Physiology of the Estrous Cycle

Sarah Moorey, University of Tennessee Aug. 20, 2019

2019 ARSBC, Knoxville, Tenn. 1

Physiology of the Estrous CycleS.E.D. Moorey1, C.C. Read1, G.A. Perry2, K.G. Pohler3, M.K. McLean4,

L.A. Ciernia5, J.N. Ketchum5, M.F. Smith5, and D.J. Patterson5

1Department of Animal Science, University of Tennessee, Knoxville, TN; 2Department of Animal and Range Sciences, South Dakota State University, Brookings, SD;

3Department of Animal Science, Texas A&M University, College Station, TX; 4Department of Animal and Avian Sciences, University of Maryland, MD; and

5Division of Animal Sciences, University of Missouri, Columbia, MO

Applied Reproductive Strategies in Beef CattleAugust 20, 2019

Knoxville, TN

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Overview1. Characteristics of the estrous cycle

2. Five hormonal changes required to establish pregnancy

3. How estrous synchronization products synchronize estrus and ovulation while promoting these hormonal changes

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Estrous cycle length• Average 21 days (range 17 to 24 days)

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• Two follicular waves – 17 to 20 days

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• Two follicular waves – 17 to 20 days• Three follicular waves – 21 to 24 days

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Estrus (standing heat)

• 12 to 18 hours (range <8 to 30 hours)



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Variation in the duration of estrus

White et al. (2002)

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Ovulation• ~ 30 hours after the onset of standing estrus• release of the oocyte (egg)• transformation of the follicle into a corpus luteum

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Ovarian structuresCorpus luteum• produces progesterone• inhibits estrus & ovulation• regressed by prostaglandin

F2α

Preovulatory follicle• produces estradiol• leads to estrus & ovulation• transformed into CL

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Overview of an estrous cycle

5 10 15Estrus

OvulationP

roge

ster

one

conc

entr

atio

n

Selection

Dominance

Atresia

Estrus

OvulationCL Progesterone

Recruitment

PGF

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Five hormonal changes required for pregnancy

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1. Elevated progesterone before estrous initiation



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2. Decrease in progesterone before AI

Prostaglandin F2α

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3. Rise in estradiol

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4. Surge of luteinizing hormone for ovulation

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5. Rapid rise in progesterone after ovulation

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Elevated progesterone before initiation of estrus

Functions• Prepares for pregnancy• Blocks estrus • Blocks ovulation• Induces cyclicity• Preserves fertility (oocyte quality)

Progestins• Controlled Internal Drug Release - CIDR• Melengestrol Acetate - MGA

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Rise in progesterone before pubertal estrus• Before normal cycles

begin, there is a silent ovulation and rise in progesterone



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Rise in progesterone before pubertal estrus• Before normal cycles

begin, there is a silent ovulation and rise in progesterone

• Progestins mimic this rise in progesterone and can induce cyclicity in peripubertal heifers or anestrous cows

Progestinor

GnRH

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FTAI pregnancy rates in anestrous cows

2341 Cows (4 Studies)Bader et al. (2005); Schafer et al. (2007); Busch et al. (2008); Wilson et al. (2010)

Estrous Cycling:

1329/2341 = 57%

Anestrous:

1012/2341 = 43%

64.8 63.8

0

10

20

30

40

50

60

70

80

90

100

Est rous Cycling Anestr us

Pre

gnan

cy R

ate

(%

)

Anestrous

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MGA-PG Protocol for Heifers• Follow synchronization protocols exactly to ensure fertility

Treatment day

PG

Feed MGA

MGA Select

14-d CIDR

GnRH + AI

72 ± 2 hr19 daysEstrus

1 14 33 36

Estrus(less

fertile)

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Progestin TreatmentMGA for 14 days

18161412108642

CL Regressed

18161412108642

18161412108642

18161412108642

18161412108642

18161412108642

Inhibition of ovulation during progestin treatment • Progestins maintain

progesterone levels to inhibit estrus and ovulation

• Progestins do not maintain CL lifespan

• Long term progestins extend the follicular wave

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Follicular wave length and fertility

0 2 4 6 8 10 12 14 16 18 20 0

Estrus

Estrus

Ovulation

Day of the Estrous Cycle

• In a normal cycle, the pre-ovulatory follicle originates from a follicular wave growing under high progesterone from the CL

• Follicle lifespan is not prolonged because the CL regresses in conjunction with final stages of ovulatory follicle growth

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0 2 4 6 8 10 12 14 16 18 20 0

Estrus

Progestin


Patterson et al, 1989

• If a progestin is used early in the cycle, luteal progesterone maintains high progesterone levels



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0 2 4 6 8 10 12 14 16 18 20 0

Estrus

Progestin

Estrus


Patterson et al, 1989

PGF

• If a progestin is used early in the cycle, luteal progesterone maintains high progesterone levels

• The follicular wave is not extended

• Pregnancy rates remain high

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0 2 4 6 8 10 12 14 16 18 20 22

Estrus

Estrus


Progestin

Preg Rate = 21%

PGF

• If a progestin is used late in the cycle, the CL regresses and progesterone levels drop

• The progestin maintains progesterone levels to inhibit estrus and ovulation

• Extended follicular wave length and lower pregnancy rates

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Synchronizing a new follicular wave

0 2 4 6 8 10 12 14 16 18 20 22

Estrus

Estrus


Progestin

GnRH PGF

• GnRH injection at start of progestin treatment starts a new follicular wave

• Fertility is restored

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Progesterone must decrease before AI

Functions• Allows increased estradiol• Permits estrous expression• Permits ovulation

Remove progesterone sources1. Remove CIDR2. Regress corpus luteum

(Prostaglandin F2α)

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PGF2α regresses the CL at certain stages of the cycle


0 2 4 6 8 10 12 14 16 18 20 0

NO

YESEstrus Estrus

• Regresses CL on days 6-16 of estrous cycle (estrus=day 0)

NO

• No effect on early CL or CL that is already regressing

• No effect on non-cycling cattle

• No jump start for anestrous cows or prepubertal heifers

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Increased estradiol before AI is essential

Functions• Estrus• Sperm transport• Induces the signal for ovulation

(LH)

Synchronize a follicular waveGonadotropin releasing hormone (GnRH)

Drop in progesteroneProstaglandin F2α +CIDR removal



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Effect of estrus on pregnancy rate• Cows that display estrus before FTAI have higher pregnancy rates

• Estrus: ~50-90%

• No estrus: ~20-40%

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GnRH injection leads to ovulation of a dominant follicle

0 2 4 6 8 10 12 14 16 18 20 22

Estrus

Estrus


CIDR

GnRH PGF

• Can be used in an estrous synchronization protocol to synchronize estrus and ovulation

• Allows a new follicular wave to begin

• Synchronizes follicular growth

YESYES NO

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GnRH induced synchronization of follicular waves

NO

0 2 4 6 8 10 12 14 16 18 20 0

Estrus


Estrus

• Only dominant follicles ovulate in response to GnRH

• Before dominance is established, follicles will not respond to GnRH

• If no response, a follicular wave is not synchronized

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A surge of LH is required for ovulation

Functions• Ovulation• Oocyte maturation• CL formation

Induce ovulatory LH surgeGonadotropin releasing hormone

(GnRH)

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GnRH at AI leads to ovulation of the dominant follicle

0 2 4 6 8 10 12 14 16 18 20 0

Estrus


AI + GnRH

• Cows that express estrus before AI have an endogenous LH surge and ovulation

• Animals that have not expressed estrus rely on GnRH injection to induce an LH surge and ovulation

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Rise in progesterone is necessary for pregnancy

Functions• Promotes embryo growth• Maintains pregnancy

Synchronize ovulation of a mature follicle to form a quality

corpus luteum



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Progesterone supports embryo growth

Fertilization (Day 1) Day 3 ~Day 6 ~Day 8

Day 16

• Progesterone promotes embryo elongation and pregnancy maintenance

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Putting it all together

Ovulation to GnRH-1

Pregnancy Rate

Estradiol at AI

Progesterone after AI

Progesterone at PGF

Atkins et al. (2013)

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Take home message• Specific hormonal changes during the estrous cycle are required for

pregnancy to be established.

• Estrous synchronization protocols mimic the preceding hormonal changes.

• Fixed-time AI protocols require synchronization of a follicular wave and controlled timing of progesterone decline.

• Therefore, estrous synchronization products must be administered at the proper dose and at the correct time.

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Take home message• Cycling and noncycling females are present at the start of the breeding

season. Consequently, a progestin-based protocol should be used.

• Nutrition, health, and reproductive management are links in a chain of reproductive success. Any weak area will break the chain and reduce pregnancy rates.

• Attention to detail is critical for success.

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Thank you!!

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Physiology of the Estrous Cycle