Efecto del uso de GnRH sobre el porcentaje de preñez en vacas Angus con cría que no presentan celo al momento de la IATF.

The objectives of the study were to evaluate: I. the predictive value of estrus expression on pregnancy rate at timed artificial insemination (TAI); II. the causal effect of administering GnRH at the time of TAI on the pregnancy rate in cows that did not express estrus; III. if GnRH administration in cows that do not exhibit estrus allows achieving pregnancy rates at TAI similar to those obtained in cows that exhibit estrus. A total of 406 suckled Angus cows were synchronized using a protocol based on intravaginal progesterone devices (IVPD), estradiol benzoate, a luteolytic agent, and estradiol cypionate (CPE). All animals were painted in the sacro-coccygeal region upon removal of the IVPD, and they were considered to be in estrus if they had at least 30% of the tail-paint rubbed-off. The cows that did not exhibit estrus were randomly allocated to receive or not receive a dose of gonadorelin acetate (GnRH) at the time of timed artificial insemination (TAI). All cows were inseminated within 48 to 54 hours after removal of the IVPD. Pregnancy diagnosis was performed 37 days post-TAI by ultrasonography. Estrus expression was associated with a higher pregnancy rate at TAI [56.1% (148/264) vs. 29.6% (21/71); P< 0.001]. GnRH administration improved the pregnancy rate at TAI in cows that had not expressed estrus [49.3% (35/71) vs. 29.6% (21/71); P= 0.026]. GnRH administration in cows that had not exhibited estrus allowed achieving pregnancy rates closer to those obtained in cows that exhibited estrus [56.1% (148/264) vs. 49.3% (35/71); P= 0.312]. In conclusion, estrus expression between IVPD removal and TAI is associated with a higher pregnancy percentage at TAI. Moreover, treatment based on GnRH application in cows that do not exhibit estrus improves the pregnancy rate at TAI, approaching the pregnancy rate achieved in cows that exhibit estrus. [ABSTRACT FROM AUTHOR]