Oestrous, ovarian and adrenal response of the ewe to fasting and cold stress

Abstract

The effect of acute stress on oestrus and ovulation rate in 40 mature Border Leicester x Merino ewes was examined in a factorial experiment utilizing the following fasting and shearing treatments: group HW, feeding ad lib., unshorn; group HS, feeding ad lib., shorn; group LW, fasted, unshorn; and group LS, fasted, shorn.

The experiment was conducted in June–July and treatments began 7 days before the expected day of onset of the second oestrus following withdrawal of a progestagen pessary. Ovulation rates were determined by laparotomy or slaughter on days 2–4 following the onset of oestrus. Climatic data were collected twice daily and the severity of the stress experienced by the ewes was assessed by macroscopic and histochemical examination of the adrenal glands in a subsample of each treatment group.

Severe conditions of cold, wind and rain occurred throughout the experimental period, and lipid accumulation in the adrenal cortex indicated that a physiological stress reaction had been invoked, particularly in the fasted groups (LW, LS). Fasting or shearing had no apparent effect on ovulation rate but a significant suppression of behavioural oestrus was observed in both treatments. While all ewes exhibited oestrus in the control group, shearing (HS) and fasting (LW) treatments reduced the incidence of oestrus to six ewes out of 10 and seven out of 10 respectively. In the fasted and shorn group, only three out of 10 ewes exhibited oestrus.