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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-induced luteal regression in November and February (mid- and late-breeding season) and June (anoestrum). Progesterone and oestradiol-17β 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 of ovulation. The remaining ewe in February and the three ewes in June failed to show elevated oestradiol-17β secretion rates after luteal regression, indicating the absence of follicles in the final stages of maturation. The preovulatory rise in the oestradiol-17β secretion rate, the LH surge and the display of oestrus all occurred earlier, with respect to the PGF-2 infusion, in November than in February, suggesting a greater stimulation of folliculogenesis, and therefore a greater availability of maturing follicles, in November than in February.