Early prenatal survival in relation to the parental environment in sheep: A review

Abstract

After multiple egg transfer litter size (LS) in recipient ewes from some breeds was greater than their potential natural size; Finnish Landrace, not Booroola Merino recipients, tended to be superior to recipient ewes from other breeds with a lower natural ovulation rate (OR). In Finnish Landrace and Merino ewes the difference in prenatal survival (PS) between purebred and crossbred litters was small. In Berrichone and Romanov breeds PS was higher in crossbred than in purebred ewes. A significant variation in PS was found between eggs from Finnish Landrace donors. A dam–embryo interaction on PS was present in high and low twinning Merino ewes. Embryos from Romney ewes selected against lamb faecal egg count (LFEC) were more likely to survive than embryos from Romney ewes selected for LFEC. Various translocation types in heterozygous and homozygous combinations in rams and ewes did not influence their overall fertility, probably because of prezygotic selection of gametes. Scrotal heating in Merino rams caused an increased loss of fetuses. Fetal survival in Merino ewes increased as both the number of spermatozoa and feeding level were higher. Lower service activity was linked to lower pregnancy rates (PR) and PS in 1.5-year-old Merino ewes, interacting with the mean duration of oestrus as compared to that in adult Merino ewes. Service activity per ewe, PR and PS in Merino flocks were lower as ram: ewe ratio decreased from 1.0 to 0.5 and 0.25%. In Merino flocks selected for skin folds the percentage of twin births per ewe lambing was lower in hot weather than in Merino flocks selected against skin folds. PR and PS tended to be higher in ewes exposed to Rambouillet rams selected for high prolificacy, compared to Rambouillet rams selected for low prolificacy. PR and PS were lower in Clun Forest ewe lambs than in Clun Forest ewes. Survival of ova from Galway and Romney ewes was greater than that of ewe lamb ova. Conditions in the uteri of both ages were equally favourable. In Galway ewe lambs plasma estrogen concentration 12–36 h after sponge removal was more than 100% greater than in adults. Progesterone levels were similar, also on day 12 of pregnancy. After day 13 progesterone concentration increased at a slower rate and remained lower on day 28 in Clun Forest ewe lambs than in Clun Forest adults. The smaller and more variable preovulatory LH surge in Clun Forest ewe lambs was considered to be evidence of inadequate progesterone priming. These and other data support the suggestion that conditions in the developing follicle and in the reproductive tract between ovulation and the 8–16 cell stage are related to the reduced fertility and PS in very young ewes. It is concluded that endogenous, paragenous and exogenous factors can have similar and interacting effects on PS in sheep.