ABSTRACT
This study investigated the effect of GnRH analogues on inducing ovulation of dominant follicle [DF] and subsequent corpus luteum [CL] lifespan in the presence of norgestomet implant in Holstein heifers. On day 6 to 8 of the estrous cycle [day 0 of the experiment], all heifers [n=15] received norgestomet implant followed by prostaglandin injection on days 0 and 1. On day 4, group 1 [n=4] and 2 [n=4] heifers received 12.5 and 25 microg Alarelin, respectively. Heifers in group 3 [n=3] received 10 microg Buserelin. Heifers in control group [n=4] did not receive any further treatment. Norgestomet was removed on day 8 in the treatment group and kept throughout the experiment in control group. From day 0, blood sampling and ultrasonography were performed. Heifers were monitored after inducing ovulation for 2 consecutive estrous cycles. DF ovulated within 33 h after GnRH injection. Progesterone started to rise on day 3 and declined on day 7.45 +/- 0.28 after GnRH injection. Heifers in treatment groups displayed estrus 7.73 +/- 0.24 days after GnRH injection followed by spontaneous ovulation. The lifespan of CLs were 5.36 +/- 0.28 and 16.9 +/- 0.37 days after inducing ovulation of DF in the presence of norgestomet and following spontaneous ovulation, respectively [P<0.05]. In conclusion, the half dose of Alarelin induced ovulation of DF and the presence of single norgestomet implant after ovulation may be associated with short luteal lifespan