Seasonal variation in the reactivity to oestrogen of the ovariectomized ewe

Abstract

Fifteen tests carried out at an average interval of 4 weeks for 14 months showed an annual rhythm in the proportion of spayed crossbred ewes which exhibited oestrus after an injection of 15.6 µg oestradiol benzoate (ODB) following a 12 day period of progesterone treatment. Reactivity was highest in late summer and early autumn and lowest in winter and early spring. The periods of minimum reactivity coincided with the periods of low environmental temperatures and low body weights. The significance of the annual rhythm is discussed with particular relation to the phenomenon of "silent oestrus". No similar rhythm was detected in the 7-aginal response to oestrogen.

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.…”

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

“…Since in spayed ewes the time of onset of oestrus decreased with increasing dose of injected oestrogen, seasonal variation in time of onset of oestrus in intact ewes may be attributed to the associated variation in oestrogen production without the need to assume a variation in the sensitivity of ewes to progesterone. Raeside & McDonald (1959) and Reardon & Robinson (1961) have reported that the incidence of oestrous behaviour in spayed ewes treated with a constant dose of oestrogen was greater during the breeding season than during the non-breeding season. Although we did not show this relationship in the case of incidence of oestrus, duration of oestrus showed significant variation not only between but also within the breeding and non-breeding seasons.…”

“…Raeside & McDonald (1959) and Reardon & Robinson (1961), using the criterion of incidence of oestrus associated with marginal doses of oestrogen, have reported a seasonal variation in sensitivity to a constant level of oestrogen. Changes in the production of endogenous oestrogen have not been investigated.…”

Neuroendocrine Regulation of the Estrous Cycle and Seasonal Breeding in the Ewe

“…From this point on, the breeding season would be effectively terminated even if ovulatory cycles continued for a short time. Seasonal changes in sensitivity to the behavioral effects of estradiol have been reported (Reardon and Robinson, 1961;Gibson and Robinson, 1971;Land et al, 1976). However, the real issue (as with the LH surge system) is whether the preovulatory estradiol rise is sufficient to induce behavioral estrus during the anestrous season.…”