Early embryo loss, morphology, and effect of previous immunization against androstenedione in the ewe

More than 30% of ova are lost by Day 30 of gestation in ewes. Understanding the natural variety of embryo morphology and the critical period where the majority of embryo loss is observed in the modern productive ewe is the first step to targeting remedies to reduce this substantial cost to livestock production industries. In a naturally mated cycle, ova and viable embryo number as well as embryo size were assessed on Day 4, 10, 14, 18, and 30 of gestation in a total of 225 Romney ewes (n = 44 to 46 per gestational group). The majority (18%) of embryo loss occurred before Day 14 of gestation with 6% loss prior to Day 4, and 12% loss between Day 4 and 14 of gestation. A similar proportion of viable embryos per number of ova ovulated were recovered on Day 14 and 18 (82%) and Day 30 (81%) of gestation. Fertilisation failure was estimated at 1%. Conceptus and embryo size was most variable on Day 14, representing a period of rapid growth (conceptus length + SD); Day 4 (169 + 8 m), Day 10 (379 + 93 m), Day 14 (23 + 32 mm), Day 18 (embryo length + SD; 5.0 + 0.7 mm). Previous seasons vaccination with commercially available fertility vaccines targeting androstenedione (Androvax and Ovastim) resulted in reduced conceptus size compared with controls. However, no difference in the proportion of viable embryos was observed between treatments signifying maternal tolerance for considerable variation at this stage of development. Further, the finding that the majority of loss occurs between Day 4 and 14 of gestation highlights the importance of the uterine environment for the embryo to successfully overcome the challenges leading up to and including pregnancy recognition in the ewe.