The effects of length of oestrus and number of inseminations on the fertility and twinning rate of the Merino ewe

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Abstract
In a flock of 512 Merino ewes all were inseminated on the first day of oestrus. On the next day half of those still in oestrus were re-inseminated, as were half of those no longer in oestrus. Based on numbers of lambs born subsequently, estimates were made of the separate and joint effects of number of times inseminated and number of days in oestrus. A second insemination significantly increased the proportion of ewes bearing twins by 4.7%. Other effects were not significant, but suggested that ewes with longer oestrous periods produced slightly more lambs, and that ewes in oestrus on a second day benefited most from re-insemination.