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Early reports suggested fetal growth is an energetically efficient process; however, more recent reports have suggested fetal growth to be a relatively inefficient process. These latter reports were based on indirect estimates obtained by the use of indirect calorimetry or comparative slaughter approaches, whereas the earlier data resulted from acute in vivo and in vitro approaches. Methodologies have been developed to directly measure rates of oxidative metabolism of tissues of the gravid uterus of cows using chronic preparations. The objective of this study was to quantify rates of oxidative metabolism of gravid uterine, fetal and utero-placental tissues of the pregnant cow and to determine how these variables change with stage of gestation.