Comparison of fertility, regular returns-to-estrus, and calving interval between Ovsynch and CO-synch + CIDR protocols in dairy cows

The main aims of the present study were to compare the pregnancy rate (PR), regular returns-to-estrus, and calving interval of a CO-Synch + controlled internal drug release (CIDR) device, commonly used to synchronize ovulations in beef cows, with the classical Ovsynch protocol in high-producing dairy cows. Holstein-Friesian cows (n = 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 = 66) received 10 μg of a GnRH analogue 7 days before and 48 hours after 25 mg PGF2α, followed by artificial insemination (AI) 16 hours after the second GnRH administration. Cows submitted to CO-Synch + CIDR (1.38 g of progesterone) inserted for 7 days beginning at the first GnRH administration (group CoS + CD; n = 62) had the second administration of GnRH concurrent with AI, 64 hours after CIDR removal/PGF2α 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 + second AI) were influenced by group treatment. Overall, cows of group CoS + CD (total PR = 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 + CD (425.9 ± 78.8 days; ±SD) than in group OS (475.3 ± 83.7 days). The CO-Synch + CIDR protocol was reliable to use in dairy herds and provided reproductive advantages when compared with Ovsynch protocol.