Multiparous Holstein cows (n = 18) were bred artificially to the same bull and then slaughtered at times ranging from 190 to 270 d postconception to assess accretion of energy, protein, fat, and ash by the conceptus. Wet weights, dry weights, and concentrations of energy, CP, crude fat, and ash were obtained for the following: fetus, combined amniotic and allantoic fluids, fetal membranes, cotyledons, caruncles, and uterine tissues. Rates of accumulation of these components in the gravid uterus (sum of all uterine contents) and fetus were described by linear or quadratic equations. Estimated rates of accretion of energy in the gravid uterus (i.e., conceptus) increased from 567 kcal/d at 190 d of gestation to 821 kcal/d at 270 d of gestation; corresponding rates of accretion of CP were 62 and 117 g/d. These daily rates represent net energy and protein requirements for conceptus growth during late pregnancy in mature Holstein cows. Conversion of predicted net energy to metabolizable energy requirements for conceptus growth, using the accepted efficiency factor of .14, yielded estimates that were consistent with current NRC recommendations. Factorial estimation of absorbed protein requirements is hampered by lack of precise information on the efficiency with which absorbed AA are deposited in conceptus tissues.

Pregnant ewes were chronically exposed to thermoneutral (TN; 20 degrees C, 30% relative humidity) or hot (H; 40 degrees C 9 h/d, 30 degrees C 15 h/d, 40% relative humidity) environments between d 64 and 136 to 141 of pregnancy. They were sampled for blood at 14-d intervals during this period for measurement of plasma metabolites and hormones, then slaughtered and dissected to measure conceptus weights, dimensions and fetal organ weights. Rectal temperatures of H ewes were elevated .3 to 1.0 C degrees above those of TN ewes throughout the experiment. Voluntary feed intakes were not altered by heat exposure except after 120 d of pregnancy, when feed intake was about 25% lower (P less than .10) by H than by TN ewes. Blood 3-hydroxybutyrate concentrations were not affected by heat, but plasma glucose concentrations were greater in H than in TN animals after 120 d (P less than .05). Placental weight, reduced by 54% (P less than .001) by heat exposure of ewes, was correlated positively with fetal weight and correlated negatively with fetal/placental weight ratio, fetal brain/liver weight ratio and fetal relative heart weight. Late in pregnancy, plasma concentrations of progesterone, cortisol and placental lactogen were reduced (P less than .01) in H ewes, whereas triiodothyronine levels were markedly lower (P less than .03) at all stages of pregnancy. Plasma concentrations of prolactin were elevated dramatically (P less than .01) and a modest increase (P less than .03) in somatotropin levels was recorded in H ewes. These results are consistent with our hypothesis that heat-induced fetal growth retardation is secondary to a primary reduction in placental growth; this could be mediated partly by reduced peripheral activity of thyroid hormones. Heat-induced reductions in secretion of progesterone and ovine placental lactogen more likely were a consequence than a cause of placental stunting.

Cellular development of muscle was studied in sheep fetuses at 85 days of gestation. Large and small fetuses were compared at 100, 115 and 130 days, and an additional group of large 130-day fetuses were studied following 7 days of maternal undernutrition. Myogenesis in the peroneus longus muscle was completed between 100 and 115 days of gestation, and myofibre number did not differ between small and large fetuses. The proportion of myofibre-related nuclei identified as entering S-phase of the cell cycle was 1.7% per hour in 85-day fetuses. In large fetuses, subsequent rates were relatively constant (=1.5% h –1 ), whereas in small fetuses cell cycle activity declined with age from 1.3 to 0.9% h –1 , and was 0.5 % h –1 in 130-day fetuses of restricted ewes. The constant rate of cell cycle activity in large fetuses was associated with an increasing estimated rate of muscle growth (peroneus longus (mg) = 0.831 ¥ 10 (0.024 ¥ age [d]) , r 2 = 0.98), which contrasted with slow and relatively constant muscle accretion in small fetuses (8.4 mg day –1 ), and slower muscle accretion at 130 days in large fetuses from restricted ewes. Differences in DNA and RNA content in the semimembranosus muscle increased with age, large fetuses having 70% more muscle DNA, 108% more muscle RNA and 104% larger muscles than small fetuses at 130 days (all P<0.001). The results demonstrate that myonuclei accumulation, but not myofibre number, is associated with fetal growth in sheep and, therefore, with fetal nutrition during mid to late gestation.

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