Effect of an additional GnRH 2 d after the initiation of the Resynch-25 program on pregnancy per artificial insemination in lactating dairy cows

Abstract

Timed-AI protocols for the reinsemination of nonpregnant cows significantly reduce the interval between inseminations (AI) in lactating dairy cows. One of the most used programs is the Resynch-25, which starts Ovsynch on d 25 after previous AI in all cows, and only nonpregnant cows continue with the pro­gram on d 32 after previous AI. Low ovulatory response to d 25 GnRH administration can compromise the proportion of cows synchronized to the program and pregnancy per AI (P/AI) after timed-AI. One of the factors that can affect ovulatory response to this d 25 GnRH is the stage of antral follicle development at the time of treatment. For instance, non-dominant follicles can­not ovulate due to a lack of LH receptors in the granulosa cells. Thus, we hypothesized that adding an extra GnRH 2 d after d25 GnRH would increase P/AI to the Resynch-25 program. Our ob­jective was to determine the effect of an additional GnRH 2 d after initiating the Resynch-25 program on P/AI.