Days postpartum at the initiation of synchronization, body condition, and parity can impact the proportion of
cows that have resumed estrous cycle activity post-calving [7,13,14]. In this study, a lesser proportion of cows calving
late in the calving season expressed estrus compared with cows that calved early in the preceding calving season
(synchronization initiation at 30–60 dpp, 49.6% vs. >60 dpp, 60.1%). Overall, 28.8% (524/1817) of cows were in the
category that had synchronization initiated at 30 to 60 dpp. Previous studies have reported that primiparous cows are
more likely to be anestrus at the initiation of the breeding season when compared with multiparous females [7,13–
16]. Body conditions score categories did affect the estrus expression in this study but age did not. More cows in the
7-day CO-Synch þ CIDR group exhibited estrus compared with cows in the 5-day CO-Synch þ CIDR group. It should
be noted that the duration of the 7-day CO-Synch þ CIDR protocol is 10 days compared with 8 days for the 5-day COSynch
þ CIDR group. Hence, cows in 7-day CO-Synch þ CIDR group had more time for the follicular growth after
the GnRH resetting of the follicular wave and it is possible that the cows in 7-day CO-Synch þ CIDR group had bigger
follicles which produced more estrogen and resulted in more cows exhibiting estrus. The variables days postpartum,
estrus expression at or before AI, and body conditions were accounted for in the effect of treatment on AI
pregnancy rates.