Lecture 4- Bovine 1 Physiology Review – The Estrous Cycle

Voluntary Waiting Period definition

Post‑calving interval during which cow is not bred; typically 45-65 days

Days to first service definition

Time from calving to first breeding; herd average ~75 days

Factors increasing days to first service

Poor estrus detection, anestrus, inadequate nutrition, poor records, heat stress

Estrus detection management

Proper cow ID, small group observation, good footing, nutrition, accurate records, trained personnel

Estrus detection aids

Tail paint, chalk, Kamar patches, Estrotect, HeatWatch, pedometers, teaser animals

Main hormones in estrus control

PGF2α (lyses CL), progesterone/progestins (extend luteal phase)

Estrus control vs follicular synchronization

Estrus control manipulates luteal phase; follicular synchronization uses GnRH to synchronize follicular waves

Advantages of TAI

Eliminates heat detection; increases service rate to 100%; improves scheduling

GnRH physiologic actions

Causes LH + FSH release; induces ovulation; synchronizes new follicular wave

Estrous cycle length

17-24 days; cows ~21 days; heifers ~20 days

Estrus duration

6-30 hours; beef ~20h; dairy ~15h; zebu

Ovulation timing

2-20 hours after end of estrus (~10h average)

Estrus signs

Standing to be mounted (definitive); clear mucus; uterine tone; restlessness; bellowing; ↓ rumination/appetite/milk

Metestrus events

Final follicular maturation; ovulation; early CL formation; metestrus bleeding (1-3 days)

Diestrus events

CL dominance; high progesterone; uterus flaccid

Proestrus events

↓ progesterone; ↑ estradiol; mounting behavior

Estrus behavior: cow

Increased locomotion/vocalization; grooming; mounting; standing heat

Estrus behavior: mare

Tail flagging; urination stance; winking; presents hindquarters

Estrus behavior: ewe

Ram seeking; urination; immobile stance

Estrus behavior: sow

Mild restlessness; immobile stance

Estrus behavior: bitch

Roaming; immobile stance; tail deflection; affectionate rubbing

Estrus behavior: queen

Vocalization; crouching; rolling; lordosis; tail deviation

Estrus behavior: llama

Acceptance; cushing

Follicular waves

2-3 waves per cycle; waves during high progesterone become atretic

First follicular wave

Occurs during rising/peak progesterone; dominant follicle becomes atretic

Second follicular wave

Occurs during peak progesterone; dominant follicle becomes atretic

Third follicular wave

Initiated before luteolysis; dominant follicle ovulates

Puberty onset

Heifers reach puberty at 50-65% adult weight

Puberty influenced by genotype

Inbred later; crossbred earlier; Bos taurus earlier than Bos indicus

Puberty influenced by nutrition

Underfed delayed; beef 600-650 lb; dairy 700-750 lb; zebu 24-36 months

Estrus cycle: non‑seasonal polyestrus

Cows cycle year‑round

Estrus cycle phases

Estrus (0), Metestrus (1-4), Diestrus (4-18), Proestrus (19-21)

Transrectal palpation: Estrus

T3 tone; clear cohesive mucus

OVD (Day 1-2)

T2 tone; ovulation depression

CH1 (Day 2-3)

T1 tone; soft CL

CH2 (Day 3-5)

T1 tone; soft CL 1-2 cm

CH3 (Day 5-7)

Flaccid uterus; soft CL >2 cm

CL3 (Day 8-17)

Flaccid uterus; firm CL >2.5 cm

CL2 (Day 18-20)

T1 tone; firm CL 1-2 cm

CL1 (Estrus-mid next cycle)

T1 tone; firm CL

Beef herd breeding

Mostly bull bred; calving interval focus; estrus detection varies

Dairy herd breeding

Mostly AI; AM‑PM rule; year‑round calving; high reliance on estrus detection

Pregnancy rate equation

PR = EDR × CR

First service conception rate

Should be ≥40%

Services per conception

Goal

Heat stress effect

Conception rate decreases in summer months

Milk production effect

Higher milk → shorter estrus duration (e.g., 110 lb milk → 2.8h standing heat)

Estrus detection improvement

New animal introduction increases activity; synchronization programs help

Estrus synchronization advantages

Reduced breeding window; reduced detection time; improved scheduling; essential for ET programs

Estrus synchronization principles

Shorten luteal phase (PGF2α); lengthen luteal phase (progestins)

PGF2α products

Lutalyse (dinoprost); Estrumate (cloprostenol)

PGF2α action

Lyses CL; estrus in 2-5 days; only effective Days 6-17

One PGF injection

~65% respond

Two PGF injections

11-14 days apart; >90% respond; Pre‑Synch program

Progestin programs

Extend luteal phase; suppress LH/FSH

MGA program

Feed 14 days; infertile estrus after withdrawal; PGF injection; fertile estrus 16-18 days later

CIDR program

Insert 7 days; remove + PGF; detect estrus; AI 12h after onset

GnRH in synchronization

Induces new follicular wave; ovulates dominant follicle; used in OvSynch

OvSynch steps

Day 0 GnRH → Day 7 PGF → Day 9 GnRH → TAI 16-20h later

Co‑Synch

Second GnRH + TAI at same time

Heat‑Synch

GnRH → PGF → ECP → TAI

Pre‑Synch + OvSynch

PGF → 14 days → PGF → 12-14 days → OvSynch

CIDR‑Synch

GnRH + CIDR → 7 days → PGF + remove CIDR → 48h → GnRH + TAI

Timed AI benefit

Eliminates heat detection; ensures all eligible cows bred; increases service rate to 100%