commonly used to synchronize ovulations in beef cows, with the classical
Ovsynch protocol in high-producing dairy cows. Holstein-Friesian cows (n ¼ 128) from six
commercial dairy herds, 40 days postpartum and not previously inseminated, were
randomly assigned to one of two treatments. Cows submitted to Ovsynch protocol (group
OS as control group; n ¼ 66) received 10 mg of a GnRH analogue 7 days before and 48 hours
after 25 mg PGF2a, followed by artificial insemination (AI) 16 hours after the second GnRH
administration. Cows submitted to CO-Synch þ CIDR (1.38 g of progesterone) inserted for
7 days beginning at the first GnRH administration (group CoS þ CD; n ¼ 62) had the
second administration of GnRH concurrent with AI, 64 hours after CIDR removal/PGF2a
administration. Nonpregnant cows with return-to-estrus between 18 and 24 days after
first AI were reinseminated (second AI). Logistic regressions were used to analyze PR and
returns-to-estrus. No effect of group or herd was observed in PR at first timed AI. However,
the sum of cows pregnant at first AI and nonpregnant cows with regular returns-to-estrus
and the total PR (first þ second AI) were influenced by group treatment. Overall, cows of
group CoS þ CD (total PR ¼ 56.5%) were 2.1 times more likely to became pregnant after AI
and until first regular returns-to-estrus than cows of group OS. The calving interval was
lower in group CoS þ CD (425.9 78.8 days; SD) than in group OS (475.3 83.7 days).
The CO-Synch þ CIDR protocol was reliable to use in dairy herds and provided reproductive
advantages when compared with Ovsynch protocol.