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

Abstract

Incomplete luteolysis after last prostaglandin F_2α (PGF_2α) 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 PGF_2α 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 PGF_2α 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-PGF_2α-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.