NUTRIENT PARTITIONING AND FETAL GROWTH IN RAPIDLY GROWING ADOLESCENT EWES

A highly controlled model to investigate nutrient partitioning and the control of fetal growth in the rapidly growing adolescent sheep is de scribed. Embryos recovered rom superovulated adult ewes inseminated by a single sire were transferred in singleton to the uterus of prepuber tal adolescent recipients induced to ovulate at 21 weeks of age (livew eight 44.4 +/- 0.38 kg). After embryo transfer, the adolescent recipie nts were individually offered a high (n = 28) or low (n = 20) quantity of a complete diet calculated to achieve rapid (RMG) or normal (NMG) maternal growth rates. After day 100 of gestation the feed intake of t he NMG group was adjusted weekly to meet the increasing nutrient deman ds of the gravid uterus. The proportion of adolescent recipients initi ally conceiving was significantly (P < 0.01) influenced by maternal nu trient intake and was lower in the RMG (0.57) than in the NMG (0.85) g roup. For adolescent dams that maintained their pregnancies, liveweigh t gain during the first 95 days of gestation was significantly (P < 0. 001) higher in the RMG compared with the NMG group (234 +/- 9.5 and 75 +/- 5.0 g day(-1), respectively). Rapid maternal growth rates were as sociated with a significant reduction in both fetal and placental weig hts as determined when the animals were killed on day 95 of gestation (n = 3 per group) or at term. For the RMG (n = 8) and NMG (n = 11) gro ups, respectively, mean lamb birthweights at term were 2.74 +/- 0.25 a nd 4.34 +/- 0.27 kg (P < 0.001), while term placental weights were 263 +/- 16.8 and 438 +/- 44.6 g (P < 0.002). The number of fetal cotyledo ns per placenta and mean fetal cotyledon weight were significantly low er in RMG compared with NMG ewes (P < 0.05). Irrespective of treatment group, lamb birthweight was highly positively correlated with placent al weight and both parameters were negatively correlated with maternal liveweight gain during the first 100 days of gestation. The incidence of non-infectious spontaneous abortion at 125 +/- 1.3 days of gestati on was higher (P < 0.001) in the RMG (4 of 12) than in the NMG (1 of 1 2) group. Similarly, duration of gestation for those ewes delivering l ive young-was shorter (P < 0.01) in the RMG compared with the NMG grou p (140 +/- 0.94 versus 143 +/- 0.28 days). Colostrum yield at parturit ion was positively related to placental weight and significantly lower (P < 0.001) in the RMG than in the NMG group (35 +/- 12.1 and 247 +/- 36.2 g, respectively). Neonatal survival rates at 72 h after parturit ion were reduced (P < 0.05) in the RMG (38%) compared with the NMG gro up (91%). These data suggest that in rapidly growing adolescent ewes, the established anabolic drive to maternal tissue synthesis is maintai ned at the expense of the gradually evolving nutrient requirements of the gravid uterus. This results in a major restriction in placental gr owth and a highly significant decrease in birthweight.