Impact of Early Resynchronization in Combination with Color Doppler Ultrasonography on Cumulative Pregnancy Rates in Postpartum Beef Cows

Abstract The objective of this study is to evaluate a GnRH and progesterone-based early resynchronization strategy combined with color Doppler ultrasonography for early pregnancy diagnosis. Postpartum Bos taurus beef cows (n = 375) from four locations were enrolled in a completely randomized design, where cows were randomly assigned to 1 of the 2 treatments: 1) Natural Service (NS), cows were exposed to a 7-d CO-Synch + CIDR protocol followed by fixed-time artificial insemination (day 0). On day 15, cows were exposed to natural service for the remainder of the study; or 2) Early Resynchronization (ER), cows were exposed to a 7-d CO-Synch + CIDR protocol followed by fixed-time artificial insemination (day 0). On day 15, a controlled internal drug release (CIDR) device was inserted. On day 20, CIDR was removed, and pregnancy was estimated based on luteal blood perfusion using color Doppler ultrasonography. Non-pregnant cows received a 25-mg injection of prostaglandin F2α at CIDR removal and were fixed-timed-AI concurrently with a 100-μg injection of GnRH 60 to 66 h later. Pregnancy diagnosis and fetal aging were performed on day 53 using conventional ultrasonography. Pregnancy rate to the first AI was greater (P = 0.03) for NS compared with ER (70.25% vs 59.29%, respectively). Pregnancy rates for the second service were also greater (P = 0.01) for NS compared with ER (65.67 vs 42.93%, respectively). Cumulative pregnancy rates to AI did not differ between treatments (P = 0.38); however, cumulative pregnancy rates (first and second service) were greater (P < 0.01) for NS compared with ER (90 vs 77.73%, respectively). In conclusion, the proposed early resynchronization strategy using color Doppler ultrasonography resulted in fewer pregnancies compared with natural service as the second service strategy. Moreover, the proposed ER decreased pregnancy rates to the first insemination, indicating a detrimental effect of ER on pregnancy establishment.