Effects of progesterone on the responses of merino ewes to the introduction of rams during anoestrus

Abstract

The effects of progesterone on the responses of Merino ewes to the introduction of rams during anoestrus were investigated in two experiments. In the first experiment, the introduction of rams induced an increase in the levels of LH in entire ewes. The mean levels increased from 0·68±0·04ng/ml (mean±s.e.m.) to 4·49±1·32ng/ml within 20min in ewes not treated with progesterone (n = 10). In ewes bearing progesterone implants that provided a peripheral concentration of about 1· 5 ng progesterone per millilitre 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/m1 after 20 min (n = 5). Progesterone treatment begun either 2 days before or 6 h after the introduction of rams and maintained for 4 days prevented ovulation. In the second 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 oestradiol implants, the introduction of rams increased the frequency of the LH pulses and the basal level of LH. In the absence of oestradiol 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 oestradiol. It is suggested that 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 progesterone. Continued progesterone treatment prevented ovulation, probably by blocking positive feedback by oestradiol. © 1983 ASEG.