Effect of an additional GnRH two days after the initiation of a resynchronization program 25 days after artificial insemination on fertility of lactating dairy cows

We investigated the effect of an additional GnRH 2d after initiation of resynchronization on d25 post-AI on the proportion of cows without a corpus luteum (CL) at nonpregnancy diagnosis (NPD) and pregnancy per AI (P/AI) of cows with a CL at NPD that were reinseminated. Once per week, cows that were inseminated 25d earlier were randomly assigned to receive either a GnRH treatment only (G25) or GnRH on this day and 2d later (G25&27). Pregnancy diagnosis and CL presence were determined d32 post-AI using ultrasonography. Nonpregnant cows with a CL were enrolled in different strategies for resynchronization of ovulation according to parity (primiparous or multiparous) and CL size. Primiparous and multiparous cows with a CL ≥ 20 mm in diameter continued Resynch-25 (PGF2α on d32; PGF2α on d33; GnRH on d34; and AI on d35). Multiparous cows with CL < 20 mm restarted a modified Ovsynch at NPD (GnRH – 7d – PGF2α – 1d – PGF2α – 1d – GnRH – 1d – AI). Primiparous cows with CL diameter < 20 mm started an Ovsynch+CIDR protocol on the day of NPD (GnRH + CIDR inserted – 7d – PGF2α + CIDR removed – 1d – PGF2α – 1d – GnRH – 1d – AI). Cows without a CL were not enrolled in any resynchronization program for a week and were excluded from fertility analyses. Only in multiparous cows, the additional d27 GnRH treatment was associated with a decreased P/AI 32d and 102d post-AI for the insemination before treatment. The overall proportion of nonpregnant cows without CL did not differ between treatments. Yet, the GnRH treatment 27d after AI tended to increase the proportion of overall nonpregnant cows with CL < 20 mm at NPD. Treatment did not affect fertility parameters of primiparous cows with a CL ≥ 20 mm that continued Resynch-25 at NPD. Primiparous cows in G25&27 with CL < 20 mm and submitted to Ovsynch+CIDR at NPD tended to have decreased P/AI 102d after AI and calving/AI. When fertility parameters of primiparous cows with a CL independent of size at NPD were analyzed combined, no effect of treatment was observed. Treatment did not affect fertility of multiparous cows with CL < 20 mm enrolled in Ovsynch at NPD. Multiparous cows in the G25&27 group that had a CL ≥ 20 mm and continued Resynch-25 at NPD tended to have a greater calving/AI due to a decreased total pregnancy loss. In summary, no treatment effect was observed on the overall fertility of primiparous cows with CL at NPD. In multiparous cows, G25&27 was associated with a decreased fertility of the previous AI, but it tended to increase calving/AI for multiparous cows with CL at NPD by decreasing the total pregnancy loss for the AI after treatment.