EARLY PRENATAL SURVIVAL IN RELATION TO THE PARENTAL ENVIRONMENT IN SHEEP - A REVIEW

After multiple egg transfer litter size (LS) in recipient ewes from so me breeds was greater than their potential natural size; Finnish Landr ace, not Booroola Merino recipients, tended to be superior to recipien t 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 an d Romanov breeds PS was higher in crossbred than in purebred ewes. A s ignificant variation in PS was found between eggs from Finnish Landrac e donors. A dam-embryo interaction on PS was present in high and low t winning Merino ewes. Embryos from Romney ewes selected against lamb fa ecal egg count (LFEC) were more Likely to survive than embryos from Ro mney ewes selected for LFEC. Various translocation types in heterozygo us and homozygous combinations in rams and ewes did not influence thei r overall fertility, probably because of prezygotic selection of gamet es. Scrotal heating in Merino rams caused an increased loss of fetuses . Fetal survival in Merino ewes increased as both the number of sperma tozoa and feeding level were higher. Lower service activity was linked to lower pregnancy rates (PR) and PS in 1.5-year-old Merino ewes, int eracting with the mean duration of oestrus as compared to that in adul t Merino ewes. Service activity per ewe, PR and PS in Merino flocks we re lower as ram: ewe ratio decreased from 1.0 to 0.5 and 0.25%. In Mer ino flocks selected for skin folds the percentage of twin births per e we lambing was lower in hot weather than in Merino flocks selected aga inst skin folds. PR and PS tended to be higher in ewes exposed to Ramb ouillet rams selected for high prolificacy, compared to Rambouillet ra ms selected for low prolificacy. PR and PS were lower in Clun Forest e we lambs than in Clun Forest ewes. Survival of ova from Galway and Rom ney ewes was greater than that of ewe lamb ova. Conditions in the uter i of both ages were equally favourable. In Galway ewe lambs plasma est rogen concentration 12-36 h after sponge removal was more than 100% gr eater 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 inadeq uate progesterone priming. These and other data support the suggestion that conditions in the developing follicle and in the reproductive tr act between ovulation and the 8-16 cell stage are related to the reduc ed fertility and PS in very young ewes. It is concluded that endogenou s, paragenous and exogenous factors can have similar and interacting e ffects on PS in sheep. (C) 1998 Elsevier Science B.V. All rights reser ved.