Evaluation of later timepoints for fixed-time artificial insemination of beef heifers and cows when using sex-sorted semen.

Two experiments were designed to evaluate later timepoints for fixed-time artificial insemination (FTAI) of beef heifers and cows, with the hypothesis that use of a later timepoint would allow a greater proportion of animals to express estrus prior to FTAI and result in greater conception rates among estrous females inseminated with sex-sorted semen. In Experiment 1, estrus was synchronized for 1640 heifers 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 F2α (PG; 500 μg cloprostenol sodium) on Day -3. Heifers were inseminated at one of three FTAI timepoints: 66 h, 70 h, or 74 h after PG administration. In Experiment 2, estrus was synchronized for 414 beef cows using the 7 & 7 Synch protocol: administration of PG coincident with CIDR insertion on Day -17, gonadotropin-releasing hormone (GnRH; 100 μg gonadorelin) on Day -10, and PG coincident with CIDR removal on Day -3. Cows were inseminated at one of two FTAI timepoints: 66 h or 72 h after PG administration. In both experiments, only animals that expressed estrus prior to FTAI were inseminated with sex-sorted semen. In Experiment 1, the proportion of heifers that expressed estrus prior to FTAI (66 h: 62 %; 70 h: 67 %; 74 h: 71 %) was greater when FTAI was performed at 74 h versus 66 h (P = 0.0097); however, conception rate of heifers that expressed estrus and were serviced with sex-sorted semen did not differ among treatments (P = 0.67; 66 h: 56 %; 70 h: 53 %; 74 h: 53 %). In Experiment 2, the proportion of cows expressing estrus prior to FTAI did not differ between treatments (P = 0.30; 66 h: 71 %; 72 h: 76 %). Additionally, conception rate of estrous cows inseminated with sex-sorted semen did not differ between treatments (P = 0.24; 66 h: 45 %; 72 h: 40 %). These 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 does not improve conception rates. Later timing of FTAI following the 7 & 7 Synch protocol was not observed to increase the proportion of cows expressing estrus prior to FTAI or improve conception rates among estrous cows inseminated with sex-sorted semen. Together, these results provide further insight into optimal timing of FTAI when using sex-sorted semen.