insemination timings for the ST protocol were compared
with insemination at 72 hours for the LT protocol to
determine the difference in AI pregnancy rates; insemination
was performed at the same time (72 hours) for both ST
and LT protocols and compared with insemination at
different periods (56 vs. 72 hours). There was no difference
in AI pregnancy between the ST and LT protocols irrespective
of the difference in the timing of insemination for
the ST protocol. In a previous study, it was reported that
there was dominant follicle regression and a new wave of
ovarian follicle development occurring after Day 25, and
the variance in follicle diameter on Day 30 was less in
heifers treated using the LT protocol indicating the presence
of a dominant follicle [10]. Similarly, the presence of a
dominant follicle occurs by 56 hours after CIDR removal
[7,15,16]. Elevated estradiol concentrations before estrus
are important for follicular and oocyte maturation and
uterine function [16]. Therefore, when using protocols to
synchronize the time of ovulation in cattle, maximizing the
proportion of females with a dominant follicle before FTAI
is critical even if estrus is not exhibited.
It should be noted that in experiment 1, four locations
had greater AI pregnancy for the ST protocol and three
locations had greater AI pregnancy for the LT protocol.
Similarly, in experiment 2, two locations had higher AI
pregnancy for the ST protocol and two locations had higher
AI pregnancy for the LT protocol. This variability in the AI
pregnancy rate, noted in this study, may not have been
caused by the synchronization treatment per se but by
other problems inherent in the AI programs in which large
numbers of animals are inseminated in a short time span
[28]. When selecting a program to synchronize the time of
estrus and ovulation in cattle, however, attention should be
focused on its monetary advantage compared with natural
service and its benefit over other synchronization protocols
in an individual herd. Utilization of protocols to synchronize
estrus and ovulation as compared with the use of
natural service in beef heifers provides for an advantage of
enhanced pregnancy rates [29] and lifetime revenue [30].
The probability of conception early in the first breeding
season is increased in beef heifers that have experienced
multiple estrous cycles before the onset of the breeding
season [31,32]. The most effective method to induce
puberty in heifers involves administration of a progestin
[33–35]. Although progesterone supplementation before
the beginning of the breeding season helped prepubertal
and peripubertal beef heifers in achieving greater AI pregnancy
rates [36], studies [23–25] have reported improved
AI pregnancy in pubertal compared with prepubertal beef
heifers after progesterone-based synchronization protocol
as noted in this study. This warrants detailed investigation
on the role of progesterone supplementation in improving
AI pregnancy in prepubertal heifers.