4. Physiology Review: The Estrous Cycle and Synchronization

When is estrus?

day 0

When is metestrus?

days 1-4

What is happening during metestrus?

time at which the follicles are growing and becoming dominant

When is diestrus?

days 4-18

What is happening during diestrus?

functional corpus luteum

What happens at day 18 during diestrus?

cow will decide if she’s pregnant or not (if not, will have an endogenous release of PGF2a which will lyse the CL and allow her to come back into proestrus and start displaying heat again)

When is proestrus?

day 19-behavioral estrus

sexual receptivity of the female

estrus

What is the most definitive sign of estrus in cows?

standing to be mounted

During estrus, there are peak levels of what hormone?

estradiol (estrogen)

What else occurs during estrus?

F
C
U
I
D

• follicular maturation

• clear vaginal mucus discharge = bulling

• uterus toned

• increased restlessness and bellowing

• decreased rumination, appetite, and milk production

What occurs during metestrus?

F
O
F
B

• final follicular maturation

• ovulation (technically occurs between estrus and metestrus)

• formation of early CL

• bleeding off of 1-3 days after estrus (not all cows do this)

What hormone is beginning to be secreted during metestrus? Why?

progesterone; formation of early CL

What happens to estrogen after ovulation?

levels drop off

What is happening during diestrus?

P
U

• period of corpus luteum dominance through luteolysis

• uterus not toned (flaccid)

What hormone dominates during diestrus? Why?

progesterone; corpus luteum dominance

During proestrus, what are the hormones doing?

progesterone is decreasing and estradiol is increasing (mounters)

What makes up the follicular phase of the estrous cycle?

proestrus and estrus

What will cause ovulation and lutenization of the follicle?

LH

What is necessary to cause the LH peak?

need a high enough level of estrogen

What is the effect of estrogen on FSH and LH?

positive feedback loop to causes them to both increase

What is considered to be the most important part of estrus?

LH spike

What hormone makes the follicle grow? When is this most important?

FSH; during proestrus and not as necessary during estrus

What is the overall hormone profile during diestrus?

progesterone dominates

What is the overall hormone profile during proestrus?

progesterone is decreasing, estrogen, LH, and FSH are increasing

What is the overall hormone profile during estrus?

estrogen predominantes, LH spike, FSH is high, progesterone is low (basically non-existent)

What is the overall hormone profile during metestrus?

progesterone beginning to increase

When does the first follicular wave occur?

either as progesterone is rising or during peak progesterone secretion (usually metestrus)

What will happen to follicles selected during the first follicular wave?

they will become atretic

In cows with three follicular waves, though most have two waves, when does the second wave occur?

during peak progesterone secretion (diestrus)

What will happen to the follicles selected during the second follicular wave?

they will become atretic

When does the third follicular wave occur?

it is initiated before luteolysis

What is the result of the third follicular wave?

a dominant follicle that will ovulate

Which follicles during the third wave will become eligible for ovulation?

only those follicles in the growing phase during luteolysis

What is true for the second wave in 2 wave cows?

it will result in a dominant follicle that will ovulate

True or false: While age is a factor for the onset of puberty, it is more determined by nutrition status and physical size.

true

When is puberty generally seen in a heifer?

when she reaches 50-65% of her adult body weight

What is puberty influenced by?

G
B
N

• genotype

• body weight

• nutrition

What are examples of genotype influences on puberty?

I
C
B
H

• inbred later onset

• cross-breeds sooner onset

• Bos Taurus earlier than Bos Indicus

• heifers from bulls with large scrotral circumference (the larger the testicles of the sire, the earlier the offspring will go into puberty)

What is a major factor when it comes to the onset of puberty?

appears to be dictated by body weight of breed

What is the effect of nutrition on puberty?

underfed will lead to delayed puberty

True or false: Heifers on various planes of nutrition will reach puberty at different ages, but at similar relative stages of physical development.

true

At what typical age range do beef cattle come into puberty?

600-650 pounds (9-12 months)

At what typical age range do dairy cattle come into puberty?

700-750 pounds (10-15 months)

At what typical age range do Zebu (Bos indicus) cattle come into puberty?

24-36 months

How is the bovine estrous cycle described?

non-seasonal polyestrus

What is the typical length of the bovine estrous cycle? How long in cows? Heifers?

17-24 days; 21 days; 20 days

How long is bovine estrus (receptivity to breeding)?

6-30 hours

How long is estrus in beef cattle?

20 hours

How long is estrus in dairy cattle?

15 hours

How long is estrus in Zebu cattle?

< 10 hours

What type of ovulation do bovine have? When does it occur?

spontaneous; 2-20 hours after the end of estrus (average ~10 hours)

In beef herds, how is most breeding done?

bull bred

What type of calving intervals are desired in beef herds? How can this be done?

light calving intervals; maximize grasses, forages, and favorable environmental conditions

True or false: There can be AI on certain beef farms, but it depends on estrus detection.

true

In dairy herds, how is most breeding done? Why?

AI; want milk year round and bulls can be dangerous

What type of calving is desired in dairy herds? Why?

year round; want milk year round

What rule is followed when it comes to breeding with dairy herds? What is it based upon?

AM-PM rule; estrus detection (if showing heat in the morning, will breed them in the afternoon and vice versa)

What is the formula for pregnancy rate?

PR = estrus detection rate (EDR) X conception rate (CR)

True or false: Estrus detection rate has a huge effect on pregnancy rate.

true

What changes are seen in the behavior of cattle when they are in estrus?

W
M
T
M
S

• walking is increased

• mucous discharge becomes watery

• time spent eating decreases

• mounting activity increases

• stranding to be mounted increases

What is of paramount importance when it comes to estrus detection?

effective heat detection

When are dairy heifers bred? Why?

P
M

~14-15 months of age

• proper heifer development

• meet proper weights at proper times

If bred at the correct time, when will dairy heifers usually calve? What happens after?

22-24 months of age; enter the milking herd

What happens once the dairy calf is born? What happens to the cow?

R
F

• removed at birth

• fed colostrum and hand-reared

she enters the milking herd

time period after calving in which the cow will not be bred

voluntary waiting period (VWP)

What is the length of the voluntary waiting period (VWP) determined by?

P
E
P

• physiology

• economics

• producers

How long is the voluntary waiting period (VWP) typically?

45-65 days

What happens after the voluntary waiting period?

cow enters the breeding group

What is the purpose for dairy monitoring parameters?

G
I
M
M

• get a picture of the herd’s reproductive history

• identify areas where improvements can be made

• monitor progress

• myriad of statistics are available from dairy computer records

Once cows are past the voluntary waiting period, what should be done with the cows? How should this all be done?

O
I
C

• observed in heat (heat detection efficiency)

• inseminated

• conceive

in an efficient manner

once heat is detected and the animal is bred (period of time from calving to first breeding)

days to first service

What do you want to days to first service to be as close to? Why?

voluntary waiting period; want her coming into heat right when we take her off of her voluntary waiting period

What should the herd average days to first service be?

75 days

What information does the days to first service give?

picture of heat detection or picture of anestrus

What does it mean if the days to first service is increased?

either the animals are not cycling or not being detected cycling

What should the conception rate on the first service be?

40% or greater

What should be the case if both estrus detection and conception rates are adequate?

few cows should have extended days open

Why is reproductive efficiency in dairy cattle poor?

low AI service rates and poor conception rates

What are reasons for poor service rates?

I
P
H

• inadequate estrus detection (ALMOST ALWAYS THE #1 REASON)

• poor expression of estrus (heat, concrete, feet, and legs)

• high number of anovulatory cows during early lactation (multifactorial)

What is the effect of heat stress on service rate?

drastically affects cattle and can drop pregnancy rates greatly (~15%)

What are reasons that conception rate is poor?

H
E
S
A
B
M
E
A
G

• high incidence of early embryonic death during early gestation

• estrus detection errors (insemination when not in heat)

• semen quality

• AI technique

• bull fertility

• milk production and nutrition

• environmental influences (temperature)

• age

• genetics

What is the equation of reproduction?

herd members detected in heat and inseminated (%) X AI technique X fertility level of herd (cow) X semen fertility level = % pregnant

How many services are required to achieve a pregnancy?

some say less than 2.5 in pregnant cows

Research has shown that the duration of estrus of a cow milking 60 pounds a day has an average time of what in standing heat? What happens to that time as milk production increases?

14.7 hours; the time is greatly decreased

Where are most dairy cows when it comes to milk production and time in standing heat?

100-110 pounds in standing heat for 5.1 hours and 110-120 pounds in standing heat for 2.8 hours

How can we increase pregnancy rates?

enhance estrus detection

What are ways to enhance estrus detection?

P
S
E
P
M
T
U
U

• proper cow ID

• separate into and observe small groups for heat (introducing a new animal increases estrus activity)

• ensure proper nutrition

• provide good footing

• maintain/assess accurate records

• train/assign heat detection to one person

• use estrus synchronization programs

• use estrus detection aids

What are examples of estrus detection aids?

T
E
P
V

• tail head pressure monitors (kymar, lotter tickets, tail paint, chalk, heatwatch)

• estrus detector animals (chin ball markers, gomer/teaser bull, androgenized heifer or steer)

• pedometry (can be used as an adjunct)

• vaginal fluid electrical resistance decreases during estrus (not practical yet)

What are the primary estrus control or synchronization protocols used in the dairy industry?

OvSynch and CIDR Synch

What type of protocols are OvSynch and CIDR Synch?

timed AI protocols (TAI)

What would the service rate be with timed AI protocols? Why?

100%; in essence, we have removed heat detection from the equation

exercised such that the onset of estrus may be hastened or delayed

control of the estrous cycle

implies the coordinated control of estrus in multiple animals

synchronization

True or false: The inherent fertility of estrus is not improved by control or synchronization and may in fact be reduced by some treatment schemes. You are not making the individual animal more fertile, just controlling the cycle.

true

When during the bovine estrous cycle is there a corpus hemorrhagicum?

between days 2 and 5

When during the bovine estrous cycle is there a functional corpus luteum? What is important to know about this time?

between days 6 and 17; longest part of the cycle and easiest to control

When during the bovine estrous cycle does the uterus naturally release PGF2a?

between days 16 and 17

Between what days of the bovine estrous cycle is there follicular development in the absence of the CL?

between days 17 and 21

What are the advantages of the control and synchronization of estrus?

R
R
E
A
P

• reduce the window of time when insemination is appropriate (less likelihood of missing the time when we should be inseminating)

• reduce the time spent on estrus detection

• enhance estrus detection potential due to anticipation by the detector and increased display of estrus due to synchrony of estrus

• allow scheduling of breedings/inseminations

• provide for synchrony essential to most embryo transfer programs