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The Estrous Cycle of Mare its Manipulation & Artificial Control. Dr. Hatem Atalla D.V.M PhD An-Najah National University Faculty of Veterinary Medicine 2009. Seasonality. Their season is initiated as the ratio of daylight to darkness increases and ends during decreasing day lengths .

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the estrous cycle of mare its manipulation artificial control

The Estrous Cycle of Mare its Manipulation & Artificial Control

Dr. Hatem Atalla D.V.M PhD

An-Najah National University

Faculty of Veterinary Medicine


  • Their season is initiated as the ratio of daylight to darkness increases and ends during decreasing day lengths.
  • The average season for horses, extending from February(2) to November(11). Peak fertility is obtained if mares are bred between May(5) and July(7).
  • Behavioral estrus occurring during short-day months (January to April) is frequently not accompained by ovulation.

The physiologic (natural) breeding season of the mare occurs in late spring and summer.

  • During the Transition from anestrus to physiologic polyestrus, the mare will frequently have variable length periods of behavioral signs of estrus without developing follicular structures or ovulating.


Increasing day length

Light Stimulus

Decreasing day length

Receptors in eye



Pineal gland

Decreasing melatonin

Increasing melatonin


Decreasing GnRH

Increasing GnRH






Anterior pituitary


  • Anestrus is caused by the secretion of melatonin, which is secreted in response to increasing darkness. The melatonin inhibits GnRH, so the FSH and LH are low.
  • Anestrus occurs around Winter solstice.
  • About 80% of mares undergo anestrus.
  • Mares in anestrus are passive to the stallion advances.
  • On rectal palpation the ovaries are small, smooth, and inactive.
  • The cervix and uterus are flaccid.
  • Vaginoscopic exam reveals a cervix that is pale and dry, and the cervix may even be open.
  • The hormones are all at very low concentrations.
  • If the is nutrition poor, the mare may not cycle back in the spring.
spring transition
Spring transition
  • The increasing daylight length in the spring brings about a series of changes in the mare. 
    • Decrease of Melatonin secretion 
    • As the melatonin decreases, GnRH resumes secretion, and FSH and LH also increase. 
  • With increased FSH, follicles start to grow. 
    • Most of these follicles are not steroidogenically competent so they do not produce estrogen.
    • They also do not ovulate. In fact, an average of 3.7 waves of follicular development occur before the first ovulation. 
    • After several waves, an estrogen producing follicle finally develops and ovulates. 
    • The first ovulation of the season, on the average, occurs about April 8
physiological breeding season
Physiological Breeding Season
  • 21 day inter-ovulatory interval (estrous cycle)
    • Estrus (heat): 5 – 7 days
      • Ovulation: 24 to 48 hours prior to end of heat
    • Diestrus: 14 – 16 days
fall transition
Fall transition
  • Fall transition mirrors spring transition.
  • You see:
    • prolonged heats, 
    • irregular cycles, 
    • large 'hung' or 'autumn' anovulatory follicles. 
      • These follicle become atretic and the mare goes into anestrus.
  • This is caused from the low LH release because melatonin is taking its grip again as the day length decreases.
  • There is no treatment for fall transition.
mating behavior estrus signs
Mating Behavior (Estrus signs)
  • The mare will allow the stallion to smell and bite.
  • She will
    • extend her hind legs,
    • lift her tail to the side and
    • lower her rump.
    • The erect clitoris will be exposed frequently by contractions (winking) of the labia.
    • The vulva will be elongate and swollen, with the labia partly everted.
  • The mare should be teased by a stallion for accurate detection.
    • Attempts to fight the stallion indicate she is not in estrus even though some other signs of estrus are apparent.
timing of insemination mares
Timing of Insemination“Mares”
  • Best results without palpation are obtained by multiple breedings starting on the third day and repeating at 48-hours intervals until the mare is no longer in estrus.
  • When only one breeding is desired it is recommended that the mare be palpated and bred when she has a 35mm follicle. She should be palpated 2 days later to see if ovulation occurred and if not, she should be rebred.
  • When two large follicles are detected by palpation, mares should not be bred, since pregnancies involving twins are usually terminated by abortion.
  • Some breeders inject LH at the time of breeding to insure ovulation while sperm are viable.
foal heat
Foal Heat
  • Mares will come into estrus from 7 to 12 days after parturition (foaling heat) and can frequently be bred with good results.
  • However, mares should be bred at this time only if they have been given a careful examination to determine if there has been adequate recovery since parturition.
  • If there is any question about recovery, wait until the next estrus, which will occur about 30 days postpartum.
criteria mares should meet in order to be bred during foal heat
Criteria Mares Should Meet In Order to Be Bred During Foal Heat
  • Delivery of foal without significant difficulty.
  • Pass placenta within 3 hrs after birth
  • A healthy foal that stands and nurses within 1 hr.
  • A cervix free from bruises and abnormal discharges.
  • A uterus significantly reduced in size, without fluid accumulations.
estrus manipulation methods
Estrus Manipulation Methods
  • Artificial lighting
  • Shortening Late Transition
  • Inducing Ovulation
  • Estrus synchronization
  • Estrus Synchronization & Ovulation Induction
manipulation methods light
200 watts for a 12 x 12 Stall X 60 days, start Dec 1

20 watts, 12 x 12 Stall X 75 days

Aged mares (greater than 10 yrs) take ~12-18 days longer

No advantage to starting before Dec 1

Manipulation Methods:Light

Light can be supplemented in the afternoon and evening to give a total of 16 hours light each day.

hormone management progestagens


Suppresses estrus by forming an artificial luteal period.

Dose - 1 ml/50 kg orally or in feed for 14 days.

Heat occurs 4-5 daysafter withdraw

Injectable progesterone

Estrus suppression

100 mg/day prevents estrus and ovulation.

Hormone Management - Progestagens
progesterone or related compounds
Progesterone or related compounds
  • Regumate – most common
  • Normalization of estrus
  • Regulation of estrus
  • Estrus synchronization
  • Long-term suppression of estrus
  • Delay foal heat
  • Pregnancy maintenance
pgf 2 lutalayse or estrumate
PGF2Lutalayse or Estrumate
  • Shorten the interval between estrous periods
  • Treatment of a maintained corpus luteum
  • After foal heat
  • Estrous synchronization with prostaglandins
hormone management

Only works on mature corpus luteum (~ 5 d after ovulation)

Mare will come into heat 2-5 days later.

Ovulation in 8 - 9 days

The time to estrus varies depending on follicular development on the ovary.


Dose is 5-10 mg (1-2 cc/mare) IM.

Side effects


Dose is 200 mcg/mare IM.

Fewer side effects

Side effects can be sever in mare


Abdominal cramps, increased motility of GI tract and/or colic

Increased heart rate

Muscle weakness and balance problems

All side effects begin at 5 - 10 minutes and last until 60 minutes after injection

Hormone Management
induction of ovulation

Dose 1,500-3,000 IU IV

3.5 cm follicle

Ovulation normally occurs within 36-48 hours.

80% of mares will ovulate within 48 hr

Older Mares loose 13-15 %

More you use it, the less effective it becomes on subsequent heats

10% less effective Feb, Mar, Apr


3.5 cm follicle, ovulate 36-48 hrs, 90%

As effective as hCG without antibodies

Induction of Ovulation
  • Limited use:
    • “Induction of estrus” for jump mare
      • Estradiol cypionate; 5-10 mg
      • Start 3-4 days prior to wanting mare showing signs of estrus
    • “Enhance signs of estrus” in silent heats
      • Estradiol 17B; 1-2 mg
suppress estrous
Suppress Estrous
  • Sterile, 30 to 35 mm diameter glass marble into the uterus within 24 hrs after ovulation
  • Increases the interovulatory interval from 21 days to ~ 90 days for about 40 to 70% of mares
  • Appears to be no discomfort, nor any detrimental effects on future breeding prospects