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Critical Period for Early Embryo Mortality in Ewes Exposed to High Ambient Temperature¹

Kentucky Agricultural Experiment Station, Lexington

Abstract

The effect on embryo mortality in ewes exposed to high ambient temperature (90° F.) at time of breeding (0-day) and on 1, 3, and 5 days was determined in two experiments. Fertility rate (69.2%) in ewes exposed to a high ambient temperature at time of breeding was non-significantly lower than it was in control ewes.

Exposure to heat resulted in an increase in morphologically abnormal ova. Only 3.7% of ova from control ewes, examined 3 days after breeding, were classified as morphologically abnormal. In the 0-day ewes 46.2% and in the 1-day ewes 30.8% of the ova were classified abnormal.

Embryo loss, estimated as the percent of fertilized ova that failed to survive, was significantly higher in all treated groups and ranged from 61.5% to 100%. Embryo loss for the combined 0- and 1-day groups was significantly higher than in the 3- and the 5-day ewes. The sheep zygote is most sensitive to the harmful effect of high ambient temperature during the initial stage of cleavage while in the oviduct.

Eighteen of 20 ewes in the 0- and 1-day groups failed to conceive and returned to estrus with an average cycle length of 17.4±0.31 days. Fourteen of the 25 ewes that failed to lamb in the 3- and 5-day groups had an average post-breeding cycle length of 17.0±3.5 days. The post-breeding cycle of four ewes ranged from 41 to 66 days, and 7 ewes failed to return to estrus. Induction of death of the embryo after it enters the uterus may result in an estrous cycle of abnormal duration.

Eighty-five percent of the control ewes lambed, compared with 10% of the ewes in the 0- and the 1-day groups. Thirty-five percent of the ewes in the 3-day group lambed, and 40% in the 5-day group lambed.

The possibility of an adrenal disturbance due to temperature stress is discussed as a mechanism by which the harmful effects on the embryo are mediated.

¹ The investigation reported in this paper is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director.

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