PSIII-6 - Evaluation of Later Timepoints for Fixed-time Artificial Insemination of Beef Heifers When Using Sex-sorted Semen Following the 14-d CIDR®-PG Protocol

Abstract: An experiment was designed to evaluate later timepoints for fixed-time 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 using the 14-d CIDR^®-PG protocol (1.38 g progesterone CIDR intravaginally inserted Day -33 and removed Day -19, PGF_2a; 500 µg cloprostenol administered Day -3) for 1640 heifers in 7 locations. Heifers were blocked by reproductive tract score and body weight and randomly assigned to one of three FTAI timepoints: 66 ± 2 h, 70 ± 2 h, or 74 ± 2 h after PGF_2a administration. Estrus detection aids (Estrotect) were applied at PGF_2a administration, with activation recorded at FTAI. Heifers that expressed estrus prior to FTAI were serviced with sex-sorted semen (SexedULTRA 4M™). Heifers that failed to express estrus were serviced with conventional semen and administered gonadotropin-releasing hormone (GnRH; 100 µg gonadorelin). The proportion of heifers that expressed estrus by FTAI (66 h: 63%; 70 h: 68%; 74 h: 72%) was greater at 74 h versus 66 h (P < 0.01). Conception rates did not differ significantly between treatments among heifers that expressed estrus and were serviced with sex-sorted semen (66 h: 55%; 70 h: 53%; 74 h: 52%). 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: 39%; 70 h: 26%; 74 h: 32%). 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 of estrous heifers inseminated at later timepoints.