Seasonal Changes in the Occurrence of Oestrus Following Progesterone Suppression of Ovarian Function in the Merino Ewe

Lamond, D. R.

doi:10.1530/jrf.0.0080101

Cited by 18 publications

(11 citation statements)

References 9 publications

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“…This is in agreement with reports of Lamond & Bindon (1962) and Lamond (1964), but they interpreted their results as an indication of seasonal variation in sensitivity to progesterone. In this experiment the shortest interval between end of progesterone treatment and onset of oestrus coincided with the time of maximum oestrogen production.…”

Section: Seasonal Variation In Oestrous Behavioursupporting

confidence: 92%

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

Fletcher¹,

Lindsay²

1971

Journal of Endocrinology

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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.

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Section: Seasonal Variation In Oestrous Behavioursupporting

confidence: 92%

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

Fletcher¹,

Lindsay²

1971

Journal of Endocrinology

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“…There was no significant interaction between dose of progesterone and season, yet interaction would be evident if ewes showed seasonal changes in sensitivity to progesterone, and priming with 10 mg/day did not condition the ewes to respond to oestrogen equally at both seasons. On the basis of this finding we are unable to confirm in the spayed ewe the suggestion by Lamond Bindon (1962) and Lamond (1964) that the sensitivity of ewes to progesterone varies with season. Seasonal changes in the expression of oestrous behaviour in spayed ewes primed with progesterone at doses between 5 and 20 mg/day can thus be attributed to changes in sensitivity to oestrogen.…”

contrasting

confidence: 93%

“…This interpretation is open to question, however, in view of suggestions from Lamond & Bindon (1962) and Lamond (1964) that intact ewes showed seasonal changes in sensitivity to progesterone. The investigation reported here was designed to determine whether changes in sensitivity to progesterone contributed to seasonal variation in the expression of oestrous behaviour in spayed ewes.…”

mentioning

confidence: 99%

Seasonal Changes in the Sensitivity of Spayed Ewes to Progesterone and Oestrogen

Fletcher¹,

Allison²,

Lindsay³

1971

Journal of Endocrinology

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Seasonal variation in the incidence and duration of behavioural oestrus in spayed ewes treated with progesterone and oestrogen has been attributed to changes in the sensitivity of ewes to oestrogen (Reardon & Robinson, 1961;Fletcher & Lindsay, 1971). This interpretation is open to question, however, in view of suggestions from Lamond & Bindon (1962) and Lamond (1964) that intact ewes showed seasonal changes in sensitivity to progesterone. The investigation reported here was designed to determine whether changes in sensitivity to progesterone contributed to seasonal variation in the expression of oestrous behaviour in spayed ewes.The experiment was of factorial design with three different doses of progesterone for priming, two breeds of ewe and two seasons of observation. Sixty spayed Merino and 60 spayed Border Leicester \ m=x\Merino cross-bred ewes were each allotted at random to three groups of 20. They were injected daily with 5, 10 or 20 mg progesterone for a total of 12 days, and treated subsequently with a single injection of 24\m=.\3 \ g=m\ g oestradiol benzoate (OB). Oestrogen was injected 24 h after the last 5 mg, 48 h after the last 10 mg, and 72 h after the last 20 mg injection of progesterone. The beginning of priming with progesterone was staggered so that all ewes were injected with OB at the same time.The ewes were teased with active rams every 4 h to determine the incidence and duration of behavioural oestrus. Duration of oestrus provides a highly sensitive measure of the response to oestrogen by spayed ewes primed with progesterone (Fletcher & Lindsay, 1971), and for this investigation differences in duration of oestrous behaviour induced by a standard dose of oestrogen were used to detect differences in sensitivity to oestrogen. Two identical tests were made with the same ewes, in December 1967 and in April 1968. These times coincide with the non-breeding and breeding seasons respectively of Border Leicester Merino cross-bred ewes in our environment (Fletcher & Lindsay, 1971). Ewes within breeds were re-allotted at random to the three dose levels of progesterone before the second test.The results are summarized in Table 1. There was no significant interaction between dose of progesterone and season, yet interaction would be evident if ewes showed seasonal changes in sensitivity to progesterone, and priming with 10 mg/day did not condition the ewes to respond to oestrogen equally at both seasons. On the basis of this finding we are unable to confirm in the spayed ewe the suggestion by Lamond &

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“…Lamond (1964) fig. 3), cyclic activity is not inhibited in those ewes which do not become pregnant and, indeed, earlier evidence fig.…”

Section: Discussionmentioning

confidence: 99%

The Seasonal Nature of Reproductive Phenomena in the Sheep

Tj¹

1971

Reproduction

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There was a marked seasonal variation in all parameters associated with fertility and best results were obtained overall with the sponges releasing the higher dose. Treatment in the spring was followed by a low incidence of ovulation, a high incidence of 'silent heats' and few pregnancies following insemination (13\m=.\6%) ; most of the ewes that were not pregnant became anoestrous. Treatment in the autumn was followed by a high incidence of ovulation and oestrus, and a high pregnancy rate following a double insemination (82\m=.\5%), and few animals became anoestrous.

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Seasonal Changes in the Occurrence of Oestrus Following Progesterone Suppression of Ovarian Function in the Merino Ewe

Cited by 18 publications

References 9 publications

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

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

Seasonal Changes in the Sensitivity of Spayed Ewes to Progesterone and Oestrogen

The Seasonal Nature of Reproductive Phenomena in the Sheep

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