PRENATAL SURVIVAL IN RELATION TO PERI-OVULATORY PHENOMENA AND THE SITE OF OVULATION IN SHEEP - A REVIEW

Differences in ovarian follicular dynamics between ewe breeds and betw een lines within breeds were shown to result from a different follicle recruitment and selection process. Synchrony and asynchrony of embryo s were shown to depend largely on luteinizing hormone (LH), progestero ne and oestrogen levels in ewes before, during and post-mating. GnRH t herapy suggested that the endocrine environment prior to ovulation has a fundamental influence on luteal function and hence on prenatal surv ival (PS). The decreasing effect of multiple ovulation on uterine effi ciency was higher in androstenedione-immune Border Leicester x Merino (BL x M) than in immunized BL x Booroola Merino (BM) ewes. Exogenous p rogesterone increased lambing rate in Polypay (P) but not in Targhee e wes. The 200% of maintenance feeding level reduced plasma progesterone concentrations on day 12 near the threshold critical for embryo survi val in M ewes but not in BL x Scottish Blackface ewes. The latter main tained pregnancy rate (PR) and PS at the same levels as the controls. Ln unilaterally twin ovulating ewes, foetal mortality was significantl y lower in ewes in which distributive embryo migration occurred. Withi n this ovarian class, survival was lower in the Cheviot, probably beca use of a lower migration frequency, than in the Blackface breed. More embryos migrated in unilaterally twin ovulating M ewes in autumn than in summer mating, with a higher incidence of loss of one embryo in the latter season. A higher incidence of successful migration was observe d in unilaterally twin ovulating BL x M than in M ewes. The higher mor tality of a migrating embryo, compared with the nonmigrating, was attr ibuted to an asynchronous environment in the contralateral hem, since ovine trophoblastine (oTP-1) might have preferentially altered the ute rine environment of the ipsilateral horn. Studies on oTP-1 and protein in uterine flushings and on conceptus development at day 13 in ewes t hat displayed short or long oestrous cycles, as well as further studie s on unilaterally ovariectomized ewes with two or three conceptuses, s uggest that the ovine embryo stimulates the migratory process locally, in interaction with the dam. This review raises the question whether and to what degree pre-, peri-, as well as, post-ovulatory phenomena a nd the site of ovulation can be interacting factors for determining PS , especially in multiple ovulating sheep as a function of breed and br eed-environment interactions. (C) 1998 Elsevier Science B.V. All right s reserved.