Patterns of late embryonic and fetal mortality and association with several factors in sheep

¹ Division of Animal and Veterinary Sciences, West Virginia University, Morgantown WV 26506-6108

^* To whom correspondence should be addressed. E-mail: einskeep{at}wvu.edu.

	Abstract

Embryonic and fetal mortality reduce lambing rates and litter sizes, thus contributing to economic loss in the sheep industry. In the present study, timing of late embryonic and fetal loss in ewes and factors with which these losses were associated were examined. Ewes lambing and lambs born were compared to pregnancy diagnosis and counts of embryos by ultrasonography near d 25, 45, 65, and/or 85 of gestation. Approximately 19.9% of ewes experienced late embryonic and/or fetal loss and 21.2% of embryos or fetuses were lost from d 25 to term. Potential offspring were lost throughout gestation; 3.7% of embryos from d 25 to 45, 4.3% of fetuses from d 45 to 65, 3.3% from d 65 to 85, and 11.5% from d 85 to parturition, thus approximately 3 to 4% for each 20-d of pregnancy beyond d 25. A greater proportion of ewes lost 1 (36.7%), but not all (20.5% single; 3.8% multiple), embryos or fetuses. Patterns of loss were similar in ewes mated during the anestrous season and the transitional period and did not vary with service period within breeding season or method of synchronization of estrus. Late embryonic or fetal losses were not related to temperature-humidity index. Maternal serum collected near d 25, 45, 65, or 85 of gestation was assayed for concentrations of progesterone, estradiol-17, and vascular endothelial growth factor (VEGF). Proportions of embryos or fetuses lost were associated with breed type (P < 0.05), as were concentrations of progesterone (P < 0.01), estradiol (P < 0.05), and VEGF (P < 0.01). Relationships of loss or retention of pregnancy to hormonal variables at the 4 stages studied were limited. Both complete and partial losses increased rapidly as maternal progesterone at d 25 decreased below 2 ng/mL (P < 0.05). Survival of individuals within a litter from d 25 to 65 was greater for ewes with medium concentrations of VEGF near d 25 and from d 65 to parturition for ewes with high concentrations of VEGF near d 45 (P < 0.05). In summary, late embryonic or fetal losses occurred from d 25 throughout gestation, and varied with breed type and with concentrations of progesterone in maternal serum on d 25.

Key Words: breed type, embryo, ewe, fetus, mortality, pregnancy

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