Pregnancy per AI differences between primiparous and multiparous high-yield dairy cows after using Double Ovsynch or G6G synchronization protocols

The objective of the present work was to determine the optimal protocol for timed AI on high-yield dairy cows. The pregnancy per AI (pregnancy/AI) arising from 7805 first timed artificial inseminations on 27 dairy farms in eastern Spain was examined after use of the Double Ovsynch (DOv; N = 6783) or G6G (N = 1022) synchronization protocols. Parity number, farm, prostaglandin analogue, and season (hot or cool) were examined for their possible association with postsynchronization fertility (in terms of pregnancy/AI). Mean pregnancy/AI overall was 36.1 +/- 8.4%, with no significant differences between the DOv or G6G protocols (36.3 +/- 8.3% vs. 34.8 +/- 9.1%). However, parity and synchronization treatment resulted in a significant interaction (P = 0.03). When DOv AIs (N = 6783) and G6G AIs (N = 1022) were analyzed separately it was observed that pregnancy/AI was significantly better in primiparous than in multiparous cows after DOv (44.3 +/- 11.4% vs. 31.4 +/- 8.2%; P < 0.0001), and pregnancy/AI was similar after G6G in primiparous and in multiparous cows (34.7 +/- 9.2% vs. 34.8 +/- 9.9%). Hot season significantly reduced pregnancy/AI to a similar extent for primiparous and multiparous cows and showed a significant interaction of protocol and season (P = 0.001). These results were similar across all farms, regardless that their overall pregnancy/AI was similar or significantly different from the average. These results indicate that G6G tended to be associated with a higher pregnancy/AI than DOv in multiparous cows, and the opposite was observed in primiparous cows. (C) 2013 Elsevier Inc. All rights reserved.