Proceedings, Western Section, American Society of Animal Science
Vol. 56, 2005
Effect of Estradiol Cypionate on conception and pregnancy rates of beef cattle synchronized with a
modified timed artificial insemination protocol1, 2
J. M. Howard*3, R. Manzo3, F. Frago3, D.G. Falk3, J. B. Glaze4, A. Ahmadzadeh3
University of Idaho, Moscow, ID, 4Twin Falls R&E Center Twin Falls, ID
Abstract: Induction of ovulation by GnRH during Introduction
the proestrus period, as occurs in the Ovsynch (OVS) and
Cosynch (COS) breeding protocols [GnRH-7d- Estrus synchronization protocols have been
Prostaglangin F2α (PGF)-2d-GnRH+AI], impair ovarian developed as a management practice to shorten the interval
estradiol (E2) secretion and may reduce fertility. It is from calving to rebreeding, decrease labor costs associated
hypothesized that incorporation of a low dose of estradiol with estrus detection, and increase the implementation of AI.
administered at the time of the second GnRH injection in Despite this fact, less than 5% of the beef herds in the U. S.
the COS would improve conception (CR) and pregnancy use artificial insemination (AI) per year with less than half
rates (PR) in beef cattle. The objective was to determine the of those implementing an estrus synchronization protocol (
effect of incorporating a low dose of estradiol cypionate Wood et al., 2001; Baker et al., 2002).
(ECP), into the COS protocol, on CR and PR in beef cattle. In an effort to further decrease the labor cost
Over the period of two years, 183 cross-bred cows were associated with estrus detection and increase reproductive
used for this experiment. All cows received 25 mg PGF efficiencies, researchers have developed the timed
(i.m.) 14 d before initiation of the COS protocol. Fourteen insemination protocols Ovsynch (OVS; Pursley et al., 1995)
days later, on d 0, 100 ug GnRH (i.m.) was administered and Cosynch (COS; Geary and Whittier, 1998), which were
followed 7 d later by 25 mg PGF (i.m.). On d 9 cows were designed to synchronize ovulation, allowing timed AI of all
paired by age, body condition score (BCS), and body cows without estrus detection (GnRH-7d → PGF2α- 2d→
weight (BW) and were randomly assigned to receive either GnRH→0 to 24 h-Timed AI).
GnRH (100 ug) + 0.25 mg ECP (COS-ECP; n = 90) or A number of studies suggest that induction of
GnRH (100 ug) + vehicle (CON; n = 93) and were ovulation by GnRH during the proestrus period, as occurs
immediately inseminated by a single inseminator. in OVS and COS breeding protocols, impairs ovarian
Pregnancy status was determined via ultrasonography 40 estradiol (E2) secretion and may reduce fertility (Lucy and
and 60 d post-insemination. Data was analyzed by logistic Stevenson, 1986; Stegner et al., 2002)
regression. There was no year by treatment interaction; the Previous research indicated that when estradiol
data was then pooled for further analysis. Conception rates cypionate (ECP®) was incorporated into the OVS
(53.3% COS-ECP vs. 57.0% CON) and pregnancy rates (administered at the time of the second dose of GnRH), the
(92.2% COS-ECP vs. 90.3% CON) did not differ between first service conception rate of the ECP-treated group
treatments. Days postpartum, age, BCS and BW had no tended to improve compared to the control group (68% vs.
effect on the odds ratio of pregnancy. In summary, the 57.5%; Ahmadzadeh et al., 2003). The use of the COS
results indicate that the incorporation of ECP into the COS protocol reduces the number of times an animal will need to
protocol did not improve CR and PR in beef cattle. handled.
We hypothesized that incorporation of a small dose of
Key words: Estradiol Cypionate, Conception Rate, Timed ECP at the time of the second GnRH, in the COS protocol,
AI would improve first service conception (CR) and pregnancy
rates (PR) in beef cattle. The objective was to determine the
Appreciation is expressed to Merial and Pfizer Animal effect of incorporating a low dose of ECP, into the COS
Health Inc. for partial support of this research. protocol, on CR and PR in beef cattle.
The authors also would like to express special thanks to
the University of Idaho’s Nancy M. Cummings Research Materials and Methods
and Extension Center management team, Mr. Heber Stokes,
Chad Cheyney and Brad Williams for the care and Animals and Treatments: This study was conducted at the
management of the animals used in this experiment. University of Idaho’s Nancy M. Cummings Research and
Extension Center in Carmen, Idaho. Over the course of two
years, 183 primiparous and multiparous crossbred beef and CON, respectively; Figure 2). Pregnancy rates (defined
cattle with normal reproductive health status were used for as the number of animals that were diagnosed pregnant in a
this experiment. The protocol used in this experiment was treatment group divided by the total number of animals
approved by the University of Idaho Animal Care and Use inseminated and exposed to a bull for 21 days in that
Committee. At initiation of the experiment (aprox. 80 days treatment group) were not different between treatment
postpartum) body condition scores (BCS) and body weights groups (COS-ECP 92.2% vs. CON 90.3%). Moreover, PR
(BW) were determined. Mean BCS were 6.34 ± 0.90 (mean were not different between treatment groups on d 40 and 70
± SD; on a scale of 1 to 9, 1 being emaciated and 9 being after insemination. There was no treatment by age, BCS,
obese. Body weights averaged 490 kg ± 58. The BW, DPP interaction (P > 0.05). The data was then pooled
experimental protocol is depicted in the figure 1. Fourteen to analyze for main effects. Age, DPP, BW, and BCS (P >
days before initiation of the COS protocol all cows received 0.05) did not affect the odds ratio of CR and PR.
a luteolytic dose of PGF (25 mg; Lutalyse®, Pfizer Animal
Health Inc., New York, NY). Fourteen d later, on d 0, 100
ug GnRH (Cystorelin, Merial, Iselin, NJ; i.m.) was 100.0%
administered followed seven d later by an injection of 25 90.0%
mg PGF (i.m.). On d 9 cows were paired by age, body
condition score, and body weight to receive either 0.25 mg 80.0%
ECP (COS-ECP; n = 93) or GnRH (100 ug) + vehicle
70.0% COS-ECP (n=90)
(CON; n = 90). All cows immediately received AI by a
60.0% CON (n=93)
Cosynch (CON) 50.0%
PGF GnRH PGF Vehicle & AI
d-14 d0 d7 d9 Conception Rate Pregnancy Rate
Modified Cosynch (COS-ECP)
PGF GnRH PGF ECP & AI Figure 2. Conception and pregnancy rates of cows in both
estradiol cypionate (COS-ECP) and control (CON)
d-14 d0 d7 d9
In the current study, incorporation of 0.25 mg of ECP
Figure 1. Schematic of the different treatments for CON (n
into the COS did not improve the first service AI CR in
= 93) or COS-ECP (n = 90). All cows received the COS
beef cattle. In contrast, research conducted by Ahmadzedeh
with the exception of ECP or vehicle treatment at AI. ECP
et al. (2003) reported that when 0.25 mg ECP was
treatment consisted of 0.25 mg estradiol cypionate in 1 ml
incorporated into OVS an 11% increase in CR was
of cotton seed oil; vehicle consisted of 1 ml cottonseed oil.
observed. A possible explanation for the difference between
these two studies is perhaps the timing of insemination
single inseminator, using semen from four sires stratified
relative to ECP administration. In the study by
across treatment groups. Fourteen d after AI, bulls were
Ahmadzadeh et al. (2003) all cows were inseminated 6 h
placed with cows for a 55-d natural service period.
after GnRH+ECP treatment, during the time of maximally
Pregnancy status was determined by ultrasonography
elevated serum E2. In contrast, in the present study AI was
(Sonovet Co., Mure, Mitaka-Shi Tokyo Japan)
performed simultaneously with GnRH+ECP administration.
approximately on d 35 and d 70 after AI.
Thereby, uterine environment of the ECP-treated cows had
not been exposed to an elevated level of E2 to possibly
Statistical Analysis: Pregnancy data was analyzed by
affect sperm sustained transport and fertility.
logistical regression using SAS (SAS Inst. Inc., Gary, NC).
Researchers have shown that exogenous E2 may cause
The statistical model included year, treatment, age, days
the development of a persistent follicle (Garcia and
postpartum (DPP), BW, and BCS.
Salaheddine, 2001). Furthermore, estradiol may inhibit
GnRH and/or LH pulses (see Wiltbank et al., 2002 for
Results and Discussion
review). Therefore, it can be argued that the lack of
response in CR of the ECP-treated group, in the present
This experiment was conducted to determine whether
study, was due to the development of persistent, non-
incorporation of 0.25 mg of ECP into the Cosynch protocol
ovulating follicles. In addition, ECP treatment could have
improved conception and pregnancy rates in beef cattle.
altered the time of ovulation and ovulation rate. However,
There was no effect of year (P = 0.60) on CR and PR;
this may not be the case. Previous research from our
therefore, data was pooled for further analysis. First service
laboratory (Sellars, 2002) showed that the rate of ovulation
CR (defined as the number of animals that were diagnosed
and time of ovulation in cows after GnRH+ECP treatment
pregnant in a treatment group divided by the total number
did not differ from that of the control group when the same
of animals inseminated in that treatment group) was not
dose of ECP was used.
between treatment groups (53.3% and 57.0% for COS-ECP
Body condition scores were recorded for each animal
at the initiation of the study. There were no interaction Hall, J. B., J. M. Dejarnette, J. C. Whittier, and T. W.
between BCS and treatments (P = 0.24). Body condition Geary. 2003. Timing of insemination and GnRH on
has been shown to influence pregnancy rates when cattle pregnancy rates in beef cows in a modified co-synch
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2000). Animals with low body reserves have a greater (Suppl. 1): 51 (Abstr.)
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A BCS of 5 at calving is important to ensure acceptable releasing hormone at estrus: luteinizing hormone,
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0.20). Our results were similar to the previous experiments.
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