Conception Rate Following Timed Artificial Insemination Protocols In Dairy Heifers Synchronised By PGF2a And Gnrh

This study aimed to develop a timed artificial insemination (TAI) protocol using PGF2a and GnRH with acceptable pregnancy rate in dairy heifers. Dairy heifers (n=290), aged between 13 and 26 months (mean 16.1 months), were used in this study. Heifers were synchronized with two PGF2a administrations by 14 d intervals. After the second PGF2a , heifers were randomly assigned into two groups. In Group 1 (n=155), GnRH was given at 56h following 2nd PGF2a and TAI was performed at 16- 18h after the GnRH. In Group 2 (n=135), GnRH was administered at 72h after 2nd PGF2a , at the time of TAI. Ultrasonography was performed at both PGF2a administrations to determine cyclicity, at the time of TAI to measure ovulatory follicle size, 7 d after TAI to determine ovulation, 31 and 62 d post-AI to determine pregnancy. Ovulatory follicle size at the time of TAI was not significantly different between groups (12.8±1.6 mm and 13.2±1.8 mm in group 1 and 2, respectively). Synchronization rate was similar in group 1 (85.2%; 132/155) and in group 2 (91.1%; 123/135). Conception rate (CR) at 31 d was not different in group 1 (59.8% ;79/132) and in group 2 (55.3%;68/123). Embryonic loss (7.6%; 6/79 and 7.4%; 5/68 in group 1 and 2 respectively) was not different. Gender ratio of the calves was found different between groups. Proportion of the female calves born were higher (P=0.03) in group 1 (63.3%; 38/60) than in group 2 (42.0%; 21/50). In conclusion, both protocols can be used in order to eliminate estrous detection in large dairy herds. Although GnRH administration at the time of TAI was found to be useful to reduce handling of heifers, GnRH can be applied 16-18 h before AI to achive higher female calves ratio in large dairy farms.