Synchronization rate, size of the ovulatory follicle, and pregnancy rate after synchronization of ovulation beginning on different days of the estrous cycle in lactating dairy cows

摘要

In 2 dairy farms in Central Utah, USA, during July-September 1996, 31 lactating Holstein cows received the first GnRH injection on days 1-4 of the oestrous cycle, 47 on days 5-9, 52 on days 10-16 and 26 on days 17-21. PG injection was given 7 days later, and a second GnRH was given 2 days after PG (Ovsynch; GnRH-7 days-PG-2 days-GnRH). Ultrasound examinations of the ovarian structures were performed on the day of each treatment and at 48 h after the second GnRH. 64% (100/156) of cows ovulated to the first GnRH and varied (P<0.01) by stage of the oestrous cycle. PG was effective, with 93% of cows having low progesterone at second GnRH. The overall synchronization rate (cows that ovulated after second GnRH) was 87% and varied by response to first GnRH (92% if ovulation to first GnRH vs. 79% if no ovulation; P<0.05). 6% of cows ovulated before the second injection of GnRH and 7% had no detectable ovulation 48 h after second GnRH. Maximal diameter of the ovulatory follicle varied by stage of the oestrous cycle, with cows in which Ovsynch was initiated at mid-cycle having the smallest follicles. Milk yield and serum progesterone concentration on the day of PG affected (P<0.05) the size of the ovulatory follicle. Pregnancy rate on days 28 and 98 afterartificial insemination for 404 cows in which Ovsynch was initiated on known days of the oestrous cycle was analysed. Pregnancy rate was lower (P<0.05) for cows expected to ovulate larger follicles than those expected to ovulate smaller follicles (32 vs. 42%).