Effects of Progesterone on the Responses of Merino Ewes to the Introduction of Rams during Anoestrus

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Abstract
The effects of progesterone on the responses of Merino ewes to the introduction of rams during anestrus were investigated in 2 experiments. In the 1st experiment, the introduction of rams induced an increase in the levels of LH [leutenizing hormone] in entire ewes. The mean levels increased from 0.68 .+-. 0.04 ng/ml (mean .+-. SEM [standard error of the mean]) to 4.49 .+-. 1.32 ng/ml within 20 min in ewes not treated with progesterone (n = 10). In ewes bearing progesterone implants that provided a peripheral concentration of .apprx. 1.5 ng progesterone per milliliter plasma, the LH response to the introduction of rams was not prevented, but was reduced in size so that the concentration was 1.38 .+-. 0.15 ng/ml after 20 min (n = 5). Progesterone treatment begun either 2 days before or 6 h after the introduction of rams and maintained fof 4 days prevented ovulation. In the 2nd experiment ovariectomized ewes were used to investigate further the mechanism by which the ram evoked increases in tonic LH secretion. In ovariectomized ewes treated with estradiol implants, the introduction of rams increased the frequency of the LH pulses and the basal level of LH. In the absence of estradiol there was no significant change in pulse frequency but a small increase in basal levels. Progesterone again did not prevent but reduced the responses in ewes treated with estradiol. Following the withdrawal of progesterone treatment, the secretion of LH pulses in response to the ram effect would be dampened. This effect could be a component of the reported long delay between the introduction of rams and the preovulatory surge of LH in ewes treated with progesterne. Continued progesterone treatment prevented ovulation, probably by blocking positive feedback by estradiol.

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