AUTHOR=Khanaki Hassan , Dewhurst Richard J. , Leury Brian J. , Cantalapiedra-Hijar Gonzalo , Cheng Long 

TITLE=Relationship Between Nitrogen Isotopic Discrimination and the Proportion of Dietary Nitrogen Excreted in Urine by Sheep Offered Different Levels of Dietary Non-Protein Nitrogen

JOURNAL=Frontiers in Animal Science

VOLUME=3

YEAR=2022

URL=https://www.frontiersin.org/journals/animal-science/articles/10.3389/fanim.2022.911673

DOI=10.3389/fanim.2022.911673

ISSN=2673-6225

ABSTRACT=<p>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 (Δ<sup>15</sup>N). This study investigated the usefulness of Δ<sup>15</sup>N 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; 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 (<italic>p</italic> &lt; 0.001), but UN/NI only increased in treatment C+1.2% compared with C (<italic>p</italic> &lt; 0.05). Plasma Δ<sup>15</sup>N was positively and significantly correlated with UN and UN/NI (<italic>r</italic> = 0.52, <italic>p</italic> = 0.028; and <italic>r</italic> = 0.68, <italic>p</italic> = 0.002, respectively) and increased linearly (<italic>p</italic> &lt; 0.001) with the highest values observed in C+1.2%. Urine δ<sup>15</sup>N changed linearly between C and C+1.2%, but plasma δ<sup>15</sup>N increased quadratically (<italic>p</italic> &lt; 0.05). Plasma urea N increased in a linear way across dietary urea levels (<italic>p</italic> &lt; 0.001). The N isotopic difference between plasma and urine (plasma δ<sup>15</sup>N–urine δ<sup>15</sup>N) of C did not vary from either of the other treatments; however, it differed between C+0.5% and C+1.2% (<italic>p</italic> &lt; 0.05). The study confirmed the potential usefulness of plasma Δ<sup>15</sup>N to estimate UN/NI from sheep. Moreover, plasma δ<sup>15</sup>N–urine δ<sup>15</sup>N can be proposed as a new biomarker of N excretion from small ruminants. These approaches, however, need to be tested in various study conditions.</p>