This document summarizes research on oestrus synchronization in cattle. It discusses how accurate detection of oestrus is a management problem, with around 50% of cycles going undetected. This limits insemination opportunities and leads to longer calving intervals and economic losses for farmers. The document then reviews research on using oestrus synchronization to group cows for insemination during specific periods, improving reproductive efficiency. It examines factors that affect follicular development and the ovarian cycle in cattle, such as energy balance and number of follicular waves per cycle.

1. Oestrus Synchronization in Cattle: A New Approach to an Old
Problem1
Dr. Punimin Abdullah2
Massey University, New Zealand
Introduction
There is an increased of desire and need to utilise economically sound reproductive
programs as part of farm management to produce a calf each year, especially in large
commercial dairy herds. A cow has to conceive by day 85 postpartum to maintain high
reproductive performance of the herd. Reproductive performance of cows affects the
efficiency of milk production in the herd because of its influence on the calving pattern,
lactation length and culling rate.
The ultimate aim of reproductive programs is to shorten the calving interval so that the 365
days calving interval required can be maintained. Mating can only take place when cow is
detected on oestrus. Undetected oestrus will limit the number of insemination, resulting in
increased days open and economic loss. Cows are inseminated at times when conception is
likely to occur if oestrous detection is accurate.
Accurate detection of oestrus is a serious management problem. Approximately 50% of
oestrus periods are not detected. This severely limits the number of insemination
opportunities, resulting in increased days open and economic loss. Errors in accuracy of
oestrus detection result in many cows being inseminated at times when conception will not
occur. Factors affecting oestrus activity include stage of the oestrous cycle, time of day of
observation, number of cows in oestrus, environmental temperature, and type of footing.
To achieve shorter calving intervals and to increase reproductive efficiency, oestrous
synchronisation is used as an aid in the reproductive management, in order to optimise
reproductive performance. Poor oestrous detection can limit optimal reproductive
1 Paper presented to DVSAI
2 Email: P.Abdullah@massey.ac.nz

2. performance in large commercial dairy herds. Cows not detected in oestrus by 60 days
postpartum have a lower conception rate and a higher risk of being removed from the herd
for failing to conceive.
The beneficial and desirable features of oestrus synchronisation
To achieve a 365 day calving interval, in artificially bred herd the cows have to conceive
by 85 days postpartum. Oestrus must be identified before they can be mated. Undetected
and falsely detected oestrus in cattle results in missed and untimely inseminations. This
lead to longer calving interval and farmers will lose from:
a. unexploited potential of milk and of calf production caused by prolonged calving
intervals,
b. expenditures on excessive replacement heifers and on infertile inseminations, and
c. a reduced rate of genetic progress.
Oestrous synchronisation can increase the usage of artificial insemination during a
designated period and contribute to improved reproductive efficiency. High producing
dairy cows show less obvious and intermittent mounting behaviour and thus are difficult to
detect in oestrus. Oestrous synchronisation can reduce management problems associated
with daily monitoring to detect oestrus in herds of cows, especially if there are suckled
calves and the presence of anoestrus cows.
The use of oestrous synchronisation, as aid in reproductive management, has it advantages
and disadvantages. In today’s large commercial herds and to reduce the escalating milk
production cost, the benefits of using oestrous synchronisation are as follows:
1. To enable the use of artificial insemination and attain genetic progress.
2. To facilitate reproductive management in seasonally calving herds.
3. To facilitate reproductive management in non-seasonally calving herds.
4. To increase the efficiency of oestrous detection and reduce labour cost.
5. To facilitate embryo transfer programmes.

3. To make use of oestrous synchronisation to its maximum advantage, the oestrous
synchronisation programmes should have these features:
a. high response rates to treatments initiated at any stage of the oestrous cycle,
b. tight synchrony in the time of oestrus and time of ovulation,
c. normal fertility at the regulated ovulation, and
d. normal return to oestrus and fertility at repeat services.
Oestrous cycle control
At present, it is not possible to achieve a high degree of fertility from fixed-time
insemination until ovulation control in cattle is fully understood. The major cause of low
fertility following the use of fixed-time insemination is due to poor synchronisation of the
timing of ovulation. Ovulation occurs close to the oestrous phase, and in cows, it occurs
about 12 hours after the end of oestrus.
Regulation of the oestrous cycle is an alternative to routine detection. The purpose is to
control the time of oestrus and, therefore, the time of ovulation. This would make it
possible to increase the percentage of cows inseminated during the desired breeding period
postpartum. If oestrous detection is not eliminated, at least the labour required for routine
daily observations for oestrus could be reduced by grouping animals and concentrating
observation efforts during periods of expected oestrus. The economic benefit of increasing
the percentage of cows that conceive during the optimum period and reducing the labour
required for routine observations must be greater than the costs of the program to regulate
the cycles.
Ovarian Follicular Development
Non-pregnant cattle
Ovarian follicular development in cattle occurs in waves and its dynamics proceeds through
an integrated stages of follicular recruitment, selection and dominance, whereby the follicle
undergoes phases of growth, maintenance and atresia. One or two anovulatory follicles
develop before the ovulatory follicle. Once the growth of a primodial follicle is initiated, it

4. is a continuos process until the follicle either ovulates or becomes atretic. Transrectal real-time
ultrasonography has confirmed that most bovine oestrous cycle have either two or
three waves. The onset of the first wave is detected as a group of 4 mm follicles just before
the day of ovulation. The second wave emerges at about day 10 postovulation and is
followed by another wave at day 16 for three-wave cycles. The corpus luteum begins to
regress earlier in 2-wave cycles (day 17) than in 3-wave cycles (day 20). Therefore, three-wave
cycles have a longer luteal phase (22.8 days) and a longer interovulatory interval than
do two-wave cycles (20.4 days). The ovulatory follicle originates from the final wave. Final
maturation and ovulation of the dominant follicle occur following luteal regression and a
preovulatory gonadotropin surge.
Bovine follicular wave dynamics in
2-wave oestrus cycle
18
16
Follicle diameter (mm)
14
12
10
0 2 4 6 8
-1 1 3 5 7 9 11 13 15 17 19 21
Days after ovulation
The first follicular wave begins with the recruitment of a cohort of follicles from which a
single follicle continues to grow while the others undergo atresia. The selected follicle
continues to grow to reach a diameter of about 15 mm (dominance) and remains at
maximum size for 2 - 3 days before regression, and there is then a new wave of follicle

5. development. The selected follicle exerts its dominance through an inhibition of additional
follicles recruitment for the next wave. Dominant follicle remains active until
approximately days 10-11 of the oestrous cycle and then the dominance of the first-wave
follicle is arrested. Thus, recruitment phase of the second follicular wave begins.
Maturation of the second or third dominant follicle of the oestrous cycle in 2-wave or 3-
wave cycles is associated with regression of the corpus luteum and the follicle is ovulated
after luteolysis.
Bovine follicular wave dynamics in
20
15
10
5
0
-1 1 3 5 7 9 11 13 15 17 19 21 23 25
Follicle diameter
(mm)
The postpartum period
3-wave oestrus cycle
Days after ovulation
Emergence of the first follicular wave postpartum ranged from 2 to 7 days after calving in
dairy cattle. Moreover, about 74% of the dominant follicle of the first postpartum wave
ovulated on mean day 27 (range from 12 to 58 days). First ovulation was not accompanied
by oestrous behaviour in 94% postpartum cows, and the length of the first postpartum

6. interovulatory interval varied depending on when the first follicle which was destined to
ovulate and emerged. The first postpartum interovulatory interval was short (mean 11.2
days) in approximately 25% of dairy cows, was of normal duration (mean 20.6 days) in
another 25%, and was long (mean 30 days) in the remaining 50% of cows. The intervals
from calving to first ovulation are 21 days (range 10-55 days) and to first oestrus is 59 days
(range 17 - 139 days). In the majority of cows (78%), ovulation occurred from the second
and subsequent postpartum follicular wave.
Short postpartum anovulatory periods (around 14 days) were followed by normal-length
cycles, whereas longer postpartum anovulatory periods (21 to 25 days) were followed by
short cycles (<14 days). Short cycles were associated with short luteal phases, a smaller
CL, and lower circulating progesterone concentrations.
Factors Affecting Follicular Development
Energy Balance
The reproductive performance of postpartum cows is often limited by the intake of dietary
energy. Negative energy balances that occur in lactating, postpartum beef and dairy cows
decrease luteinizing hormone (LH) secretion and delay return to oestrus. Amplitude of LH
pulses as well as diameter of the dominant follicle increased with positive energy balance.
Lactating cows placed in negative energy balance before ovulation had preovulatory
follicles that grew more slowly than follicles in cows that were in positive energy balance.
Feeding diets that contain fats and that are higher in energy may partially alleviate negative
balance and stimulate ovarian function. Feeding of additional dietary energy stimulated the
development of follicles. This is associated with the movement of follicles into larger size
classes. High milk production causes a decrease in energy balance, as the cows require
several weeks for energy intake to increase to compensate for additional milk energy
output.

7. The Post Partum Period
Follicular development was first characterized by the development of follicles <8 mm in
diameter. Then, one of these follicles continued development and become the dominant
follicle on day 11 (range 5 - 39 days) postpartum. The first dominant follicle ovulated, or
became cystic.
The duration of the first ovarian cycle was affected by postpartum interval. Cows that
ovulated within 9 days of calving had either normal or long first cycles (>25 days); cows
ovulating after day 20 had short (9 - 13 days) first cycles. Cows with short cycles
developed only one dominant follicle, which became the ovulatory follicle; cows with
normal cycles had mainly 2 dominant follicles while those with long cycles had either 3, 4
or more dominant follicles.
Heat Stress
Low fertility in cattle induced by heat stress is a multifactorial problem, because
hyperthermia of various organs and tissues results in diverse functional alterations and
impairments. Recent studies have indicated that ovarian follicles are susceptible to thermal
stress. The first-wave dominant follicle in heat-stressed, lactating cows was found to be
smaller in diameter and to contain less fluid on day 8 of the cycle. Developing small
follicles that damage by heat stress may ovulates an infertile oocyte or develop
subfunctional corpora lutea. A chronic effect of heat stress also was evidenced by lower
concentrations of oestradiol in plasma and follicular fluid. Low concentrations of oestradiol
in heat stress cows caused delay in luteolysis. This was as result of failure of follicular
oestradiol to initiate the series of endocrine events leading to luteolysis.
The length of the luteal phase increased in heat stress cows because of a delay in luteolysis.
The longer luteal phases resulted in more follicular waves per estrous cycle in heat stress
cows. The interovulatory interval is increase with longer luteal phase.

8. Oestrus Synchronization Programs
Prostaglandin (PGF)
Studies showed that the maturity of corpus luteum (CL) at the time of PGF treatment
influenced the luteolytic response. PGF did not effectively induce luteolysis during the first
5 or 6 days following oestrus. However, recent work found that luteolysis and ovulation
were successfully induced if PGF was given twice, on the 4th or 5th day after oestrus.
In cows in which luteolysis is effectively induced by PGF treatment, the ensuing oestrus is
distributed over 6 days. The least variability in interval to oestrus resulted from PGF
treatment during early dioestrus (7 and 8 days after oestrus) and later dioestrus (15 days
after oestrus) than from treatment during the mid-dioestrus (10-13 days after oestrus). This
variation is due to follicular status at the time of treatment. Treatment when the dominant
follicle of a wave is in its late growing or early static phase will result in ovulation of that
follicle within 2 to 3 days. Whereas, treatment after the mid- to late-static phase will result
in ovulation of the dominant follicle of the next wave 4 to 5 days after treatment.
Progesterone and Oestradiol
Progestogen, if given long enough to allow normal regression of the corpus luteum (>14
days), would induce synchronous oestrus, but was associated with lowered fertility.
Lowered fertility after prolonged progestogen treatment (> 14 days) observed has been
attributed to aging of the oocyte within the long-lived oversized follicle. Shorter treatment
periods (8 to 10 days) with progestogens resulted in normal fertility at the controlled
oestrus, but synchrony of oestrus was lower. If administration begins early in the oestrus
cycle, the existing CL might outlive the exogenous progestogens treatment period and
synchronization will not occur. For this reason, it is necessary to incorporate a luteolytic
agent with the short-term progestogen treatment to obtain both efficient synchrony and
normal fertility. Oestrogens are luteolytic when administered to cattle during the early stage
of the oestrous cycle. Progestogen plus oestradiol is highly effective in suppressing
follicular growth and results in the emergence of a new follicular wave at a consistent time

9. after treatment. The interval from oestradiol treatment to wave emergence occurred on
average was 3.1 days. Oestradiol treatment combined with progestogen only effective if
given at the early stage of the follicular wave cycle (day 3 of each wave cycle). The oestrus
synchrony response from such treatment is tighter after the withdrawal of the exogenous
progestogen treatment.
Conclusions
Despite many years of research, a protocol has not been found that will consistently
synchronize oestrus with sufficient precision to permit high levels of success. The
variability in oestrus synchronization response is because the difficulty for veterinarians to
recognize the exact stage of the follicular development and the number of follicular wave
presence at the time of treatment. In practical terms, results of in-herd synchronization
programs are variable and the decision to undertake such a program will depend on the
individual herd management, and on factors other than effectiveness of response or even
simple economics.

10. Current CIDR-B Treatment Programme
1. Lactating Dairy Cow
· Insert CIDR for 7 days
· Inject 2 mg Oestradiol Benzoate or insert 10 mg Oestradiol Benzoate gelatins
capsule at CIDR insertion.
· Inject prostaglandin on Day 7 and remove CIDR. Inseminate to detected oestrus.
· On Day 9, inject cows not showing oestrus with 1 mg Oestradiol Benzoate.
· Inseminate to detected oestrus.
2. Anoestrous Cow
· Insert CIDR for 7 days. Treat cow at 40 days postpartum.
· Inject 1 mg Oestradiol Benzoate 24 hours after CIDR removal.
3. Beef Cow
· Insert CIDR for 7 days.
· Inject 2 mg Oestradiol Benzoate or insert 10 mg gelatins capsule at insertion.
· Inject prostaglandin on Day 6.
· Inject 1 mg Oestradiol Benzoate 24 hours after CIDR removal.