Assessment of reproductive wastage in sheep. 1. Fertilization failure and early embryonic survival

Abstract

The measurement of fertilization rate and embryonic survival in ewe flocks has been examined from both a theoretical and practical viewpoint. An hypothesis of independence (of survival) for ova or zygotes in multiple ovulating ewes was tested in data obtained from four adult and four maiden ewe flocks mated during the autumn, both crossbred and Merino ewes being examined. Only the adult ewe flocks, with reasonable numbers of multiple ovulating ewes, provided adequate information to test the hypothesis. The data for fertilization rate showed significant deviation from the hypothesised binomial distribution (independence hypothesis). Fertilization appeared to be 'all or none'; that is, irrespective of the number of eggs present in the ewe, they were either all fertilized or all not fertilized. An 'all or none' model for fertilization is described and parameter estimates given. Data for embryonic survival in single and twin ovulating ewes were in agreement with the binomial distribution and the independence hypothesis was accepted. Methods are described for using the 'all or none' and the 'independence' models to obtain estimates of fertilization rate and embryonic survival in investigations of ewe infertility.