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EMBRYO MORTALITY IN THE HEAT STRESSED EWE

Summary.: The possible rôles were examined of the corpus luteum, thyroid and adrenal cortex in the embryo mortality experienced by heat stressed Merino ewes.

Heat stress during early pregnancy did not significantly affect the histology, size or weight of the corpus luteum, and progesterone therapy did not enhance embryo survival in heated ewes. Heat stress did depress (P<0·001) thyroid epithelial cell height and hence, presumably, thyroxine secretion rate. However, thyroxine therapy was without effect on embryo survival in both heated and unheated ewes. Raised cortisol secretion rates were not indicated in heated ewes. Cortisol administered to unheated ewes did not affect embryo survival, although it did depress embryo growth rate (P<0·10).

Changes in the luminal fluids of the Fallopian tubes and uterus are proposed as the most likely mechanism of heat-induced embryo mortality in the ewe.