J. Dairy Sci. 88:957–968 American Dairy Science Association, 2005. Use of Intravaginal Progesterone-Releasing Inserts in a Synchronization Protocol before Timed AI and for Synchronizing Return to Estrus in Holstein Heifers

Holstein heifers (n = 189) were submitted to a 42-d artificial insemination (AI) period in which they under-went AI after once-daily evaluation of rubbed tail chalk. At the onset of the AI period (d 0), heifers were assigned randomly to receive synchronization of ovulation and timed AI (TAI; d 0: 100 µg of GnRH; d 6: 25 mg of PGF2α; d 8: 100 µg of GnRH+TAI) either without (GPG; n = 95), or with inclusion of a CIDR insert (CIDR; n = 94) from d 0 to 6. No CIDR heifers received AI before d 8 compared with 24 % of GPG heifers, and pregnancy rate per AI (PR/AI) at 30 d after TAI did not differ between treatments. To synchronize return to estrus for heifers failing to conceive after TAI, heifers (n = 166) receiving TAI to first service were randomly assigned to receive no further treatment (control; n = 85) or receive a new CIDR insert between 14 and 20 d after TAI (Re-synch; n = 81). No Resynch heifers received AI during CIDR treatment compared with 35 % of control heifers, and the proportion of heifers receiving AI within 72 h after the day of CIDR removal was 78 vs. 50 % for Re-synch vs. control heifers, respectively. No treatment × inseminator interaction was detected at first or second AI; however, overall PR/AI was modest for heifers throughout the experiment due to poor performance of 2 of the 3 herd inseminators (14, 6, and 58 % PR/AI, respectively). Inclusion of CIDR inserts suppressed es-trus during the TAI protocol with no reduction in PR/ AI. Resynchronization of estrus using CIDR inserts re-sulted in tighter synchrony of estrus among nonpreg-nant heifers compared with untreated controls. (Key words: controlled internal drug-releasing insert, dairy heifer, timed artificial insemination, resynchroni-zation) Abbreviation key: CIDR = controlled internal drug-releasing insert,GPG = synchronization regimen using sequential injections of GnRH and PGF2α to contro