In several previous studies,
though not in contemporaneous comparisons, the AI
pregnancy rates for the 5-day CO-Synch þ CIDR protocol
ranged from 54.9% to 69% with a mean of 56.6% [9,17] and
AI pregnancy rates for the 7-day CO-Synch þ CIDR protocol
ranged from 50.4% to 58.8% with a mean of 53.4% [18–20].
Cows that exhibited estrus had a greater pregnancy rate
compared with cows that did not. It is interesting to note
that more cows in the 7-day CO-Synch þ CIDR group
exhibited estrus compared with cows in the 5-day COSynch
þ CIDR group; nevertheless, a greater proportion
of cows in the 5-day CO-Synch þ CIDR group became
pregnant. There was no treatment by estrus expression
interaction for AI pregnancy rate (P > 0.1). Location greatly
influenced AI pregnancy rate (P < 0.0001), which may have
been a result of differing management, nutrition, genetics,
production goals, and environment.