Development of the Bovine Fetus1

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Abstract
Heifers were fed three dietary energy levels to provide for gains of 0 to .1 (Low), .5 to .6 (Medium), and 1.0 to 1.1 (High) kg/day from 35 to 42 days of gestation until slaughtered. Animals on each dietary treatment were slaughtered at 90, 120, 150, 180, 210, 240 and 255 days of gestation in a study to characterize bovine fetal growth as a basis for assessing the stage of gestation at which fetal development might be altered without suppressing neonatal growth. Level of maternal dietary energy did not influence (P>.10) fetal weight or composition. Initial instantaneous growth rate (weight) was 7.4%/day and this rate declined at a rate of .0125% for each day of gestation. The instantaneous rate of increase of fetal weight reached a maximum at 232 days of gestation and declined thereafter. The rate of DNA synthesis reached a .maximum at 218 days of gestation followed by RNA (227 days), protein (243 days) and ash (250 days). The rate of growth due to hyperplasia (DNA per fetus) decreased relatively more with increasing fetal age than did the rate of growth due to hypertrophy (protein to DNA and RNA to DNA ratios). Correlations among fetal weight, fetal fluid weight, cotyledon area, placental weight and uterine weight were high and positive, but there was no indication that fetal growth can be effectively mediated indirectly by restriction of uterine or placental development. Copyright © 1979. American Society of Animal Science . Copyright 1979 by American Society of Animal Science.