Effect of exogenous oestradiol-17 on gonadotrophin secretion in post-partum beef cows

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Abstract
Twelve cows between days 10 and 17 post partum received 1 or 2 silicone rubber implants, each containing 45 mg estradiol-17.beta.. Plasma estradiol concentrations increased from 4.25 .+-. 0.51 pg/ml before implant insertion to 16.57 .+-. 3.66 pg/ml (1 implant) and 37.10 .+-. 9.1 pg/ml (2 implants) during the 5 days of implant treatment. Preovulatory-type LH [luteinizing hormone] and FSH surges were induced in 11/12 cows between 15 and 79 h after implant insertion. There were no relationships among plasma estradiol concentrations, LH pulse frequency, pretreatment FSH concentrations, timing of the LH surge or time to 1st rise in milk progesterone concentration. There was a negative correlation (r = -0.58; P < 0.05) between the stage post partum and the timing of the peak of the LH surge after implant insertion. Although the positive feedback mechanism was functional in most cows, the data may reflect a continuing recovery of the responsiveness of the hypothalamo-pituitary axis to the positive feedback effects of the estradiol-17.beta. between days 10 and 17 post partum.