Relationship between nitrogen isotopic discrimination and the proportion of dietary nitrogen excreted in urine by sheep offered different levels of dietary non-protein nitrogen.

Hassan Khanaki, RJ Dewhurst, Brian Leury, Gonzalo Cantalapiedra-Hijar, Long Cheng*

*Corresponding author for this work

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Abstract

Urinary nitrogen (N) excretion (UN) as a proportion of N intake (NI; UN/NI) is a major determinant of N excretion from ruminants and could be predicted from the N isotopic discrimination occurring between dietary and animal proteins (Δ15N). This study investigated the usefulness of Δ15N and other plasma biomarkers to reflect changes in UN/NI from sheep offered different levels of dietary urea. Eighteen Merino rams (age: 1-2 years old; live weight: 41 ± 3 kg) were allocated to three dietary N treatments for a N balance study. Treatments were control (C), control + 0.5% urea (C+0.5%), and control + 1.2% urea (C+1.2%) and designed to provide maintenance, maintenance plus an additional 15%, and maintenance plus an additional 33% NI, respectively. The urea effect term was used for one-way ANOVA and regression analysis. As NI increased, the UN and retained N (RN) increased linearly (P < 0.001) but, UN/NI only increased in treatment C+1.2% compared with C (P < 0.05). Plasma Δ15N was positively and significantly correlated with UN and UN/NI (r = 0.52; P = 0.028 and r = 0.68; P = 0.002, respectively) and increased linearly (P < 0.001) with the highest values observed in C+1.2%. Urine δ15N changed linearly between C and C+1.2%, but plasma δ15N increased quadratically (P <0.05). Plasma urea N increased in a linear way across dietary urea levels (P < 0.001). The N isotopic difference between plasma and urine (plasma δ15N ‐ urine δ15N) of C did not vary from either of the other treatments; however, it differed between C+0.5% and C+1.2% (P < 0.05). The study confirmed the potential usefulness of plasma Δ15N to estimate UN/NI from sheep. Moreover, plasma δ15N ‐ urine δ15N can be proposed as a new biomarker of N excretion from small ruminants. These approaches, however, need to be tested in various study conditions.
Original languageEnglish
Article number911673
JournalFrontiers in Animal Science
Volume3
Early online date4 Jul 2022
DOIs
Publication statusFirst published - 4 Jul 2022

Keywords

  • sheep production
  • Nitrogen utilization
  • nitrogen isotope
  • urinary nitrogen
  • urea supplementation

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