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/ml 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.