Short communication: Optimization of a timed artificial insemination program for reproductive management of heifers in Canadian dairy herds.

Pregnancy per AI (P/AI) following the use of 1 of 2 timed AI (TAI) protocols and 2 different intervals between TAI and resynchronization were compared in heifers that were inseminated with either conventional or sex-sorted semen. Holstein heifers (n = 317; 527 inseminations) were submitted to a 5-d Cosynch protocol with (+) or without (-) GnRH at the time of controlled internal drug release (CIDR) insertion on d 0, CIDR removal and a single PGF2α treatment on d 5, and TAI plus GnRH on d 8 (72 h later). Visual estrus detection (ED) was conducted on d 6 in the afternoon and d 7 in the morning and heifers observed in estrus were artificially inseminated on d 7 in the afternoon. Heifers were alternately assigned conventional or sex-sorted semen. Pregnancy was diagnosed by ultrasound 27 and 42 d after AI, and heifers diagnosed as nonpregnant were resynchronized, up to 3 times, starting on d 27 or 34 to provide an interbreeding interval of 35 or 42 d. Overall, TAI protocol had no effect on P/AI at 27 or 42 d after artificial insemination or on pregnancy loss, but P/AI following the first service tended to be higher in the -GnRH TAI group (66.3 vs. 56.8%). Pregnancy per AI at 27 d (61.9 vs. 55.5%) tended to differ between conventional and sex-sorted semen. Heifers artificially inseminated based on ED tended to have a greater P/AI (67.6 vs. 58.2%) and had decreased pregnancy loss (0.0 vs. 4.1%) than those submitted to TAI. A greater number of heifers in the -GnRH TAI protocol were artificially inseminated on ED than the +GnRH TAI protocol (21.5 vs. 13.7%). No difference in P/AI was observed between the 35- and 42-d interbreeding intervals; however, more heifers in the 42-d group were artificially inseminated based on ED than in the 35-d group (22.7 vs. 7.8%). A 5-d Cosynch+CIDR TAI protocol without the initial GnRH and with a single PGF2α at CIDR removal is an acceptable alternative to achieve high P/AI when either conventional or sex-sorted semen is used in Holstein heifers. Breeding heifers based on detected estrus increases labor, but has the potential to increase fertility.