8 Efficacy of Sequential Estrus Synchronization and Timed Artificial Insemination Protocols in Beef Cattle

Abstract

Abstract The objective of this study was to assess the efficacy of sequential estrous synchronization and timed artificial insemination (TAI) in beef cattle using transrectal ultrasonography (US) for pregnancy diagnosis 26d after the initial TAI (TAI-1;d0) to determine pregnancy status and eligibility for the second TAI (TAI-2) on d 29. Angus and angus cross primiparous and multiparous cows were randomly assigned to one of two treatment regimens; either single TAI (CON; n = 468) or two sequential TAI (RS; n = 473). Estrus was synchronized for both treatment groups using the 7-day CO-synch + CIDR protocol forTAI-1 at 60h - 66h post- Prostaglandin injection (PGf2α). A second 7-day CO-synch + CIDR protocol was initiated for RS groups on d 19 and pregnancy status determined by US coincident with CIDR removal on d 26 post TAI-1. Non-pregnant RS cows received an injection of PGf2α and Estrotect® estrous detection aid was applied. All non-pregnant cows received a second TAI (TAI-2) and GnRH injection 60-66h after PGf2α (day 29). The probability of pregnancy at 30d post TAI-2 was higher (P < 0.01) for RS (82%) than CON cows (56%). Overall pregnancy rate on d 120 post TAI-1 did not differ between treatments (92%; P < 0.823). The probability of pregnancy to AI on d 120 was higher (P < 0.0001) for RS (78%) than CON cows (50%). Conversely, the probability of pregnancy sired by natural service was higher (P < 0.03) for CON (41%) than RS (15%) by design. Mean fetal age at 120d pregnancy diagnosis did not differ (P < 0.5547) between treatments (RS, 94 days; CON ,96 days). In conclusion, re-synchronization for a second TAI beginning 19 d after the initial TAI increased total AI-sired pregnancies and has the potential to reduce the required number of natural service sires without negatively influencing calving distribution.