Influence of GnRH analogue (fertirelin acetate) doses on synchronization of ovulation and fixed-time artificial insemination in lactating dairy cows

The effect of using a dose of 50 μg rather than 100 μg fertirelin in an ovulation/fixed-time insemination protocol for Holstein–Friesian dairy cows was investigated in three experiments. In each experiment, fertirelin was administered at the beginning of the protocol followed 7 days later by 500 μg cloprosterol. Two days later, a second dose of fertirelin was given and AI performed 16–19 h later regardless of the incidence of behavioral oestrus. fect on conception rate was studied in experiment 1 using 114 postpartum anoestrus cows. There was no significant difference in the age, parity or number of days after parturition in each treatment groups. The conception rate did not differ between the 50 μg fertirelin group (61.1%; n=72) and the 100 μg group (59.5%; n=42; NS). In experiment 2, a further 12 cows at 40–60 days postpartum were treated with 100 or 50 μg fertirelin (n=6 per dose) with treatment commencing in the follicular or luteal phase of the oestrous cycle. The plasma concentration of luteinizing hormone (LH) reached similar peaks of over 5 ng/ml 120 min after the intramuscular administration of fertirelin in both groups. There were no significant differences in LH levels between treatments or phase of the oestrous cycle when treatment commenced. Doses of 50 and 100 μg fertirelin were compared in experiment 3 using 17 cows to study follicular wave development and synchronization by transrectal ultrasonography, conception rate and corpus luteum function. There were no significant differences between treatments for these factors. concluded that using a dose of 50 μg fertirelin enabled the drug costs to be reduced without affecting the efficiency of a synchronization of ovulation/fixed-time AI protocol for dairy cows.