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EMBRYO MORTALITY IN THE HEAT STRESSED EWE I. THE INFLUENCE OF BREED

Summary.: In two separate experiments during the autumn of 1965 mature ewes of the Merino and Southdown breeds were continuously exposed to elevated temperatures in a hotroom for the first 20 days of pregnancy. Hotroom conditions in the two experiments were adjusted so as to stress similarly the ewes of both breeds. In both experiments hotroom treatment elevated rectal temperatures by approximately 2·5° F and respiratory rates by approximately 150 respirations/min. Such conditions led to the death of 100% of embryos in treated ewes. In both experiments approximately 75% of this embryonic death occurred at an early stage which did not interfere with subsequent returns to service after an oestrous cycle of normal length. The remaining 25% of the embryonic deaths occurred at later developmental stages and resulted in the presence of degenerating embryos in utero at autopsy on Day 23. It would appear that breed differences in heat-induced embryo mortality under similar environmental conditions are largely a consequence of breed differences in heat tolerance.

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