The document discusses the estrous cycle in mares, detailing their reproductive organs, the phases of the estrous cycle, and the hormonal changes involved. It outlines how the cycle is characterized by behavioral and gonadal events, highlighting the differences in mare reproductive behavior, including their classification as seasonal polyestrous animals. Key aspects include the length of the estrous cycle, the role of estrogen and progesterone, and the impact of seasonality on reproductive cycles.

1. Estrous cycle in mare
By: Mohammad
Sangary
Kabul University
Veterinary Science Faculty
Master Program of Veterinary
Medicine
Date: Saturday, November 2,
2024

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Topics to be discussed
Reproductive organs of a mare
Introduction to reproductive cycle in animals
Estrous cycle in mare
 Estrus phase
 Nonestrus/ Diestrus phase
Hormonal changes during mare estrous cycle
Reference

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Reproductive organs of a mare
It can be divided into external and internal reproductive organs
 External: Vulva (Labia, Clitoris, Vestibule)
Internal: Vagina, Cervix, Uterus, Oviduct & Ovaries
 Ovaries are located in the abdominal cavity of the mare near the kidneys
 Ovaries volume vary from 35 to 120 cm3
Its weight vary form 40 to 80 grams
The presence of fossa in the site of ovulation in ovary
 Inverted location of its cortex and medulla

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Reproductive cycle
Two types of ovarian cycles are recognized:
I. Estrual cycle
II. Menstrual cycle
 They are recognized based on the external characteristic to
identify a particular stage of reproductive cycle (ovulation).
 Domestic animals have limited periods of estrus (sexual
receptivity), thus, the term estrous cycle in used.
 The proestrus is the onset of cycle.

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Reproductive cycle
 Primates are sexually receptive during most of
the reproductive cycle.
Thus, the only stage of the reproductive cycle that can be
recognized is the menstruation (vaginal discharge of
blood-tinged fluids).
 That's why we use the term menstrual cycle in these
animals.

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Reproductive cycle
 Estrous cycle: Is the interval between two successive ovulation
(estrus)

The estrous cycle has been classically divided into two stages:
 Based on the behavioral events:
1) Animal is in estrus (sexually receptive)
2) Animal in not in estrus (Proestrus, Metestrus, Diestrus)
 Based on the gonadal events (dominant structure in the ovary)
1) Follicular phase
2) Luteal phase

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Proes
trus
Estr
us
Metes
trus
Diestr
us
Follicular
phase
Luteal
phase
Estrous cycle based
on behavioral
events and
gonadal events
4
3
1
2

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Estrous Cycle
Polyestru
s
Monoestrus
Seasonal-
Polyestrus
Nonseasonal-
Polyestrus
Short-day Seasonal
Polyestrus
Long-day Seasonal
Polyestrus

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Estrous cycle in mare
The corpus luteum (CL) of the mare enlarges within the internal part of the
ovary and does not protrude to the outer ovarian surface as in other species.
 As the equine CL is relatively difficult to identify by palpation per rectum,
horses are usually classified by sexual behavior:
I. Estrus
II. Nonestrus (Diestrus)
 Behavioral classification is also used in other domestic species like, sheep, goat…
The ovarian status of cattle can be determined accurately by rectal palpation (follicular phase &
Luteal phase

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Estrous cycle in mare
They reach to puberty by 12-18 months where the experience
estrous cycle.
 It gives the mare chance of conception
 In winter they go to anestrous (follicle < 10mm)
Mares are Seasonal polyestrous animals
 Mares have 2 follicular waves each cycle
The first wave occurs during diestrus, and these follicles become atretic.
The second wave occurs after luteolysis and is associated with estrus.

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Estrus in mare
 Estrus cycle length is 21 days
Monovulatorey (7-25% Double ovulation, but when? 👇)
 Long-day breeder (related to photoperiod, but how?)
When two dominant follicles (>28mm)
develop in the same follicular wave,
double ovulations occur in about 40%
of mares.

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Estrus in mare
 The length of this phase is 5-7 days
 The graafian follicle produce large amount of Estrogen
 Mare has interest to stallion (sexual receptivity of stallion)
 Because of high level of the estrogen
 Relaxed facial muscles
 Ears turned to side and a lowered head

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Estrus in mare
 Turn hind quarter towards the stallion
 Lower the pelvis & straddled hinds
 Deviation of the tail
 Frequent urination in small quantities
 Winking (eversion of the clitoris)
 Mucous secretion is seen under the tail

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Estrus in mare
 Follicular estrogen stimulate the hypophysis to
produce LH
 LH initiate the ovulation
 Ovulation occurs 24-48 hours before the end of the
estrus
 Thus, some times we may see heat after ovulation
 After ovulation the remnants of follicle transform into

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Diestrus in mare
Once ovulation occurred, 1-2 days after that diestrus is stared.
Length of the this phase is 14-16 days
CL is the dominant structure of the ovary (producing
progesterone)
Progesterone Reduce GnRH secretion from hypothalamus
No interest to stallion is seen in this phase
Because of high level of progesterone and low level of estrogen

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Diestrus in mare
 If the egg is not fertilized with a sperm the CL begins to
lyse (Luteolysis).
 It is mainly lysed by the PGF2 produced by the uterus.
 After luteolysis, proestrus in started and mare enter another
estrous cycle.
 There is no uterine edema
 Cervix is now tightly closed

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Proes
trus
Estr
us
Metes
trus
Diestr
us
Primordial Follicles
Primary Follicles
Secondary Follicles
Tertiary Follicles (FSH dependent)
Graafian Follicles
Estrogen and Inhibin
producing follicle
C
H
Ovulatio
n
CL
C
A
FSH
LH
Progesterone
GnRH
FSH
FSH

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‫منابع‬
Cortés-Vidauri, Z., Carlos , A.-F., Melba , R.-D., Rochín-Berumen, F., López-Carlos, M., &
Flores-Flores, G. (2018). Mare Reproductive Cycle: A Review. 8(3), ABANICO VETERINARIO.
Aurich, C. (2011). Reproductive cycles of horses. Animal Reproduction Science, Animal
Reproduction Science, 200-228. doi:doi:10.1016/j.anireprosci.2011.02.005
Nagy, P., Guillaume, D., & Daels, P. (20000). Seasonality in mares. Animal Reproduction
Science, 60(61), 245-262. Retrieved from www.elsevier.comrlocateranireprosci
Pryor, P. (2005). Management of Estrus in the Performance Mare. Clinical Technique in Equine
Practice. doi:doi:10.1053/j.ctep.2005.07.001

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Thanks for your attention!
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