Effect of Oestrogen on Oestrous Behaviour and Its Variation With Season in the Ewe

Abstract

The effects of dose of oestrogen and season on the expression of oestrous behaviour was studied in 60 spayed and 16 intact ewes.In progesterone-primed spayed ewes the incidence and duration of oestrous behaviour increased and the interval between injection of oestrogen, and onset of oestrous behaviour decreased as the dose of oestrogen was increased. The linear regression of duration of oestrus on log dose of injected oestrogen provided the most precise measure of the effect of changing dose level of oestrogen on oestrous behaviour.The sensitivity of spayed ewes to constant levels of oestrogen increased to a maximum near the middle of the breeding season and then declined to a minimum during the non-breeding season. In progesterone-synchronized intact ewes the interval between the end of progesterone treatment and onset of oestrus was at a minimum, and duration of oestrus at a maximum, near the middle of the breeding season. By bioassay it was estimated that endogenous oestrogen production was greater near the middle ofthe breeding season than at either the beginning or end.It is concluded that seasonal variation in the expression of oestrous behaviour in intact ewes is controlled by changes in both the production of endogenous oestrogen and the sensitivity of ewes to it.

Effect of Rams on the Duration of Oestrous Behaviour in Ewes

“…In this experiment, variation in ovarian oestrogen production was eliminated by using spayed ewes and injecting controlled doses of oestrogen. Lindsay (1966) In the first experiment, the linear regressions of incidence, time of onset, and duration of oestrous behaviour on log dose of oestrogen were similar to those already reported (Fletcher & Lindsay, 1971). Although measurements of oestrous behaviour, and duration of oestrus in particular, varied with method of observation, the slopes of the regression lines were not significantly different.…”

“…The main effects are shown in (Donovan & Harris, 1966;Lisk, 1966), and duration of oestrus increases with increasing level of oestrogen (Fletcher & Lindsay, 1971). Association with rams could influence the duration of oestrus in entire ewes either through a direct effect on the neural mechanism controlling oestrous behaviour, or through an indirect effect on the elaboration of oestrogen from the ovaries.…”

Reproductive performance of Coopworth ewes following oral doses of zearalenone before and after mating

“…In neither experiment was there any effect of zearalenone before mating on subsequent embryo recovery rates. The decrease in cycle length and increase in duration of oestrus with increasing dose of zearalenone confirm its oestrogenic action in the ewe (Fletcher & Lindsay, 1971). This is further supported by the increase in uterine weight and reduction in ovarian weight recorded.…”

Feedback basis of seasonal breeding: test of an hypothesis

“…The infusion rate was increased progressively over a 3-day period in a manner calculated to reproduce the sustained increase in tonic LH secretion that normally precedes the preovulatory LH surge. This con¬ tinuous infusion pattern did not, however, mimic the pulsatile character of the endogenous tonic LH rise (Baird, 1978 (Land, Wheeler & Carr, 1976), much as oestradiol appears to be less effective in eliciting oestrous behaviour at that time (Raeside & McDonald, 1959;Reardon & Robinson, 1961;Fletcher & Lindsay, 1971;Gibson & Robinson, 1971). These findings, however, are difficult to evaluate in the context of our proposed scheme for seasonal breeding because the response to oestradiol concentrations within the physiological range of 1-10 pg/ml has not been studied.…”