The partition of nutrients in ewes maintained in a moderate compared with a lean body condition in late pregnancy

The effect of ewe fatness on fetal weight at term was tested without the confounding effects of placental weight and feed intake. We hypothesised that fetal weights should be similar in fat or lean ewes with placentas of a similar size, and tested the hypothesis by manipulating nutrition so that, at mating, Merino ewes carrying a single fetus were in a medium (score 2.9, liveweight 46.6 kg) or lean (score 2.0, liveweight 40.6 kg) condition. They were maintained at this fatness difference until slaughter at Day 146 of pregnancy when fetal, placental, and maternal tissues were weighed and analysed for composition. Subgroups (n = 8 per fatness group) slaughtered at Day 110, a stage when most placental hypertrophy is complete but the majority of fetal hypertrophy is yet to occur, confirmed that the treatments differed in ewe fatness (3.82 v. 9.19 kg empty-body fat, s.e.m. = 0.960, P <0.001) but not placental weight (487 v. 538 g, s.e.m = 41.5, P > 0.05). By Day 146, fatness differences (4.77 v. 9.56 kg empty-body fat, s.e.m. = 0.960, P <0.001) and placental similarities (434 v. 502 g, s.e.m. = 38.3, P > 0.05) were maintained, and both groups produced fetuses of similar size (4408 v. 4382 g, s.e.m. = 204.6, P > 0.05). However, the fetuses in the lean ewes had 20% less fat/kg fat-free body weight (24 v. 30 g/kg, s.e.m. = 1.3, P <0.01). Fetal weight was correlated with placental weight (r = 0.70, P <0.01) but not with ewe fatness. Fetal fatness, however, was correlated with ewe fatness (r = 0.69, P <0.01). Ewe fatness per se did not influence fetal size but did influence the deposition of fat in the fetus, possibly via a greater ability of fatter ewes to partition more glucose toward their fetus.