RESUMENSe comparó la tasa de gestación (TG) en vacas lecheras inseminadas a tiempo fijo mediante dos protocolos. El día 32 posparto, 275 vacas recibieron dos inyecciones de PGF2 con intervalo de 14 días. Doce días después de la segunda inyección, se asignaron a los siguientes tratamientos: 1) Ovsynch (n=144) recibieron GnRH; siete días después PGF2; 56 h más tarde GnRH y se inseminaron 16 h después 2) PRID5d (n=131) recibieron GnRH y un dispositivo intravaginal liberador de progesterona durante cinco días; al retirar el dispositivo, se aplicaron dos inyecciones de PGF2 con intervalo de 24 h; 56 h posterior al retiro del dispositivo, se aplicó GnRH y se inseminaron 16 h después. La gestación se diagnosticó por palpación rectal el día 45 posinseminación. Se comparó la TG mediante regresión logística. Las variables independientes fueron: tratamiento (Ovsynch vs PRID5d), producción láctea (40 vs >40 kg), técnico inseminador (1 vs 2), tipo de puerperio (normal vs anormal), partos previos (primíparas vs multíparas) y si mostraron estro en la presincronización. La TG fue similar (P>0.1) entre los tratamientos (27 vs 21 %; Ovsynch y PRID5d, respectivamente). Ninguna de las otras variables independientes afectó la TG (P>0.1); no hubo efecto de la interacción de dichas variables con el tratamiento en la TG (P>0.1). Se concluye que la TG fue similar entre las vacas inseminadas con un protocolo Ovsynch o mediante un protocolo Ovsynch modificado que incluyó 5 días de progesterona y la reducción de 7 a 5 días entre la inyección de GnRH y la PGF2.PALABRAS CLAVE: Ovsynch, Progesterona, Fertilidad, Vacas lecheras. ABSTRACTThe objective was to compare the pregnancy rate (PR) between two timed AI protocols in dairy cattle. Two hundred seventy-five (275) cows were presynchronized with PGF2 given 14 d apart beginning at d 32 postpartum; 12 d after second PGF2, cows were assigned to two treatments: 1) Ovsynch (n=144) received GnRH, followed 7 d later by PGF2 and then GnRH 56 h after PGF2 and cows were inseminated 16 h later 2) PRID5d (n=131), received GnRH and a progesterone-releasing intravaginal device, which remained in place 5 d. Cows received an injection of PGF2 at the time of progesterone device removal and a second injection of PGF2 24 h later. At 56 h following progesterone device removal, the next injection of GnRH was given and cows were inseminated 16 h later. Pregnancy diagnosis was performed 45 d after AI by transrectal palpation. PR was compared by logistic regression analysis. Independent variables were: treatment (Ovsynch vs PRID5d), milk yield (40 vs >40 kg), technician (1 vs 2), puerperium (normal vs abnormal), parity (primiparous vs multiparous) and standing estrus during presynchronization. PR was similar (P>0.1) between treatments (27 vs 21 %; Ovsynch and PRID5d, respectively). There was no effect of the other independent variables on PR. The interaction of these variables with the treatment did not affect PR (P>0.1). In conclusion, PR was similar in dairy cows inseminated by an Ovsynch protocol or t...
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