Ultrasonic Evaluation for the Time of Ovulation in
Ewes Treated with Norgestomet and Pregnant Mare Serum Gonadotropin During
the Spring and Fall Breeding Seasons
B.E. Cardwell, G.Q. Fitch
and R.D. Geisert
Story In Brief
Ovaries of sixty Dorset and Rambouilet X Dorset ewes were evaluated during
Spring and Fall breeding seasons (n = 30 per season). Ewes were randomly
assigned to one of three treatment groups (n = 20/group): controls were
given prostaglandin F2a 9 days apart,
10 day norgestomet implant only, and 10 day norgestomet implant + 500 IU
i.m. pregnant mare serum gonadotropin (PMSG) at implant removal. Onset
of estrus was detected with the HeatWatch Estrus Detection System. Ovaries
were monitored via rectal ultrasonography every six hours to determine
time intervals from implant removal to onset of estrus, implant removal
to ovulation, and onset of estrus to ovulation. Ovulation occurred on average
70 to 80 hours after implant removal in ewes treated with norgestomet,
and treatment with PMSG at implant removal advances time of ovulation 16-18
hours.
(Key Words: Synchronization, Ultrasound, Ewes, Ovulation, Norgestomet.)
Introduction
In the sheep industry, use of artificial insemination (AI) could greatly
enhance the efficiency of genetic gain as demonstrated in the dairy, beef
and swine industries. While AI of ewes has been utilized, wide variability
in conception and fertility rates have been problematic and hindered its
acceptance in the industry. Laparoscopic AI (LAI) and Transcervical AI
(TAI) are two artificial insemination techniques that have been utilized
with some success in sheep. Laparoscopic AI has been somewhat effective,
however, costs and required surgery has slowed reception for use by sheep
producers. Transcervical AI could potentially be the most convenient practice
for producers to use, but ease of the procedure using the recommended equipment
and inconsistencies in fertility rates must first be revised before it
will become widely practiced. In an overview by Salamon and Maxwell (1995)
causes of reduced fertility rates with AI were method, depth of insemination,
semen handling and treatment of ewes which are all key factors for successful
fertility rates. Timing of insemination plays a key role in the success
of artificial insemination as well. Utilization of progestins and follicular
stimulants have provided reasonable techniques for estrous synchronization,
but the time of ovulation relative to removal of the progestin is unclear.
Insemination of ewes has been accomplished, on a timed basis, from when
progestagen implant was removed, usually between 50-65 hours (Eppleston
and Maxwell, 1995).
The goals of this study were: 1) To observe the time interval from onset
of estrus to ovulation and its variability among ewes. 2) To observe the
time interval from onset of estrus to ovulation in ewes synchronized with
progestagens and follicular stimulants.
Materials and Methods
Sixty, 5 to 7 year old, Dorset and Dorset x Rambouilet cross ewes were
synchronized and onset of estrus and ovulation were recorded during Spring
and Fall breeding (n = 30 per breeding season). Ewes were divided into
three treatment groups (n = 20/group). Control (C) ewes were given a 5
mg dose of prostaglandin F2µ 12
and 3 days prior to the beginning ( implant removal = day 0) of the trial.
An implant only group (I) consisted of ewes who were implanted with norgestomet
for 10 days. A third group of ewes (PI) were implanted with norgestomet
for 10 days and given 500 IU Pregnant Mare Serum Gonadotropin upon removal
of implants.
On the day of synchronized estrus, the ewe’s ovaries were observed transrectally
using an Aloka 500 with a 7.5 MHz human prostate transducer (linear array,
Corometrics Medical Systems, Inc., Wallingford, CT) while in dorsal recumbancy.
Ovaries were evaluated at the onset of estrus, and every six hours until
ovulation. Location of ovary, and size and location of the follicle on
the ovary were recorded.
Exact time for initiation of estrous behavior was detected by activation
of the HeatWatch system. Mounting by a vasectomized ram was recorded by
a transducer adhered to the rump of the ewe that reported radiowave signals
to the main computer program. The computer program recorded the time, duration
and identity of the ewe mounted.
Onset of estrous (EST), ovulation (OVUL) and interval from onset of
estrous to ovulation (INT) were examined for effects of season, treatment
group and interaction by analysis of variance using GLM and least-square
means procedures of SAS (1985).
Results
Treatment comparisons are presented in Table 1. Only 7 out of 20 (35%)
C ewes responded to the PGF2a synchronized
estrus, compared to 13 out of 20 (65%) I and 14 out of 20 (70%) PI ewes
(P<.10). Onset of estrus (range=26-68h) and time interval form estrus
to ovulation (range=12-62h) were not effected by breeding season (P>.10),
however, time to ovulation after implant removal (range=54-100h) was longer
(P<.10) during Fall breeding (79.3h) compared to Spring breeding (70.8h)
in I and PI ewes. The mean estrus to ovulation interval was reduced (P<.05)
in the C ewes (20.9h, range=16-27h) in comparison with I (35.9h, range=12-62h)
and PI (38.0h, range=17-53h) ewes. Time of ovulation from implant removal
was prolonged (P<.10) in the I group (79.4h, range=64-100h) compared
with the PI group (70.6h, range=54-82h), and mean time of estrus onset
after implant removal was also greater (P<.10) for I ewes (43.5h, range=26-68h)
compared with PI ewes (34.9h, range=24-45h).
Discussion
Time of ovulation after implant removal is shortened when using follicular
stimulants (PMSG) in combination with progestins to synchronize ewes. Previous
research has recommended that semen be placed in the reproductive tract
10 to 12 hours, not more than 36 hours prior to ovulation (Salamon and
Maxwell, 1995). Present data indicates that ovulation after implant removal
occurred between 70 and 80 hours. Since most timed AI is performed either
50 to 65 hours after implant removal or 20 to 30 hours after estrous detection,
data from this study indicates that insemination is occurring at a time
appropriate for sufficient fertility rates to be obtained.
In this study, Control ewes had a tendency to have a shortened time
to ovulation after estrous detection (range=16-27h). This would indicate
that insemination of natural estrous ewes should occur sooner after detection
of estrus. Results from this study suggest 10 to 12 hours after estrous
detection would be an appropriate time to obtain optimum fertility rates.
Literature Cited
Eppleston, J. and W.M.C. Maxwell. 1995. Theriogenology 43: 777-788.
Salamon, S. and W.M.C. Maxwell. 1995. Anim. Reprod. Sci. 38: 1-36.
SAS. 1985. SAS User’s Guide: Statistics (Version 5 Ed.). SAS Inst. Inc.,
Cary, NC.
Acknowledgment
A very special thanks to the USDA-ARS Research Laboratory, Ft. Reno, OK
for the use of their ewe flock, and David Jones, Herdsman of the OSU Dairy
Center for the use of their facilities and HeatWatch system during this
trial.
| Table 1. Percent response to synchronization
and means for ESTa, OVULb and INTc by
treatment in mature ewesd. |
|
Treatment
|
No.
|
% estrus
|
EST(h)
|
OVUL(h)
|
INT(h)
|
| Control |
20
|
35e
|
N/A
|
N/A
|
20.9e
|
| Implant only |
20
|
65f
|
43.5e
|
79.4e
|
35.9f
|
| PMSG+implant |
20
|
70f
|
34.9f
|
70.6f
|
38.0f
|
aEST = implant removal to onset of estrus.
bOVUL = implant removal to ovulation.
cINT = onset of estrus to ovulation.
d5 to 7 years in age.
e,fLS Means within a column lacking a common superscript
letter differ (P<.1).
1997 Research Report