PSX-19 Critical points for optimizing timed AI outcomes in beef cattle.

Abstract This study aimed to (1) assess the effect of GnRH treatment 7 days after artificial insemination (AI) and (2) identify factors contributing to pregnancy failure in beef cattle following the 7-day CO-Synch+CIDR protocol. Brangus heifers (n=41) and nonlactating cows (n=148) were submitted to the 7-day CO-Synch+CIDR timed AI (TAI) protocol, which included CIDR insertion and GnRH administration on d-10, CIDR removal and PGF administration on d-3, followed by GnRH and TAI on d0. Estrus patches were applied on d-3, and all animals were inseminated with conventional semen on d0. Prior to initiating the protocol, all animals received a pre-synchronization PGF dose on d-12 or -13. Seven days after AI, heifers and cows were randomly assigned to one of two treatments: GnRH group (n = 95, received GnRH) or Control group (n = 94, no GnRH). Blood samples were collected on d0 and d7 to assess circulating progesterone and cortisol via chemiluminescence. Animals were classified as having complete luteolysis (< 0.30 ng/mL progesterone) or partial luteolysis (≥ 0.30 ng/mL progesterone). Cortisol levels at AI categorized animals into high stress (≥ 3.0 ng/mL) or low stress (< 3.0 ng/mL). Ovulatory follicle size and corpus luteum (CL) presence were evaluated via ultrasonography on d0 and d7. Ovulation rate was determined based on CL presence on d7. Pregnancy was diagnosed on d33. Data were analyzed using SAS. GnRH administration 7 days post-AI did not affect pregnancy per AI (P/TAI) (53.3 ± 5.3% vs. 49.6 ± 5.5% for GnRH and Control, respectively; P = 0.62). Group of cows that activated the heat patch had higher P/TAI than those that did no activated the heat patch (64.1 ± 5.0 vs. 35.1 ± 5.6%; P < 0.01). Complete luteolysis occurred in 36.5% (69/189) of animals. Those undergoing complete luteolysis had a higher ovulation rate (97.5 ± 1.8% vs. 89.4 ± 3.0%; P = 0.05) and improved P/TAI compared to partial luteolysis (66.1 ± 6.4% vs. 44.9 ± 4.8%; P < 0.01). No difference was found in ovulation rate between low- and high-stress groups (92.3 ± 3.3% vs. 91.1 ± 2.6%; P = 0.78). However, the low-stress group tended to have higher P/TAI than the high-stress group (61.5 ± 6.0% vs. 49.2 ± 4.5%; P=0.10). Animals classified as synchronized (complete luteolysis + ovulation) had significantly higher P/TAI than non-synchronized animals (68.3 ± 6.4% vs. 44.2 ± 4.7%; P < 0.01). In conclusion, administering GnRH 7 days post-AI did not improve pregnancy rates. However, increasing the proportion of animals undergoing luteolysis and optimizing ovulation significantly enhanced P/TAI outcomes in beef cattle under a TAI program.