title: The effects of dietary nitrogen to water-soluble carbohydrate ratio on isotopic fractionation and partitioning of nitrogen in non-lactating sheep.
abstract: The main objective of this study was to investigate the relationship between partitioning and isotopic fractionation of nitrogen (N) in sheep consuming diets with varying ratios of N to water-soluble carbohydrate (WSC). Six non-lactating sheep were offered a constant dry matter (DM) allowance with one of three ratios of dietary N/WSC, achieved by adding sucrose and urea to lucerne pellets. A replicated 3 dietary treatments (Low, Medium and High N/WSC) × 3 (collection periods) and a Latin square design was used, with two sheep assigned to each treatment in each period. Feed, faeces, urine, plasma, wool, muscle and liver samples were collected and analysed for ¹⁵N concentration. Nitrogen intake and outputs in faeces and urine were measured for each sheep using 6-day total collections. Blood urea N (BUN) and urinary excretion of purine derivative were also measured. Treatment effects were tested using general ANOVA; the relationships between measured variables were analysed by linear regression. BUN and N intake increased by 46% and 35%, respectively, when N/WSC increased 2.5-fold. However, no indication of change in microbial protein synthesis was detected. Results indicated effects of dietary treatments on urinary N/faecal N, faecal N/N intake and retained N/N intake. In addition, the linear relationships between plasma δ¹⁵N and urinary N/N intake and muscle δ¹⁵N and retained N/N intake based on individual measurements showed the potential of using N isotopic fractionation as an easy-to-use indicator of N partitioning when N supply exceeds that required to match energy supply in the diet.
authors.affiliation: Faculty of Agriculture & Life Science, Lincoln University, PO Box 84, New Zealand. long.cheng@lincolnuni.ac.nz
authors.forename: L
authors.initials: L
authors.lastname: Cheng
authors.forename: A M
authors.initials: AM
authors.lastname: Nicol
authors.forename: R J
authors.initials: RJ
authors.lastname: Dewhurst
authors.forename: G R
authors.initials: GR
authors.lastname: Edwards
mesh_terms: D000704:Analysis of Variance; D000821:Animal Feed / Q000032:analysis; D000818:Animals; D001806:Blood Urea Nitrogen; D005591:Chemical Fractionation; D004040:Dietary Carbohydrates / Q000378:metabolism*; D005243:Feces / Q000737:chemistry; D008297:Male; D013058:Mass Spectrometry / Q000662:veterinary; D009584:Nitrogen / Q000378:metabolism* / Q000652:urine; D009587:Nitrogen Isotopes / Q000378:metabolism / Q000652:urine; D034561:Sheep, Domestic / Q000378:metabolism*; D013395:Sucrose / Q000378:metabolism*; D014508:Urea / Q000378:metabolism*
chemical_list: D004040:Dietary Carbohydrates; D009587:Nitrogen Isotopes; D013395:Sucrose; D014508:Urea; D009584:Nitrogen