EARLY EMBRYONIC DEATH

In many cases, it is difficult to differentiate between early embryonic death and conception failure. Fertilization failure and embryonic loss prior to day 15 of gestation both result in return to service at the normal time of the next expected estrus. Most embryonic loss takes place before day 30 of gestation.^1,² Although fertilization rates may approach 95%, embryonic mortality rate ranges up to 85% with a mean of 20% to 30%.² A great amount is known about embryonic death in sheep, but there is a paucity of data for goats. Embryonic losses of 6% to 42% suggest that survival of goat and sheep embryos is similar.¹

The reason for loss of morphologically normal embryos remains obscure but is probably attributable to physiologic or environmental factors. The selective loss of genetically abnormal embryos is unavoidable. Genetically abnormal embryos contribute a relatively constant, and significant, proportion of observed losses.² This loss takes place early in gestation and offers a chance for rebreeding in a relatively short time. In humans, pregnancy occurs in only 18% to 28% of the menstrual cycles.³ The majority of losses are from embryos with gross chromosomal abnormalities, with trisomy in almost half of these cases.⁴ Chromosomal abnormalities, such as centric fusions and translocations, cause an unstable karyotype. Animals are inconsistent with humans, having a lower incidence of chromosomal abnormalities. One study showed that animals had an incidence of chromosomal abnormalities of 8.7%, with mosaicism predominating.⁵ The incidence of abnormalities, especially polypoids, increases with aging of the female gametes.

The maternal environment may be unable to support normal development of the embryo or an inappropriate relationship may exist between the embryo and its dam.⁴ The maternal environment may be inadequate to support normal gestation either because of an inherent abnormality of the reproductive tract or as a result of an inappropriate hormonal pattern or other outside factors. Progesterone and estrogen determine the proper function of the uterus in preparation for embryonic development. A steroid imbalance may induce asynchrony between the embryo and uterus² as shown when embryos are transferred to recipients that are not synchronous with the donor. In these cases, pregnancy is not established and the embryos are lost. The variability of progesterone concentration around estrus and the timing and extent of the luteal rise in progesterone concentration account for a proportion of embryonic deaths.²

Ovulation rate seems to play a role in embryo survival in goats. The chance of an embryo surviving in any doe decreases as the ovulation rate rises above two.^2,⁶ Embryo transfer in the goat has shown that transferring two embryos results in better pregnancy rates and embryo survival than transferring one or three embryos.⁷ Prolificacy may be lowered if there is a significant partial loss of multiple embryos with the female remaining pregnant.

Nutrition and stress are two environmental factors that may adversely affect embryo survival. Heat-induced embryonic death is at times an important factor in goat productivity.² Does exposed to high ambient temperatures around breeding and during the early cleavage stages experience some embryo loss. Poor nutrition is often implicated when there appears to be no other reason for embryo loss. In sheep, overfeeding or underfeeding may reduce embryo survival.