Date of Graduation


Document Type


Degree Type



Davis College of Agriculture, Natural Resources and Design


Animal and Nutritional Sciences

Committee Chair

E. Keith Inskeep.


Very few authors have studied the effect of the sire on the outcome of pregnancy in sheep beyond differentiation, development, spermatogenesis, fertilization and early embryo survival. A major limitation in direct selection for increased sire competence is that males need to be selected mainly on the performance of their female relatives and females do not express lambing traits until at least one or two years of age. This selection process can be accelerated by studying prenatal survival ability of individual ram's progeny.;The objective of the present study was to identify occurrence of late embryonic and fetal mortality in relation to individual sires. The data set included 980 ewes on 10 farms; each ewe pregnant to one of 67 rams of 12 breeds. Number of embryos or fetuses was determined by transrectal or transabdominal ultrasonography. Records of lambs born showed that potential litter size was underestimated in at least 166 of the ewes. Factors investigated for effects on numbers of lambs born were number of embryos at diagnosis (single or multiple), individual ram, breed of ram, month ewe was bred, breed of the ewe, age of the ewe, embryo type (purebred or crossbred), farm and interactions among these factors. The GLM procedure of SAS was used to examine the main effects and interactions of these factors on the pregnancy outcome. Individual rams that ewes were bred to had a significant effect on the number of lambs born from the pregnancy.;Individual rams sired from 0.70 lambs to 2.45 lambs born per pregnant ewe. In addition, litter size varied with the interaction of breed-type of ram and number of diagnosed embryos. Ewes diagnosed with twins were more often underestimated due to an unusually high number of triplet and quadruplet births. The number of lambs born increased linearly with the age of the ewe. The number of lambs born varied significantly (<0.05) with farm as well as with the farm by number of diagnosed embryos interaction.;Losses of embryos and fetuses sired by hair-type rams (42%) or black-faced rams (30%) were greater than for white-faced rams (20%) or dairy-type rams (21%). Fewer partial losses were observed among embryos and fetuses sired by white-faced rams, but complete losses were greater for the hair-type and white-faced rams. Hair-type ewes (46%) lost significantly more documented embryos or fetuses from the time of examination to birth than black-faced (27%), white-faced (20%), or dairy-type (25%) ewes. Surprisingly, purebred embryos had fewer documented deaths than crossbred embryos. Losses varied with farm and the farm by number of diagnosed embryos interaction (P<0.001), reflecting both expected flock differences and the variation in breed-type composition of the flocks.;In conclusion, the sire influenced the conceptus's ability to develop to term. Although, much of the paternal role is still a mystery, careful ram selection based upon previous records of pregnancy losses in ewes they were bred to may decrease embryonic and fetal wastage by improving conceptus competence genetically. The individual sire's influence on the conceptus needs to be investigated further to determine repeatability and heritability of survival or loss and to better understand the mechanisms controlling the paternal factors influencing embryonic and fetal wastage in sheep.