The relationship between the duration of induced oestrus and the characteristics of the progesterone-oestrogen treatment used for its induction was investigated and described. The regression of the duration of oestrus on the dose of oestrogen was found to be 0·0032 and 0·0045 days/μg in the Blackface and Finnish Landrace ewes, respectively. The duration of oestrus was also increased by approximately 40% following the division of a particular dose of oestrogen into two units given 24 hr apart.
The mean durations of induced oestrus over all treatments, 1·52 and 1·92 days, respectively, in the Blackface and Finnish Landrace ewes, were significantly different.
It was concluded that the relationship between ewe prolificacy and the duration of natural oestrus arose at least in part from differences in the pattern of oestrogen secretion around the time of ovulation.
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