Estrous Synchronization in Seasonal Calving Systems

38 Lactating dairy cows (n=1,538) were enrolled in a randomized complete block 39 design study to evaluate protocols to synchronize estrus and ovulation. Within each 40 herd (n=8), cows were divided into three calving groups: EARLY, MID and LATE 41 based on days in milk (DIM) at mating start date (MSD). EARLY calving cows 42 (n=1,244) were  42 DIM at MSD, MID calving cows (n=179) were 21 to 41 DIM at 43 MSD, and LATE calving cows (n=115) were 0 to 20 DIM at MSD. Cows in the 44 EARLY, MID and LATE calving groups were synchronized to facilitate estrus or 45 timed AI (TAI) at MSD (Planned Breeding 1; PB1), 21 d (PB2) and 42 d (PB3) after 46 MSD, respectively. For each PB, cows in the relevant calving group were stratified by 47 parity and calving date and randomly assigned to one of four experimental groups: 1) 48 d -10 GnRH (10 μg i.m. Buserelin) and CIDR (Controlled Internal Drug Release) 49 insert (1.38 g P4); d -3 PGF2α (25 mg i.m. dinoprost); d -2 CIDR out and AI at 50 observed estrus (CIDR_OBS); 2) same as CIDR_OBS, but GnRH 36 h after CIDR 51 out and TAI 18 h later (CIDR_TAI); 3) same as CIDR_TAI, but no CIDR 52 (OVSYNCH) or 4) untreated Controls (CONTROL). CIDR_OBS, CIDR_TAI and 53 OVSYNCH had shorter mean intervals from calving to first service compared with 54 CONTROL (69.2 d, 63.4 d, 63.7 d vs. 73.7 d, respectively). Both CIDR_OBS 55 (predicted probability; PP of pregnancy = 0.59) and CIDR_TAI (PP of pregnancy = 56 0.54) had increased odds of conceiving to first service compared with OVSYNCH (PP 57