Equine Reproduction Seasonality (Week 4 ASCI 333)

Seasonal polyestrous

Mares cycle repeatedly but only during certain seasons; cyclic activity peaks in summer with minimal cycling in winter

Long day breeder

Mare's reproductive activity triggered by increasing and long daylight hours; photoperiod is the most important cue for cycling

Winter anestrus (deep anestrus)

December-February period with quiescent ovaries, little to no follicular activity, no follicles >10 mm, and low estrogen and progesterone

Spring transition (vernal transition)

March-April period when follicles grow to 20-30 mm through 3-4 waves but do not ovulate; ends with first ovulation of breeding season

Ovulatory season

April/May through September; cyclic follicular activity with regular ovulations approximately every 21 days

Fall transition

October-December period as photoperiod decreases; follicles may grow but fail to ovulate; mare gradually returns to winter anestrus

What percentage of mares ovulate during winter?

Approximately 30% may still ovulate during winter, though not the norm

Photoperiod effect on melatonin

Melatonin production by pineal gland decreases as day length increases; prolonged melatonin secretion inhibits GnRH release

How does melatonin affect reproduction?

Prolonged melatonin secretion inhibits GnRH release from hypothalamus and decreases pituitary sensitivity to GnRH, suppressing LH and FSH release

Artificial photoperiod manipulation

Exposing mares to 16 hours of light per day can advance breeding season, but requires 8-10 weeks for response

Nutritional influence on seasonality

Low body condition score (BCS) associated with greater acyclicity; mares on pasture transition earlier than those on hay alone

Age effects on seasonality

Young mares exhibit greater periods of anestrus; older mares (≥20 years) begin cycling later in spring

Breed differences in seasonality

Ponies more likely to show pronounced anestrus compared to light horse breeds

Housing effects on cyclicity

Mares housed indoors have lower probability of anovulation during transition compared to those on pasture

GnRH seasonal patterns

Absent or very low during anestrus; increases with extended photoperiod; key hormone controlling reproduction

FSH and LH during anestrus

Both remain low due to lack of GnRH stimulation

FSH and LH during spring transition

FSH rises significantly driving follicular growth, but LH remains relatively low preventing ovulation

FSH and LH during ovulatory season

Both rise sufficiently to support complete follicular development, ovulation, and corpus luteum formation

Estrogen and progesterone during anestrus

Both remain at low baseline levels due to lack of follicular and luteal structures

Estradiol role in ovulation

Rises in late transition as follicles mature; initiates positive feedback LH surge at hypothalamic "surge center" which drives ovulation

Progesterone seasonal pattern

Remains low during anestrus and transition; rises only after ovulation and corpus luteum formation

Inhibin function

Produced by ovarian follicles; provides negative feedback to pituitary to regulate FSH secretion; increases as follicles develop

Hair coat seasonal changes

Long coat in winter, short coat in summer; driven by photoperiod

Seasonally anestrous mare behavior

Typically shows no interest in stallions

Transitional (vernal) mare behavior

Exhibits erratic heat behavior with inconsistent, prolonged but irregular sexual receptivity without ovulation

Ovulatory mare behavior

Displays distinct estrous and diestrous behaviors at regular intervals with clear receptivity during estrus

Anestrous mare follicular dynamics

Little to no follicular activity

Vernal transition follicular dynamics

Waves of follicles growing to 20-30 mm diameter; all regress without ovulating; 3-4 waves of development

Ovulatory mare follicular dynamics

Multiple follicles develop; single large dominant follicle (>35 mm) suppresses smaller follicles before ovulating and forming corpus luteum

Anestrous mare uterus and cervix

Small, flaccid uterus and small, closed cervix due to lack of hormonal stimulation

Ovulatory mare in estrus uterus and cervix

Enlarged, edematous uterus with prominent endometrial folds; flaccid, hyperemic "open" cervix with abundant clear mucus (estrogen influence)

Ovulatory mare in diestrus uterus and cervix

Toned uterus and tight "closed" but prominent cervix (progesterone influence)

Transitional mare uterus and cervix characteristics

Intermediate characteristics; developing follicles produce some estrogen but without corpus luteum formation, progesterone remains low

Puberty definition in fillies

Age at first ovulation

When do fillies reach puberty?

First ovulation between 11-24 months of age, most around 12-18 months; generally begin ovulating in second summer of life

Puberty timing dependency

Heavily depends on month of birth; fillies generally ovulate in second summer when photoperiod is increasing

Pre-puberty hormone profiles

Similar to anestrous adults with low GnRH, FSH, LH, estrogen, and progesterone

Stallion year-round breeding capability

Unlike mares, stallions can breed all year and are not limited to specific breeding season

Seasonal effects on stallion reproduction

Season strongly affects stallion behavior and sperm characteristics due to photoperiod effects on HPG axis

Daily sperm production seasonal variation

Lowest production (50% decline) September-February; increases in March, peaks May-June, declines July-August

Breeding vs non-breeding season stallion semen

Breeding season (March-July): larger volume, higher total sperm count; Non-breeding season (September-December): higher sperm concentration

Stallion sexual maturity

Fertile by approximately 3 years of age but not fully reproductively competent until 5 years of age

Spermatogenesis duration

Requires approximately 57 days from initiation to mature spermatozoa production

LH targets in stallion

Leydig cells in testis to produce testosterone

FSH and testosterone targets in stallion

Sertoli cells in seminiferous tubules; FSH initiates spermatogenesis and testosterone supports sperm maturation

Stallion vs mare HPG axis outcomes

Stallion HPG ensures continual sperm production during breeding season; Mare HPG produces cyclical variation to synchronize mating and ovulation

Evolutionary significance of equine seasonality

Ensures fertilization occurs during breeding season, resulting in offspring born in late spring/early summer when environmental conditions favor foal survival