ANS 431: Estrous Cycle
I. Introduction
A. Estrous cycle
1. Period of reproductive cyclicity
B. Anestrus
1. Periods of no estrous cycles
a. Pregnancy
b. Nursing
c. Season of year
d. Poor Nutrition
c. Pathological Conditions
C. Other Terminology
1. Estrus
a. Noun
b. Period of sexual receptivity
c. Heat = Common name
d. Oestrus = British equivalent
2. Estrous
a. Adjective to describe events associated
with estrous cycle
b. Oestrous = British equivalent
3. Seasonal Breeders
a. Animals that have one or more estrous cycles
during certain periods of the year
4. Polyestrous Animals
a. Estrous cycles throughout the year
b. Cattle, Pigs, Humans
c. Are sometimes only bred at certain periods
of the year, but will cycle all year
5. Seasonally Polyestrous Animals
a. Animals that have multiple estrous cycles
only during certain periods of the year
b. Short-Day Breeders
i. Cycle when days are getting shorter
--Fall
ii. Sheep, Goats, Deer, and Elk
iii. Anestrus in spring and summer
c. Long-Day Breeders
i. Cycle when days are getting longer
--Spring
ii. Horses and Hamsters
iii. Anestrus in fall and winter
6. Monestrous animals
a. Animals that have one estrous cycle per year
b. Dogs, wolves, foxes, and bears
c. Extended periods of estrus (days)
i. Increase probability of mating
and pregnancy
II. Length of Estrous Cycle in Different Animals
Length
Ewe 17 d (13-19)
Goat 21 d (15-24)
Sow 21 d (17-25)
Cow 21 d (17-24)
Mare 21 d (15-25)
Woman 28 d (Menstrual Cycle)
Rodents 4-6 d
III. Divisions of the Estrous cycle
A. Luteal and Follicular Phase
1. Luteal Phase = Period of the Corpus Luteum
2. Follicular Phase = Period of the Follicle
3. Order
a. Non-menstruating animals
Estrus to Luteal Phase
to Follicular Phase to Estrus
b. Menstruating animals
Menses to Follicular Phase
to Luteal Phase to Menses
B. Proliferative and Secretory Phase
1. Proliferative Phase = Thickening of endometrium
a. Due to estrogen secretion
b. Menses + Proliferative Phase = Follicular Phase
2. Secretory Phase = Endometrium highly secretory
a. Due to progesterone secretion
b. Luteal Phase
C. Metestrus, Diestrus, Proestrus, and Estrus
1. Metestrus + Diestrus = Luteal Phase
2. Proestrus + Estrus = Follicular Phase
3. Metestrus
a. Begins at cessation of estrus
b. Lasts for about 3-5 days
c. Period of the formation of the
Corpus Luteum
d. Some Bleeding in Farm Animals
--Declining Estrogen causes
capillary vessels to break
--Not an indicator of conception
--Not the same as menstruation
4. Diestrus
a. Lasts between 10-14 days
b. Period of a functional C.L.
c. Progesterone increases
d. Some follicular growth
5. Proestrus
a. Lasts 2 to 4 days
b. C.L. regresses (apoptosis begins)
c. Rapid follicular growth
d. Estrogen increases
6. Estrus
a. Period of sexual receptivity
b. Usual period of ovulation
III. Duration of Estrus and Time of Ovulation
A. Ewe
1. Estrus Duration--30 h (18-48)
2. Ovulation--24-30 h from beginning of estrus
3. LH surge to Ovulation--26 h
B. Sow
1. Estrus Duration--50 h (12-96)
2. Ovulation--36-44 h from beginning of estrus
3. LH surge to Ovulation--40 h
C. Cow
1. Estrus Duration--15 h (6-24)
2. Ovulation
--24-32 h from beginning of estrus
--10-12 h after end of estrus
3. LH surge to Ovulation--28 h
D. Mare
1. Estrus Duration--7 d (2-12)
2. Ovulation
--5 d from beginning of estrus
--1-2 d before end of estrus
3. LH surge to Ovulation--2 d
V. Anestrus
A. Gestational Anestrus
1. Inhibition of GnRH by Progesterone
a. From CL verum (early pregnancy)
b. From Placenta (later pregnancy)
B. Postpartum Anestrus
1. Reproductive System needs a Period of Recovery
2. Uterus need to undergo involution
a. Time Required
i. Cow--35-40 d
ii. Sheep--25-30 d
iii. Pig--25-28 d
3. Postpartum Ovulations
a. Sows
i. Estrus w/o ovulation
--few days after parturition
b. Horses
i. Fertile Estrus
--1-3 weeks after parturition
c. Ruminants
i. Silent Ovulation
--2-3 weeks after parturition
--Not very fertile
4. Fertility Maximum after Parturition
a. Cows--60-90 d
b. Sows--After Weaning
c. Sheep--Next Season
d. Horse--Weeks after parturition
C. Lactational Anestrus
1. Prevents a new pregnancy before young are weaned
a. Weaning induces Estrus
2. Sow
a. No cycles during lactation
b. Estrus and ovulation 4-8 days after weaning
3. Cow
a. Suckling can delay cycles up to 60 days
i. Very variable
b. Needs to be at least 2 encounters per day
i. Effect not increased
with more sucklings
--Threshold effect
4. LH pulsatility is decreased
a. Weaning causes increased episodic pulses
5. Mammary stimulation is not totally responsible
for lactational anestrus
a. Weaning causes same effect in denervated cows
b. Encounter with offspring appears
to be necessary
i. Visual, Olfactory, and/or Auditory
ii. Dairy cows that have offspring
removed early don't encounter
lactational anestrus
iii. Needs to be own offspring
c. Negative energy balance related to lactation
can also cause anestrus
i. Especially in primiparous
(first time birth) females
ii. Still growing
D. Other factors associated with anestrus
1. Nutrition
a. Poor nutrition reduces reproduction
2. Pathology
a. Uterine infection
b. Persistent corpus luteum
c. Mummified fetus
VI. Dogs and Cats
A. Dogs
1. Mono- or Biestrus depending on breed.
a. 3-9 month estrous cycle
i. 5 months anestrus (major variant)
--large increases in LH pulsatility before proestrus
ii. 9 day proestus
iii. 9 day estrus (4-21 days)
iv. 2 month diestrus
2. Ovulation
--4-24 days from beginning of estrus
3. LH surge to Ovulation--2-3 days
B. Cats
1. Induced Ovulators
a. Only ovulate if mated
2. Estrous cycle length--17 days (4-30)
3. Duration of estrus--9 days
4. LH surge to Ovulation--30-40 hours
VII. Seasonal Breeders
A. Introduction
1. Wild species have a breeding season that
is initiated at a time when the environment
will allow for the best survival of the young.
--Spring is usually the optimal season
for parturition
2. Time of breeding is then dependent
on Gestation length.
3. Domestication has stopped seasonal breeding
in some species
a. Cattle & Swine
4. Of the farm species, Sheep, Goats and Horses
are still seasonal breeders
B. Short Day Breeders: Sheep and Goats
1. Most sheep and goats are seasonal breeders
a. Breeds native to the tropics may cycle
throughout the year
b. Arctic natives shorter season
than temperate natives
2. Ewes are short-day or fall breeders
a. Cyclicity dependent on photoperiod
i. Starts Daylight to Darkness ratio
decreases
ii. Ends Daylight = Darkness again
iii. Between Autumn and Spring Equinox
iv. Time of change called Transition
v. Some breeds have extended breeding
--Horned Dorset, Merino, Rambouillet
b. Silent ovulations occur more frequently at
beginning and end of season
c. Introduction of Ram during fall transition
causes synchrony of estrus
i. 1st estrus 15 to 20 days
after introduction of male
3. Does are also short-day breeders
a. Cycle between Late June and April
b. Peak Breeding between September and January
c. Introduction of Buck during transition also
causes synchrony of estrus
i. 1st estrus 5-10 days
after introduction
ii. Lactation independent
4. Rams and Bucks
a. Also affected by photoperiod
(Less than females)
b. Highest breeding activity and fertility
in fall
i. As photoperiod increases
(Spring & Early Summer)
ii. Sperm production decreases
iii. Abnormal Spermatozoa increases
iv. Fertility decreases
c. Rams have some sexual activity year around
d. Bucks usually become sexually inactive
during anestrus
5. Other influencers
a. Season can be reversed by changing animals
from Northern to Southern hemisphere
b. Control of Light artificially has also been
used to reverse anestrus
i. 16 hrs dark/8 hr light will induce
early transition
ii. Both sexes most be exposed to changes
C. Long Day Breeders: Horses
1. Mares are long-day breeders
a. Cyclicity dependent on photoperiod
(opposite sheep & goats)
i. Starts when Daylight
to Darkness ratio increases
ii. Ends when Daylight Decreases
iii. Ponies: May to October
iv. Horses: February to November
v. Peak fertility: May to June
--a lot of variation
vi. Artificial light will increase
earlier transition
--16 hrs on/8 hrs off
2. Stallion not as sensitive as Mare
a. Fertile year around
b. Sexual activity and Semen production
decreases during mare anestrus period
D. Endocrinology
1. Pineal Gland
a. Responds to environmental lighting
(Photoperiod)
i. Retina receptors are stimulated
ii. Signals transmitted
to Suprachiasmatic nuclei
in Hypothalamus
iii. Signal then transmitted
via sympathetic nerves to Pineal
b. Secretes Melatonin
i. Tryptophan Derivative (Amine)
ii. Darkness stimulates synthesis
& release
iii. Inhibits reproduction
in long day breeders
--Horses, Hamsters
iv. Stimulates reproduction
in short day breeders
--Sheep and Goats
v. Probably acts on Hypothalamus
vi. Affects GnRH synthesis and release
2. Pituitary
a. LH increases in pulsatility just before
breeding season
b. Similar to puberty
ANS 431 Homepage
~~~~~Revised 2/25/04~~~~~ TAW