed in spring during vernal transition, short, smooth and glossy in the summer, hair coat changes are driven by photo period, and change as ovarian activity changes. Although hair coat is sometimes used as an indicator of ovarian activity, the association is not always perfect; under experimental conditions, mares have grown long hair coats while cycling normally.
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3. Follicular dynamics-Stage of the annual reproductive cycle is most clearly reflected in follicular activity. A few small (<15 mm) follicles may be present during anestrus. During early vernal transition growth and regression of multiple follicles up to about 20 mm occurs. Later in transition multiple large follicles (> 30 mm) grow and regress, but do not ovulate. As the mare approaches the ovulatory season a single large follicle may develop, suppressing growth of other follicles, but regresses without ovulating. Eventually a single large follicle (>35mm) develops, suppresses other follicles, and ovulates. Ponies will develop a series of 3-4 anovulatory follicles prior to the first ovulation of the season. The number appears more variable in mares.
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4. GnRH- Gonadotropin releasing hormone (GnRH) is the master switch of reproductive function in the mare. Low to undetectable during winter, return of GnRH secretion in the spring is driven by photo period. Increased GnRH secretion drives changes in FSH and LH which stimulate follicular development and reproductive cyclicity.
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5. FSH and LH- Gonadotropins (FSH and LH) act directly on the ovaries to drive ovarian function. Both are low in anestrus. In vernal transition FSH is high, promoting follicular development, but LH is still low. Rise in LH at the end of transition drives the first ovulation of the season.
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6. Estradiol and progesterone- Levels of estrogen and progesterone reflect ovarian activity. Both are low in anestrus. Estradiol rises when a dominant follicle in late transition starts to synthesize estradiol. Estradiol synthesis by the follicle is a hallmark of its maturity, and likely ability to ovulate. In the series of 3-4 anovulatory transitional follicles in ponies, the first two do not synthesize estradiol, but the third or fourth acquire "steroidogenic competency". Estradiol rise initiates the ovulatory surge of LH. Other estrogenic substances known as "conjugated estrogens" rise earlier in vernal transition. Origin and function of these conjugated estrogens is unknown. Progesterone is baseline throughout anestrus and transition, but rises when a corpus luteum is formed after ovulation.
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7. Tubular genitalia- Changes in the tubular genitalia are driven by changes in estradiol and progesterone. Lack of these hormones in anestrus and early transition results in a small, flaccid uterus. Estradiol causes edematous enlargement of the uterus, and a flaccid , hyperemic, cervix. These changes in a transitional mare would suggest that ovulation will occur soon if a large follicle is present. Progesterone causes a toning of the uterus due to loss of edema and tonic myometrial contraction. Cervix is pale, and toned under progesterone. A toned uterus and cervix in a mare thought to be in anestrus or transition suggests that she is cycling, and currently in diestrus.
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PUBERTY
1. Age- Age at puberty is defined as the age at the first ovulation in females. Fillies have their first ovulation at 1-1.5 years of age, during the summer in the year after their birth. Horses born late in the summer may have to wait an additional year.
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2. Hormonal changes- Hormonal changes are similar to those encountered in vernal transition. In the first summer of life a filly will have elevated FSH, but not LH. Only in the second spring does LH rise, causing ovulation.
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3. Effect of artificial light- Inappropriate light can delay puberty. Fillies exposed to artificial light during their first winter had a delayed onset of puberty. This is in contrast to other long-day species in which puberty is accelerated by long photo periods.
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4. Follicular activity in young mares- Mares aged 2 years or less enter fall transition earlier in the season than mares aged 6 -15 years.