Initiation of the breeding season in a grazing-based dairy by synchronization of ovulation

Lactating dairy cows (n = 228) in a semiseasonal, grazing-based dairy were subjected to artificial insemination (AI) to start the 23-d breeding season (d 0 to 22) followed by natural service (d 23 to 120). Cows were randomly assigned to: 1) Ovsynch (GnRH, d -10; PGF(2alpha), d -3; GnRH, d -1; timed AI, d 0) followed by AI at estrus (tall paint removal) on d 1 to 22 (Ovsynch; n = 114); or 2) AI at estrus (tail paint removal) throughout 23 d of AI breeding (tall paint; n = 114). Days to first AI service were greater and the 23-d AT service rate was less for tail paint vs. Ovsynch cows (12.0 +/- 0.6 d vs. 0 d; and 84.2 vs. 100%, respectively). However, conception to first AT was greater for tail paint vs. Ovsynch cows (47.3 vs. 27.3%, respectively). Cows in the tall paint group received only one AI, during 23 d of AI, but 46.4% of Ovsynch cows received a second AI, with similar conception (43.1%) to that of tall paint cows at first AI (47.3%). Based on serum progesterone, incomplete luteal regression after PGF(2alpha), and poor ovulatory responses to GnRH contributed to lower conception to timed AT in the Ovsynch group. Cumulative pregnancy rates for tail paint and Ovsynch cows did not differ after 23 d of AT breeding (47.3 vs. 46.3%, respectively) nor after 120 d of AI/natural service breeding (80.5 vs. 83.3%, respectively). Lactating cows in this grazing-based dairy synchronized poorly to Ovsynch resulting in reduced conception to timed AT compared with AI after tail paint removal.