title: The effect of dietary water soluble carbohydrate to nitrogen ratio on nitrogen partitioning and isotopic fractionation of lactating goats offered a high-nitrogen diet.
abstract: The objective of this study was to investigate the relationship between nitrogen (N) partitioning and isotopic fractionation in lactating goats consuming diets with a constant high concentration of N and increasing levels of water soluble carbohydrate (WSC). Eight lactating goats were offered four different ratios of WSC : N in the diet. A two-period incomplete cross-over design was used, with two goats assigned to each treatment in each period. N balance measurements were conducted, with measurement of feed N intake and total output of N in milk, faeces and urine. Treatment, period and infusion effects were tested using general ANOVA; the relationships between variables were analysed by linear regression. Dietary treatment and period had significant effects on dry matter (DM) intake (g/day). DM digestibility (g/kg DM) and N digestibility (g/kg N) increased as the ratio of WSC : N increased in the diet. No treatment effect was observed on milk urea N concentration (g/l) or urinary excretion of purine derivatives (mM/day). Although dietary treatment and period had significant effects on N intake, the change of N intake was small; no effect was observed for N partitioning among faeces, milk and urine. Milk, plasma and faeces were enriched in 15N compared with feed, whilst urine was depleted in 15N relative to feed. No significant relationship was established between N partitioning and isotopic fractionation. This study failed to confirm the potential to use N isotopic fractionation as an indicator of N partitioning in dairy goats when diets provided N in excess to requirements, most likely because the range of milk N output/N intake and urinary N output/N intake were narrow.
authors.affiliation: 1Faculty of Agriculture and Life Sciences,Lincoln University,PO Box 85084,New Zealand.
authors.forename: L
authors.initials: L
authors.lastname: Cheng
authors.affiliation: 1Faculty of Agriculture and Life Sciences,Lincoln University,PO Box 85084,New Zealand.
authors.forename: G R
authors.initials: GR
authors.lastname: Edwards
authors.affiliation: 2Scotland's Rural College,West Mains Road,Edinburgh EH9 3JG,UK.
authors.forename: R J
authors.initials: RJ
authors.lastname: Dewhurst
authors.affiliation: 1Faculty of Agriculture and Life Sciences,Lincoln University,PO Box 85084,New Zealand.
authors.forename: A M
authors.initials: AM
authors.lastname: Nicol
authors.affiliation: 3AgResearch Grasslands Research Centre,Private Bag 11008,Palmerston North,New Zealand.
authors.forename: D
authors.initials: D
authors.lastname: Pacheco
mesh_terms: D000822:Animal Husbandry; D000818:Animals; D004032:Diet / Q000662:veterinary*; D004040:Dietary Carbohydrates / Q000378:metabolism*; D005243:Feces / Q000737:chemistry; D005260:Female; D006041:Goats / Q000378:metabolism*; D007774:Lactation; D008892:Milk / Q000737:chemistry; D009584:Nitrogen / Q000378:metabolism* / Q000652:urine; D009587:Nitrogen Isotopes / Q000378:metabolism / Q000652:urine
chemical_list: D004040:Dietary Carbohydrates; D009587:Nitrogen Isotopes; D009584:Nitrogen
keywords: dairy goats; nitrogen utilisation; stable isotopic discrimination; urea nitrogen