Split-time artificial insemination following synchronization of estrus with the 14-d CIDR-PG protocol in primiparous two-year-old beef cows

Abstract Text: This experiment was designed to test the hypothesis that estrous response and pregnancy rate following synchronization of estrus with the 14-d CIDR-PG protocol in primiparous two-year-old beef cows would be improved using split-time AI (STAI) compared to fixed-time AI (FTAI). Estrus was synchronized for 523 primiparous, postpartum beef cows at five locations. Cows received a CIDR insert (1.38 g progesterone) on d 0 with removal on d 14. On d 30, 16 d after CIDR removal, cows were administered PGF_2α (25 mg) and estrus detection aids (Estrotect) were applied. All cows were administered GnRH (100 µg) on d 33, 72 h after PGF_2α administration. Treatments were equally represented across locations, and cows within each location were assigned to one of two treatments based on days postpartum and body condition score. Cows assigned to the FTAI treatment (n = 266) were inseminated at a fixed-time concurrent with GnRH administration at 72 h after PGF_2α regardless of estrus expression, while cows assigned to the STAI treatment (n = 257) were inseminated based on estrus expression observed at 72 h. Cows assigned to STAI that expressed estrus by 72 h were inseminated; however, AI was delayed 24 h until 96 h after PGF_2α for cows that failed to express estrus prior to the standard fixed time. Estrus detection aids remained attached following GnRH at 72 h for all non-estrous cows assigned to the STAI treatment, and estrus expression during the delayed time period was recorded. Estrous response at 72 h did not differ between treatments (FTAI = 42%; STAI = 40%; P = 0.33). Delayed insemination to 96 h after PGF_2α of STAI treated cows that failed to exhibit estrus prior to the standard FTAI at 72 h increased total estrous response (FTAI = 42%; STAI = 64%; P < 0.0001); however, pregnancy rates resulting from AI were similar between treatments (FTAI = 56%; STAI = 55%; P= 0.60). In summary, estrus expression was increased when STAI was used following synchronization of estrus with the 14-d CIDR-PG protocol in primiparous two-year-old beef cows; however, this strategy did not result in significant improvements in AI pregnancy rates compared to FTAI.

Keywords: artificial insemination, estrus synchronization, primiparous two-year-old beef cow