An experiment (Chapter 2) was designed to evaluate later timepoints for fixedtime artificial insemination (FTAI) of beef heifers, with the hypothesis that use of a later timepoint would allow a greater proportion of heifers to express estrus prior to FTAI and result in greater conception rates among estrous heifers inseminated with sex-sorted semen. Estrus was synchronized for 1640 heifers in 7 locations using the 14-d CIDR-PG protocol: insertion of an intravaginal progesterone-releasing insert (CIDR; 1.38 g progesterone) on Day -33 and removal on Day -19, and administration of prostaglandin F2a (PG; 500 [mu]g cloprostenol sodium) on Day -3. Within location, heifers were blocked based on reproductive tract score and body weight and were randomly assigned to one of three FTAI timepoints: 66 h, 70 h, or 74 h after PG administration. Estrus detection aids (Estrotect) were applied at PG administration, with activation recorded at FTAI. Heifers that expressed estrus prior to FTAI were inseminated with sex-sorted semen (SexedULTRA 4M[trademark]). Heifers that failed to express estrus were inseminated with conventional semen. Gonadotropin-releasing hormone (GnRH; 100 [mu]g gonadorelin) was administered coincident with FTAI for heifers that failed to express estrus. The proportion of heifers that expressed estrus prior to FTAI (66 h: 62 percent; 70 h: 67 percent; 74 h: 71 percent) was greater when FTAI was performed at 74 h versus 66 h (P < 0.01). Treatments did not differ (P > 0.10) with respect to the conception rates of heifers that expressed estrus and were serviced with sex-sorted semen (66 h: 56 percent; 70 h: 53 percent; 74 h: 53 percent). Among heifers that failed to express estrus and were serviced with conventional semen, conception rates were greater (P = 0.02) when FTAI was performed at 66 h versus 70 h (66 h: 37 percent; 70 h: 25 percent; 74 h: 31 percent). Results indicate that performing FTAI later following the 14-d CIDR-PG protocol increases the proportion of heifers that express estrus and are serviced with sex-sorted semen, but later timing of FTAI does not improve conception rates. A series of experiments (Chapter 3) was designed to evaluate treatment schedules for control of the estrous cycle in which luteolysis is induced prior to atresia of the first follicular wave following a long-term progestin presynchronization treatment in beef heifers. The overarching hypothesis was that the proportion of heifers undergoing luteolysis and expressing estrus would be affected by the duration of treatment with an intravaginal progesterone-releasing insert (CIDR[registered trademark ownership]; 1.38 g progesterone) and/or by the interval from CIDR removal to prostaglandin F2[alpha] (PG; 500 [mu]g cloprostenol) administration. In Experiment 1, heifers (n = 91) were blocked by reproductive tract score (RTS) and body weight (BW) and were assigned randomly within block to one of four treatments in a 2 x 2 factorial design: presynchronization via CIDR treatment for either 14 d (Treatments 14-8 and 14-9) or 18 d (Treatments 18-8 and 18-9) with administration of PG either 8 d (Treatment 14-8 and 18-8) or 9 d (Treatment 14-9 and 189) following CIDR removal. In Experiment 2, heifers (n = 63) were blocked by RTS and BW and randomly assigned to receive administration of PG either 9 d or 10 d following a 14 d CIDR presynchronization. In Experiment 3, heifers (n = 83) were blocked by RTS and BW and randomly assigned to receive administration of PG either 9 d or 16 d following a 14 d CIDR presynchronization. In all three experiments, estrus detection aids (Estrotect) were applied at the time of PG administration. In Experiments 1 and 2, heifers were observed for expression of estrus three times daily for 4 d following PG administration and received artificial insemination (AI) based on estrous response. For heifers that failed to express estrus by 96 h after PG administration, timed AI was performed and gonadotropin-releasing hormone (GnRH; 100 [mu]g gonadorelin) was administered. In Experiment 3, Estrotect patch activation was recorded at 66 h, and heifers that expressed estrus by 66 h were inseminated at that timepoint. Heifers that failed to express estrus by 66 h were inseminated at 90 h. For heifers that failed to express estrus by 90 h, GnRH was administered at timed AI. In all three experiments, blood samples were collected at the time of PG administration and, for heifers that failed to express estrus, at timed AI to determine serum progesterone concentrations via radioimmunoassay. In Experiment 1, serum estradiol concentrations were also determined via radioimmunoassay. Transrectal ovarian ultrasonography was performed to determine CL status and to measure largest follicle diameter (LFD) at PG administration and, for heifers that failed to express estrus, at timed AI. Transrectal ultrasonography was performed 60-80 d after AI to determine pregnancy status. In Experiment 1, there was a tendency for a greater proportion of heifers to express estrus by 96 h after PG administration when PG administration occurred 9 d after CIDR removal versus 8 d after CIDR removal (P < 0.07; 8 d: 57 percent [26/46]; 9 d: 76 percent [34/45]). Additionally, serum E2 levels were greater at the time of PG administration when PG was administered 9 d after CIDR removal versus 8 d after CIDR removal following an 18-d CIDR treatment (P < 0.006; 18-8: 5.7 [plus-minus sign] 1.0 pg/mL; 18-9: 9.6 [plus-minus sign] 0.9 pg/mL). In Experiment 2, pregnancy rate to AI (P/AI) was greater among heifers receiving PG administration 9 d after CIDR removal versus 10 d after CIDR removal (P < 0.03; 14-9: 58 percent [18/31]; 14-10: 28 percent [9/32]). There was a tendency for LFD to be greater among non-estrous heifers in treatment 14-10 versus non-estrous heifers in treatment 14-9 when measured at timed AI (P < 0.08; 14-9: 11.9 [plus-minus sign] 1.6; 14-10: 13.6 [plus-minus sign] 1.3). In Experiment 3, there was a tendency for serum progesterone concentrations at the time of PG administration to be greater among heifers in treatment 14-16 versus heifers in treatment 14-9 (P = 0.07; 14-9: 1.2 [plus-minus sign] 0.2 pg/mL; 14-16: 1.8 [plus-minus sign] 0.2 pg/mL). These results provide a preliminary evaluation of the feasibility of inducing luteolysis prior to atresia of the first follicular wave following long-term progestin presynchronization.
