Embryo Mortality in the Heat Stressed Ewe

Abstract

Continuous hot-room exposure during the first 15 days of pregnancy raised the rectal temperatures and respiratory rates of mature Merino ewes to about 104 to 105\s=deg\F and 170 respirations/min respectively and led to the death of 83% of embryos.Similar ewes subjected to a diurnally variable heat stress, which raised their rectal temperatures and respiratory rates to about 104 to 105\s=deg\F and 220 respirations/min during an 8-hr 'day' and allowed them to fall to about 102\s=deg\F and 140 respirations/min respectively during a subsequent 16-hr 'night', experienced an embryo mortality rate of only 35%.Control ewes, housed similarly, but at natural temperatures (33 to 65\s=deg\ F), had rectal temperatures and respiratory rates of about 101 to 102\s=deg\F and 20 respirations/min, respectively, and suffered the loss of 19% of their ova.The results illustrate the mitigating effects of a daily period of respite from high temperatures and indicate that this, rather than differences in acclimatization to heat and lack of radiant heating in the hot-room, is the major factor to be considered when comparing the effects on embryo mortality of heat stress in the field with those of periods of continuous heat stress in the hot-room.

Embryo Mortality in the Heat Stressed Ewe

“…Similarly, in Exp. 100 C. J. Thwaites The relative numbers of lutein cell types and the proportion of lutein cells in the cl did not differ significantly between groups (Thwaites, 1968a) and group mean values ( + S.E.) It was, however, associated with a reduction in embryo size (Table 4) and, for embryo weight, this reduction approached significance.…”

“…Full details of conditions experienced by ewes in the various experiments have been reported elsewhere (Thwaites, 1968a(Thwaites, , b, 1969, it is sufficient to record here that prevailing temperatures ranged from 33°to 72°F and that mean hot-room temperatures in different experiments varied between 96·5°and 100°F with approximately 50% relative humidity. Full details of conditions experienced by ewes in the various experiments have been reported elsewhere (Thwaites, 1968a(Thwaites, , b, 1969, it is sufficient to record here that prevailing temperatures ranged from 33°to 72°F and that mean hot-room temperatures in different experiments varied between 96·5°and 100°F with approximately 50% relative humidity.…”

“…1 to 5 have been presented elsewhere (Thwaites, 1968a(Thwaites, , b, 1969. 1 to 5 have been presented elsewhere (Thwaites, 1968a(Thwaites, , b, 1969.…”

“…Embryo death on or before Day 12 would not be expected to interfere with the normal cyclic life-span of the cl (Moor & Rowson, 1964Edey, 1967) and, on Day 15, such cl could be expected to be decreasing in size (Restall, 1964;Hutchinson & Robertson, 1966). There were, however, only five ewes in each group and the fact that the progesteronetreated ewes had lower body temperatures than controls may be sufficient to explain the former's slightly better reproductive performance (Thwaites, 1968c). Other studies using the same method of evaluation have shown that luteal regression in the cyclic Merino ewe occurs between Days 15 and 17 (Thwaites, 1967a).…”

Early Reproductive Failure in Ewes in a Tropical Environment*

“…Thwaites (1969) has reviewed the results of a number of hot room studies which have demonstrated that ewes subjected to continuous heat stress during mating and early pregnancy suffer high levels of fertilisation failure and embryonic mortality. The relevance of these studies to the field situation is somewhat doubtful, since it appears clear that the diurnally variable component of natural heat stress has an ameliorating effect on fertilisation failure and embryonic mortality (Thwaites 1969).…”