Comparison of pregnancy outcomes using either an Ovsynch or a Cosynch protocol for the first timed AI with liquid or frozen semen in lactating dairy cows.

The objective of this study was to evaluate fertility to the first timed AI (TAI) using either liquid semen or frozen semen after an Ovsynch or a Cosynch protocol in lactating dairy cows. The hypothesis was that there is an increase in fertility to the first TAI when cows are inseminated with liquid semen compared to that when frozen semen is used in a Cosynch protocol. Lactating dairy cows (n = 1724; 540 primiparous, 1184 multiparous) from 9 commercial dairy farms were enrolled on a weekly basis to facilitate first timed AI. Two experiments were conducted. In experiment 1, all cows received GnRH, 7 d later PGF, and then received one of the following treatments: 1) GnRH + TAI with liquid semen 56 h after PGF; 2) GnRH + TAI with frozen semen 56 h after PGF; 3) GnRH 56 h after PGF + TAI with liquid semen 12-16 h after the second GnRH; 4) GnRH 56 h after PGF + TAI with frozen semen 12-16 h after the second GnRH. In experiment 2, all cows received GnRH, 7 d later PGF, and then received treatments 3 or 4 as described for experiment 1. Number of sperm per straw was 20 × 10 sperm/straw and 10 × 10 sperm/straw for frozen and liquid semen, respectively. Pregnancy diagnosis was performed by ultrasound scanning at 39 d after TAI. In experiment 1 (n = 1263), there was an interaction of semen preservation method by TAI protocol. Cows inseminated with liquid semen concurrently with the second GnRH (Cosynch-56) achieved greater pregnancy per AI (P/AI) than cows inseminated with frozen semen using the same synchronization protocol (20.0% vs. 27.5%; P = 0.032). There was no effect of semen preservation method (liquid semen 32.3% vs. frozen semen 28.6%; P = 0.330) when cows were inseminated approximately 16 h after the second GnRH injection (Ovsynch-56). Parity affected P/AI with primiparous having a greater P/AI than multiparous cows (34.8% vs. 20.2%; P = 0.001). In experiment 2 (n = 377), there was no effect of semen preservation method (liquid semen 26.5% vs. frozen semen 25.5%; P = 0.846) when cows were inseminated approximately 16 h after the second GnRH injection (Ovsynch-56). Parity affected P/AI with primiparous having a greater P/AI than multiparous cows (37.0% vs. 17.3%; P = 0.001). The results of this study provide evidence that liquid semen achieved greater P/AI in a TAI protocol with a long time interval between insemination and ovulation (Cosynch-56) compared with frozen semen indicating that liquid semen might have a longer viability in the reproductive tract.