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.
 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.

Seasonality
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.
J
F
M
Neuropathway
A
M
J
J
A
Receptors in eye
S
O
N
D
Neuropathway
Decreasing melatonin
Pineal gland
Increasing melatonin
Increasing GnRH
Hypothalamus
Decreasing GnRH
Increasing
Gonadotropins
(FSH, LH)
Anterior pituitary
Ovaries
Decreasing
gonadotropins
Anestrus

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

The increasing daylight length in the spring brings
about a series of changes in the mare.

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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

21 day inter-ovulatory interval (estrous
cycle)
 Estrus
(heat): 5 – 7 days
 Ovulation:
 Diestrus:
24 to 48 hours prior to end of heat
14 – 16 days
Fall transition
Fall transition mirrors spring transition.
 You see:


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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.

Estrous Cycle of the Mare
Endocrinology of the Estrous Cycle
Mating Behavior
(Estrus signs)


The mare will allow the stallion to
smell and bite.
She will
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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”
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
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
 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
 Artificial
lighting
 Shortening Late Transition
 Inducing Ovulation
 Estrus synchronization
 Estrus Synchronization & Ovulation
Induction
Manipulation Methods:
Light
Light can be supplemented in the afternoon and evening to
give a total of 16 hours light each day.
 Aged mares (greater than 10
1. 200 watts for a 12 x 12
yrs) take ~12-18 days longer
Stall X 60 days, start Dec 1
 No advantage to starting
2. 20 watts, 12 x 12 Stall X
before Dec 1
75 days
Hormone Management
- Progestagens

Regumate
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Oral
Suppresses estrus by
forming an artificial luteal
period.
Dose - 1 ml/50 kg orally or
in feed for 14 days.
Heat occurs 4-5 days after
withdraw

Injectable
progesterone
 Estrus
suppression
 100 mg/day prevents
estrus and ovulation.
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

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

Prostglandins
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Only works on mature corpus  Lutalyse
luteum (~ 5 d after ovulation)
 Dose is 5-10 mg (1-2 cc/mare)
Mare will come into heat 2-5
IM.
days later.
 Side effects
Ovulation in 8 - 9 days
 Estrumate
The time to estrus varies
 Dose is 200 mcg/mare IM.
depending on follicular
 Fewer side effects
development on the ovary.
 Side effects can be sever in
mare
Sweating
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

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Induction of Ovulation
 hCG
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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

Deslorelin
3.5 cm follicle, ovulate 36-48
hrs, 90%
 As effective as hCG without
antibodies

Estrogen
 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
 Estradiol
signs of estrus” in silent heats
17B; 1-2 mg
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
