Heat stress and development of the conceptus in domestic sheep

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Abstract
In four series of experiments Merino ewes were exposed to ambient temperatures of about 44 °C and water vapour pressure of 33 mmHg for 9 h daily, and to 32 °C and 18 mmHg for the remaining 15 h daily, during the middle third, the final third or the final two-thirds of pregnancy. Birth weight and the weight of the placenta were considerably reduced by the treatments and the reductions were considerably more than could be accounted for by the partial loss of appetite produced by heating.There was a close inverse relationship between birth weight and the elevated rectal temperature of heated ewes in several series; but the absence of foetal dwarfing in ewes with elevated rectal temperatures due to daily heating for only 9 h at 44 °C indicates that the elevated rectal temperature of the ewe is not the main cause of dwarfing.Contrary to previous suggestions, the dwarfed lambs were not proportional miniatures, for the head components, the body lengths, the kidneys and adrenal glands were disproportionately large in heated lambs, while the liver, thyroid and thymus glands and the biceps femoris muscle were disproportionately small, and the ratio of secondary to primary wool fibres was very much reduced. There were cavities in the white matter of the cerebral hemispheres of heat-dwarfed lambs.The hypothesis that foetal dwarfing is due to stunting of the placenta was examined, but conflicting evidence was obtained in the different series. However, it appears that under some circumstances a heat-stunted placenta is capable of considerable growth in the absence of heating during the final third of pregnancy, when the placenta is normally shrinking, and that placental shrinkage in late pregnancy may be greatly accelerated by the application of heat.Neither thyroxine nor a preparation of ovine growth hormone, injected into heated ewes, prevented foetal dwarfing; the injections of growth hormone appeared to increase foetal mortality.