Comparing strategies to synchronize estrus before fixed-time artificial insemination in primiparous 2-year-old beef cows

Two experiments evaluated controlled internal drug release (CIDR)-based protocols to synchronize estrus in primiparous 2-year-old beef cows. In each experiment, treatments were balanced according to body condition score and days postpartum. Experiment 1 compared the 14-day CIDR-PG (14-d) and 7-day CO-Synch + CIDR (7-d) protocols on the basis of estrous response, pregnancy rates after fixed-time artificial insemination (FTAI), and final pregnancy rate. Cows assigned to 14-d (n = 355) received a CIDR insert on Day 0 with removal on Day 14. Cows assigned to 7-d (n = 349) received gonadotropin releasing hormone (GnRH) and a CIDR insert on Day 23. On Day 30, CIDRs were removed from 7-d cows, and PGF_2α was administered to all cows in each treatment. On Day 33, GnRH was administered concurrent with FTAI at 66 and 72 hours after PGF_2α for 7-d and 14-d treated cows, respectively. Estrous response before FTAI was higher for 7-d compared with 14-d cows (74% vs. 43%, respectively; P < 0.0001); however, pregnancy rates resulting from FTAI were similar (14-d 63%; 7-d 64%; P = 0.52). Ovarian follicular dynamics and serum estradiol-17β concentrations were evaluated among a subset of cows assigned to each protocol. Dominant follicle diameter was smaller at PGF_2α (P = 0.04) and FTAI (P = 0.002) among 14-d cows compared with 7-d cows; however, estradiol-17β at PGF_2α (P = 0.06) and FTAI (P = 0.001) was greater for 14-d versus 7-d treated cows. Experiment 2 compared estrous response and pregnancy rates in 2-year-old beef cows after FTAI- or split-time artificial insemination (STAI) following synchronization of estrus with the 14-day protocol. Cows assigned to FTAI (n = 266) were inseminated at a fixed time concurrent with GnRH at 72 hours after PGF_2α regardless of estrus expression, whereas cows assigned to STAI (n = 257) were inseminated based on estrus expression as determined by activation of an estrus detection aid. Cows assigned to STAI that exhibited estrus by 72 hours were inseminated; however, AI was delayed until 24 hours after GnRH (96 hours after PGF_2α) for nonestrous cows. Total estrous response was increased for STAI- versus FTAI-treated cows (STAI 64%; FTAI 42%; P < 0.0001); pregnancy rates resulting from AI were similar (STAI 55%; FTAI 56%; P = 0.60). In summary, the 14-day CIDR-PG and 7-day CO-Synch + CIDR protocols can be used effectively to synchronize estrus before FTAI in primiparous 2-year-old beef cows. Although expression of estrus was increased using STAI in conjunction with the 14-day protocol, this approach did not increase pregnancy rates compared with FTAI.