Embryo survival in relation to number and site of ovulations in the ewe

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Abstract
The incidence of single-ovulating, and bilateral and unilateral twin-ovulating ewes was determined for 12 flocks that included three breeds of sheep; these were observed at four times within a year. Twin ovulations among these breeding ewes had approximately equal probabilities of being unilateral or bilateral. This suggests that ovulation rate is unlikely to be determined from within the ovary itself. An examination of several sets of published data revealed that the probabilities of embryo survival are consistently highest for single-ovulatory ewes and least for unilateral twin-ovulators. The higher level of embryo losses in unilateral twin-ovulating ewes may be associated with a high incidence of transuterine migration by embryos in these ewes. The probability of successful migration varied with breed and time of year. A binomial model is postulated which takes account of these different probabilities of embryo survival. The model fitted three out of five sets of data examined; the other two sets indicated homeostatic and ewe effects, respectively, acting on embryo survival.