The objectives of this study were to evaluate the effects of (1) double doses of PGF2α administration or (2) an exogenous progesterone (CIDR) applied concurrently with, or (3) the day after, first GnRH of Ovsynch (GnRH-1), on synchronisation and fertility during the Ovsynch protocol. All cows (n = 378) received the Ovsynch protocol (GnRH-7d-PGF2α-56h-GnRH-18h-TAI). The 'OVS' group (n = 105) received only the Ovsynch protocol. The 'OVS-PGF' group (n = 118, GnRH-7d-PGF2α-12h-PGF2α-44h-GnRH-18h-TAI) received an extra dose of PGF2α 12 h later on Day 7. The 'OVS-7CIDR' group (n = 78, GnRH+CIDR-7d-PGF2α-56h-GnRH-18h-TAI) received a CIDR for 7 days between GnRH-1 and PGF2α. In the 'OVS-6CIDR' group (n = 77, GnRH-24h-CIDR-6d-PGF2α-56h-GnRH-18h-TAI), CIDR was applied one day after GnRH-1 and removed 6 days later. When all cows were evaluated, the responses to GnRH-2 were higher (P = 0.005) in cows that responded to GnRH-1 (95.4%) compared to the cows that did not respond (87.6%). The pregnancy rates at 31 and 62 days for each group were 48.6% and 42.9% in the OVS, 54.2% and 52.5% in the OVS-PGF, 52.6% and 48.7% in the OVS-7CIDR, and 55.8% and 49.3% in the OVS-6CIDR groups. Thus, none of the three different treatments has an effect on increasing the out-comes of the Ovsynch protocol in cyclic lactating dairy cows.
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