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

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Abstract
Ewes with cervical ovarian autotransplants were studied after PG[prostaglandin]F-2.alpha.-induced luteal regression in Nov. and Feb. (mid- and late-breeding season) and June (anestrum). Progesterone and estradiol-17.beta. secretion rates and LH [luteinizing hormone] concentrations were determined in serial ovarian venous blood samples and the ewes were frequently tested for estrus. All 4 ewes in Nov. and 3 of the 4 ewes in Feb. exhibited estrus and endocrine changes indicative of ovulation. The remaining ewe in Feb. and the 3 ewes in June failed to show elevated estradiol-17.beta. secretion rates after luteal regression, indicating the absence of follicles in the final stages of maturation. The preovulatory rise in the estradiol-17.beta. secretion rate, the LH surge and the display of estrus all occurred earlier, with respect to the PGF-2.alpha. infusion, in Nov. than in Feb., suggesting a greater stimulation of folliculogenesis, and therefore a greater availability of maturing follicles, in Nov. than in Feb.