Lecture 4: The Estrous Cycle and Synchronization

bovine estrous cycle

• estrus = day 0

• metestrus = days 1-4 → follicles growing to become dominant

• diestrus = days 4-18 → functional CL, trying to decide if pregnant or not

• proestrus = day 19 → behavioral estrus → release of PGF2alpha that lyses the CL and allows dam to come back into pro-estrus → heat again

estrus

• sexual receptivity of the female → standing to be mounted

• follicular maturation

• peak estradiol levels

• clear vaginal mucus discharge = bulling

• uterus toned

• increased restlessness and bellowing, decreased rumination, appetite, and milk production

metestrus

• final follicular maturation

• ovulation

• formation of early CL → progesterone secretion

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

diestrus

• period of corpus luteum dominance through luteolysis → not pregnant and release of PGF2alpha

• uterus not toned (flaccid)

proestrus

• progesterone levels decreasing

• estradiol levels increasing → mounters

what hormonal changes occur during the follicular phase and lead to ovulation?

• decreased P4 (progesterone) → allows phase to begin

• FSH → stimulates follicular development

• growing follicles → increased E2 (estrogen)

• GnRH surge → LH surge → ovulation

• after ovulation → Cl develops and P4 increases

how does progesterone change throughout the estrous cycle?

• metestrus → increases as CL develops

• diestrus → high because of functional CL

• late diestrus → luteolysis occurs so Cl regresses

• proestrus → P4 rapidly decreases

• estrus → P4 is low

what happens during the 1st follicular wave of the estrous cycle?

• recruitment → selection → dominance → atresia

• P4 is high so it prevents the LH surge and dominant follicle cannot ovulate so atresia occurs

what happens during the 2nd follicular wave in a cow with 3 follicular waves?

• occurs during diestrus while P4 is still high

• follicles go through recruitment → selection → dominance

• dominant follicle develops and becomes atretic because P4 prevents LH surge

what happens during the final follicular wave of the estrous cycle?

• final wave begins before luteolysis

• follicles undergo recruitment → selection → dominance

• luteolysis occurs and P4 decreases

• low P4 removes the inhibition of LH surge so dominant follicle continues growing → ovulation

puberty

• generally seen when the heifer reaches 50-65% of her adult body weight

• influenced by genotype

• influenced by body weight and nutrition

• beef 600-650 pounds (9-12 mo)

• dairy 700-750 pounds (10-15 mo)

• zebu 24-36 mo

what is a major factor for the onset of puberty?

body weight

non-seasonal polyestrus

• length → 17-24 days, cows are 21 days, heifers are 20 days

• estrus (receptivity) → 6-30 hours

• ovulation (spontaneous) → 2-20 hours after the end of estrus

beef herd breeding

• most bull bred

• desire light calving intervals → maximize grasses, forages, and favorable environmental conditions

• AI on certain farms → depends on estrus detection

dairy herd breeding

• most practice AI

• not many bulls

• year round calving

• follow AM-PM rule for breeding → based upon estrus detection

estrus detection

effective heat detection is of paramount importance

dairy herd heifer breeding

• bred at 14-15 months of age → proper heifer development, meet proper weights at proper times

• calve at 22-24 months of age

• enter milking herd

dairy cattle

• calf is born → removed at birth, fed colostrum and hand-reared

• cow enters milking herd

• voluntary waiting period

• then cow enters breeding group

what is the voluntary waiting period for dairy cattle?

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

• typically 45-65 days

• cows should be observed in heat, inseminated, conceive

• once heat is detected and the animal is bred, this is days to first service

dairy monitoring parameters

• used to get a picture of the herd’s reproductive history

• used to identify areas where improvements can be made

• monitor progress

• a myriad of statistics are available from dairy computer records

what is it called once heat is detected and the animal is bred?

days to first service

days to first service

• period of time from calving to first breeding

• herd average should be 75 days

• gives picture of heat detection

• or gives picture of anestrus

if days to first service is increased, then what does that mean?

either animals are not cycling or not being detected cycling

what is conception rate on the first service?

should be 40% or greater (how many get pregnant the first time they’re bred)

if both estrus detection and conception rates are adequate, then

few cows should have extended days open

reproductive efficiency in dairy cattle is poor due to

low AI service rates and poor conception rates

service rate is poor due to

• inadequate estrus detection

• poor expression of estrus → heat, concrete, feet, and legs

• high number of anovulatory cows during early lactation → multifactorial

conception rate is poor due to

• high incidence of EED during early gestation

• estrus detection errors → insemination when not in heat

• semen quality

• AI technique

• bull fertility

• milk production and nutrition

• environmental influences

• age

• genetics

equation of reproduction

• A x B x C x D = % pregnant

• A = herd members detected in heat and inseminated

• B = AI technique

• C = fertility level of herd

• D = semen fertility level

how many services are required to achieve a pregnancy?

some say less than 2.5 in pregnant cows

what did Milo Wiltbank’s research show?

the duration of estrus of a cow milking 60 pounds a day has an average time of 14.7 hours in standing heat

pregnancy rate equation

PR = EDR x CR

enhancing estrus detection

• proper cow ID

• separate into and observe small groups for heat

• 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

estrus detection aids

• tail-head pressure monitors

• estrus detector animals → chin ball markers, gomer/teaser bulls, androgenized heifer or steer

• pedometry

• vaginal fluid electrical resistance decreases during estrus

what is this?

tail head paint

what is this?

chin ball

improving service rate

• estrus control or synchronization → OvSynch and CIDR are primary ones used in dairy industry (timed AI protocols)

• service rate would now be 100% because all eligible cows were bred → removed heat detection from the equation

estrus synchronization and control

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

• synchronization implies the coordinated control of estrus in multiple animals

• inherent fertility of estrus is not improved by control or synchronization

advantages of control and synchronization of estrus

• reduce the window of time when insemination is appropriate

• 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

general principles of control and synchronization of estrus

• shorten the luteal phase

• lengthen the luteal phase

estrus synchronization methods

• prostaglandin programs

• progestin programs

• luteal and follicular synchronization programs

estrus synchronization method choice is based on

• cycling vs not cycling

• ability to detect estrus

• facilities and labor to handle cattle

• drug costs

• program interval from inception to breeding

prostaglandin F2alpha products

• lutalyse (dinoprost tromethamine) or estrumate (cloprostenol sodium)

• causes regression of the CL

• estrus displayed in 2-5 days

• only works days 6-17 of the estrus cycle

• after day 17, it doesn’t influence onset of estrus

most common methods of estrus synchronization with prostaglandins

• one injection

• two injections 11-14 days apart

prostaglandin F2alpha injection

• no effect from days 0-6

• window to shorten estrus cycle is day 6-16 → effect due to injection

• day 16-21 → effect but not due to injection

response rate of one PGF2alpha injection

65%

second PGF2alpha injection

• most animals will respond

• good choice for beef cows and heifers

heat detect then PGF2alpha

• most animals will respond

• lower drug costs

• longer heat detection requires more labor

proestagens

• extend luteal phase

• suppresses gonadotropin release (FSH and LH)

• melengestrol acetate (MGA) → fertility is poor at the initial estrus, but synchrony is maintained through one cycle

• controlled intravaginal drug-release (CIDR)

melengestrol acetate

• don’t breed on first heat after withdrawal of MGA

• used in beef heifers

• feed MGA for 14 days then withdraw feed and inject PGF2alpha from days 16-18

CIDR program

• implant on day 0

• implant removed and PGF2alpha injection on day 7

• days 9-11 observe for estrus → inseminate 12 hours after onset of estrus

GnRH

• causes the release of FSH and LH

• FSH → synchronous emergence of a new follicular wave

• LH → ovulation

follicular synchronization

• inducing a synchronous emergence of a new follicular wave with GnRH

• cows must be handled more

• more expensive than other synchronization programs

• OvSynch → no estrus detection needed

• heavy use in dairy industry

• all cows undergo timed insemination (TAI)

OvSynch timeline

• GnRH at day 0 with new follicular wave

• day 7 → PGF2alpha with luteolysis

• day 9 → GnRH given to induce LH surge and ovulation

OvSynch

an attempt to refine hormonal synchronization for appointment breeding/timed insemination (TAI)

pregnancy rate = estrus detection rate x conception rate

• PR may be improved by inflating EDR to 100%

• there are now many variations of OvSynchs

traditional OvSynch

• injection of GnRH to grow follicles

• 7 days and then injection of PGF

• 2 days and then injection of GnRH to cause ovulation

• breed 16 hours later

follicular synch combined with CIDR = CIDR synch

• day 0 → CIDR implant only + GnRH

• day 7 → CIDR out + PGF2alpha

• in between 7 and 9 → heat detection ± calf withdrawal in beef cattle

• day 9 → GnRH

• day 10 → inseminate 16 hours after GnRH injection