DIETARY EXCESSES OF UREA INFLUENCE THE VIABILITY AND METABOLISM OF PREIMPLANTATION SHEEP EMBRYOS AND MAY AFFECT FETAL GROWTH AMONG SURVIVORS

In the first of two experiments investigating the effect of dietary ur ea on the survival and metabolism of ovine embryos, 30 Border Leiceste r x Scottish Blackface ewes received a maintenance diet (milled hay, m olasses, minerals, vitamins) with no urea (control, C; n = 10) or with added urea at 15 g (low urea, LU; n = 10) or 30 g (high urea, HU; n = 10) kg(-1) feed for a 12 week period. The degraded nitrogen (N) statu s relative to estimated rumen microbial N requirements was -2, +9 and +20 g per day, respectively. One week after allocation to diets, proge sterone priming (12 days) commenced. Ewes received 800 IU of equine ch orionic gonadotrophin at progesterone withdrawal, were inseminated 52 h later (Day 0) and embryos were collected from five ewes per group at Day 4 and from five ewes at Day 11. If available, one embryo was retu rned to each ewe; the rest were cultured in vitro. There was no effect of treatment on progesterone, luteinizing hormone (LH), or time of oe strus onset. C, LU and HU plasma urea (P < 0.001) and ammonia levels ( C vs. HU, P < 0.01; LU vs. HU, P < 0.05) differed. Day 4 HU embryos we re retarded relative to C and LU embryos. After 3 days of culture, 70% , 66% and 0% of C, LU and HU embryos, respectively, were viable. Mid-t erm pregnancy rates following transfer were 63%, 43% and 33%. Only one HU lamb (male) was born following embryo transfer; its birthweight (1 0.1 kg) exceeded that of its C (n = 3; 7.0, 7.0, 7.5 kg) and LU (n = 2 ; 7.3, 8.2 kg) counterparts (P < 0.025). In the second experiment, C2 (2.5 g urea kg(-1); n = 5) and HU2 (30 g kg(-1); n = 7) diets which pr ovided similar intakes of degraded N relative to microbial requirement s as those for C and HU ewes in Experiment 1 were fed to Border Leices ter x Scottish Blackface ewes superovulated with 16 mg of porcine foll icle-stimulating hormone. Urea and ammonia levels in utero-oviductal s amples were elevated in HU2 ewes (P < 0.05). At collection (Day 3), HU 2 embryos used more glucose (P < 0.01) and, following culture, some ex hibited up to a 2.8-fold increase in metabolism. In conclusion, excess numen degradable N in ewe diets elevates urea and ammonia in plasma a nd in utero, with an associated increase in embryo mortality. Neverthe less, metabolism appears to be up-regulated in some embryos and, among those that survive, fetal growth appears to be enhanced. (C) 1997 Els evier Science B.V.