Effect of fertirelin acetate or buserelin on conception rate at first or second insemination in lactating dairy cows

The objective of this study was to determine if fertirelin acetate and buserelin, two GnRH agonists, improve conception rate when administered at the time of first or second AI in lactating dairy cows. The study consisted of a common protocol conducted at 10 commercial dairy farms. Approximately 150 cows within each dairy were assigned randomly in replicates to receive intramuscularly either no injection or injection of 25, 50, 75, or 100 micrograms fertirelin acetate or 10 micrograms buserelin immediately after AI. Cows were subjected to the reproductive management practices normal for each location. Cows at each location were palpated for pregnancy status at 35 to 60 d postinjection. No improvement in conception rate was detected in response to either agonist (control = 48%, fertirelin acetate = 41.5%, buserelin = 39.7%). Conception rate was unaffected by either days postpartum at injection or overall fertility of the individual herds. These observations do not support the routine use of doses of 25 to 100 micrograms fertirelin acetate or 10 micrograms buserelin at the time of first or second AI as a means to improve conception rate in lactating dairy cows.