J. Dairy Sci. 85:1752–1763 American Dairy Science Association, 2002. 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 insemi-nation (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; PGF2α, d −3; GnRH, d −1; timed AI, d 0) followed by AI at estrus (tail paint removal) on d 1 to 22 (Ovsynch; n = 114); or 2) AI at estrus (tail paint removal) throughout 23 d of AI breeding (tail paint; n = 114). Days to first AI service were greater and the 23-d AI 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 AI was greater for tail paint vs. Ovsynch cows (47.3 vs. 27.3%, respectively). Cows in the tail paint group received only one AI, during 23 d of AI, but 46.4 % of Ovsynch cows received a secondAI,with similar concep-tion (43.1%) to that of tail paint cows at first AI (47.3%). Based on serum progesterone, incomplete luteal regres-sion after PGF2α, and poor ovulatory responses toGnRH contributed to lower conception to timed AI in the Ov-synch group. Cumulative pregnancy rates for tail paint andOvsynch cows did not differ after 23 d ofAI 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 synchro-nized poorly to Ovsynch resulting in reduced conception to timed AI compared with AI after tail paint removal