Seasonal variation in oestrus, the secretion of oestrogen and progesterone and LH levels before ovulation in the ewe

Summary. Ewes with cervical ovarian autotransplants were studied after PGF-2\g=a\\x=req-\ induced luteal regression in November and February (mid- and late-breeding season) and June (anoestrum). Progesterone and oestradiol-17\g=b\secretion rates and LH concentrations were determined in serial ovarian venous blood samples and the ewes were frequently tested for oestrus. All 4 ewes in November and 3 of the 4 ewes in February exhibited oestrus and endocrine changes indicative ofovulation. The remain-ing ewe in February and the three ewes in June failed to show elevated oestradiol-17\g=b\ secretion rates after luteal regression, indicating the absence of follicles in the final stages of maturation. The preovulatory rise in the oestradiol-17\g=b\secretion rate, the LH surge and the display of oestrus all occurred earlier, with respect to the PGF-2\g=a\ infusion, in November than in February, suggesting a greater stimulation of folliculo-genesis, and therefore a greater availability of maturing follicles, in November than in February