Treatment with progesterone and 17beta-oestradiol to induce emergence of a newly-recruited dominant ovulatory follicle during oestrus synchronisation with long-term use of norgestomet in Brahman heifers.

摘要

50 Brahman heifers were injected with 100 or 200 mg of progesterone, 10 mg of oestradiol + 100 mg of progesterone (n = 10), or 10 mg of oestradiol only or received no treatment (controls) on day 10 of a 17-day treatment with a norgestomet implant. Plasma concentrations of LH, progesterone and oestradiol were determined during the time when the norgestomet implant was in place. All treatments resulted in the emergence of a new cohort of follicles within 5 days of administration. The day of emergenceof the ovulatory follicle tended to be delayed after treatment with 100 (2.7±0.3 days after treatment) and 200 mg of progesterone (3.7±0.5 days after treatment), 10 mg of oestradiol + 100 mg of progesterone (4.4±0.2 days after treatment) and 10 mg of oestradiol (4.6±0.4 days after treatment) compared with control heifers (1.4±1.4 days after time of treatment). The mean interval from implant removal to onset of oestrus was significantly shorter after treatment with 100 mg of progesterone (38.4±2.6 h) than after treatment with 200 mg of progesterone (61.5±3.9 h) but otherwise, the mean interval from implant removal to onset of oestrus did not differ. Oestrus synchrony, measured by the sample standard deviation of oestrus onset, was tighter in all treated groups compared with untreated controls. The mean interval from implant removal to ovulation did not differ significantly between groups. The synchrony of ovulation was significantly tighter after treatment with 100 mg of progesterone, 200 mg of progesterone and 10 mg of oestradiol compared with controls. Treatment with 10 mg of oestradiol resulted in ovulation in 7 of 10 heifers before implant removal, 3 of which failed to ovulate after implant removal. Progesterone administered on day 10 lowered plasma LH concentrations (P<0.05), whereas treatment with oestradiol caused a surge of LH and ovulation. Progesterone administered with oestradiol prevented the LH surge.