Effect of level of food intake on hypothalamo-pituitary sensitivity to ovarian Steroid feedback in ewes

Abstract

Effects of level of food intake and body condition on ewe hormone status, ovulation rate and reproductive performance may be mediated either through a direct effect on hypothalamic or pituitary activity or through an indirect effect on the sensitivity of either or both of these organs to steroid feedback. The aim of this experiment was to determine the effect of level of food intake of ewes on hypothalamic activity and pituitary responsiveness to gonadotrophin-releasing hormone (GnRH) in the presence of ovarian steroids. Sensitivity to steroid feedback cannot be measured in an entire animal, owing to the variable and irregular nature of ovarian steroid secretion. Ovariectomised ewes were therefore used together with steroid replacement therapy.Two groups of 16 ewes were fed to achieve body condition scores of either 2.00 or 2.25/2.50 by 3 weeks before the period of study. Thereafter, ewes of the lower condition score were fed dried grass pellets ad libitum and 250 g hay/head/day (H) while those of the higher condition score were fed 1050 g pellets and 250 g hay/head/day (M), a ration designed to maintain liveweight and condition score so that at the time of study, ewes of the two groups had a similar level of body condition but different levels of intake.