161 Increasing the dose of cloprostenol sodium reduced pregnancy losses but did not increase pregnancies per AI in lactating dairy cows

Incomplete luteolysis after last prostaglandin F2α (PGF2α) of Ovsynch in fertility programmes decreases the likelihood of pregnancy per AI (P/AI) in lactating dairy cows (Martins et al. 2011 J. Dairy Sci. 94, 2815-2824; https://doi.org/10.3168/jds.2010-3652). Two doses of PGF2α 24 h apart increased the percentage of cows with complete luteolysis (Brusveen et al. 2009 J. Dairy Sci. 92, 1412-1422; https://doi.org/10.3168/jds.2008-1289). Increasing the dose of cloprostenol sodium (CLO) from 0.5 to 0.75 mg enhanced luteolytic properties and increased P/AI (Giordano et al. 2013 Theriogenology 80, 773-783; https://doi.org/10.1016/j.theriogenology.2013.07.003). We hypothesised that a 1.0-mg dose of CLO would not have different P/AI compared with two 0.5-mg doses 24 h apart but would have greater P/AI than a single dose (0.5 mg) in a timed AI programme. We also hypothesised that dose of CLO would not affect pregnancy losses. The objective was to determine the effect of cloprostenol dose on P/AI and pregnancy losses in lactating dairy cows. Lactating dairy cows (n = 944) were randomly assigned to three treatments of various CLO doses at final PGF2α of Ovsynch: single dose (0.5 mg; n = 338), two doses 24 h apart (0.5 mg each; n = 311), or double dose (1.0 mg; n = 295). Cows were randomly assigned to treatments by parity, synchronisation status, and type of AI (Double-Ovsynch (n = 546) for first AI or GGPG (GnRH-GnRH-PGF2α-GnRH; n = 398) for second and third AI). Cows were classified as either synchronised or non-synchronised based on evaluation of follicle and corpora lutea (CL) development during synchronisation protocols using ultrasound. Synchronised cows were defined as having at least one Day 7 and Day 14 CL at time of treatment. About 72% of all cows were classified as synchronised. Synchrony was not different between treatments. Within-cow pregnancy-specific protein B levels on Days 17 and 24 post-AI were used to determine pregnancy at 24 days. Pregnancy was confirmed via ultrasonography 34 days post-AI. All binomial variables were analysed using chi-square with PROC FREQ in SAS (SAS Institute Inc.). There were no differences between treatments on P/AI on d 24 post-AI (55, 50, and 53% for 0.5 mg, two doses of 0.5 mg 24 h apart, and 1.0 mg, respectively; P = 0.35). At 24 days post-AI, P/AI were greater in synchronised versus non-synchronised cows (57 vs. 42%; P < 0.0001). Older cows (third parity or greater) had reduced P/AI (44%) compared with first-parity (60%) and second-parity (54%) cows at 24 days post-AI (P = 0.0001). At 24 days post-AI, P/AI in cows inseminated following Double-Ovsynch were greater than those in cows that were resynchronised using GGPG (58 vs. 46%; P = 0.0001). Pregnancy loss between 24 and 34 days post-AI was greater for cows receiving 0.5 mg (single dose) versus two 0.5-mg doses 24 h apart or 1.0 mg (12, 6, and 5%, respectively; P = 0.02). In summary, cows receiving a double dose (1.0 mg) of CLO had similar P/AI to cows receiving two doses of 0.5 mg 24 h apart. Unexpectedly, P/AI at 24 days post-AI in cows from the single-dose group were not different from those of cows treated with a double dose and two doses 24 h apart. Despite numerically higher P/AI, cows that received a single dose had greater pregnancy loss between 24 and 34 days post-AI. Further analyses of effects of treatment on luteal function assessed with colour Doppler may provide information about fertility outcomes.