title: Dietary excesses of urea influence the viability and metabolism of preimplantation sheep embryos and may affect fetal growth among survivors.
abstract: In the first of two experiments investigating the effect of dietary urea on the survival and metabolism of ovine embryos, 30 Border Leicester x Scottish Blackface ewes received a maintenance diet (milled hay, molasses, 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) status relative to estimated rumen microbial N requirements was -2, +9 and +20 g per day, respectively. One week after allocation to diets, progesterone priming (12 days) commenced. Ewes received 800 IU of equine chorionic 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 returned to each ewe; the rest were cultured in vitro. There was no effect of treatment on progesterone, luteinizing hormone (LH), or time of oestrus 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 were 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-term pregnancy rates following transfer were 63%, 43% and 33%. Only one HU lamb (male) was born following embryo transfer, its birthweight (10.1 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 provided similar intakes of degraded N relative to microbial requirements as those for C and HU ewes in Experiment 1 were fed to Border Leicester x Scottish Blackface ewes superovulated with 16 mg of porcine follicle-stimulating hormone. Urea and ammonia levels in utero-oviductal samples were elevated in HU2 ewes (P < 0.05). At collection (Day 3), HU2 embryos used more glucose (P < 0.01) and, following culture, some exhibited up to a 2.8-fold increase in metabolism. In conclusion, excess rumen degradable N in ewe diets elevates urea and ammonia in plasma and in utero, with an associated increase in embryo mortality. Nevertheless, metabolism appears to be up-regulated in some embryos and, among those that survive, fetal growth appears to be enhanced.
authors.affiliation: Rowett Research Institute, Bucksburn, Aberdeen, UK.
authors.forename: T G
authors.initials: TG
authors.lastname: McEvoy
authors.forename: J J
authors.initials: JJ
authors.lastname: Robinson
authors.forename: R P
authors.initials: RP
authors.lastname: Aitken
authors.forename: P A
authors.initials: PA
authors.lastname: Findlay
authors.forename: I S
authors.initials: IS
authors.lastname: Robertson
mesh_terms: D000641:Ammonia / Q000032:analysis / Q000097:blood; D000818:Animals; D001755:Blastocyst / Q000378:metabolism / Q000502:physiology*; D004032:Diet / Q000662:veterinary*; D004624:Embryo Transfer / Q000662:veterinary; D005314:Embryonic and Fetal Development / Q000187:drug effects* / Q000502:physiology; D005260:Female; D005640:Follicle Stimulating Hormone / Q000494:pharmacology; D005947:Glucose / Q000378:metabolism; D007315:Insemination, Artificial / Q000379:methods / Q000662:veterinary; D007328:Insulin / Q000097:blood; D007986:Luteinizing Hormone / Q000097:blood; D008297:Male; D009584:Nitrogen / Q000378:metabolism; D010062:Ovulation Induction / Q000379:methods / Q000662:veterinary; D011247:Pregnancy; D018873:Pregnancy Rate; D011374:Progesterone / Q000097:blood; D011863:Radioimmunoassay / Q000662:veterinary; D012417:Rumen / Q000378:metabolism; D012756:Sheep / Q000097:blood / Q000196:embryology* / Q000502:physiology; D014508:Urea / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D007328:Insulin; D011374:Progesterone; D000641:Ammonia; D014508:Urea; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone; D005947:Glucose; D009584:Nitrogen