The efficacy of animal-based biomarkers for nitrogen partitioning in beef cattle : A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science (Honours) at Lincoln University

Abstract

Ruminants have been associated with a negative environmental footprint, including greenhouse gas emissions and nitrogen (N) leaching. Identifying animals more efficient in N metabolism presents as a possible mitigation strategy. However, individual N efficiency requires quantification of individual animal intake, which is impractical under grazing systems. Therefore, biomarkers should be used to estimate N intake. The objective of this study was to investigate the efficacy of multiple animal-based biomarkers for N partitioning in beef Hereford heifers. 32 heifers, fed lucerne (Medicago sativa) silage, were measured for liveweight gained from the start of the 14 days. On four nonsequential days during the second measurement week, spot samples of blood plasma, faeces, and urine were collected in the morning (0900 h) and afternoon (1400 h). Samples collected from this were bulked across the measurement period and further analysed using nitrogen isotopic fractionation. The difference between the 15/14N (‰) content of plasma and feed was expressed as Δ15N. Urine N excretion (g/d) was estimated using two different equations from Chizzotti and Pacheco, giving two divergent estimations of N partitioning and use efficiency. Plasma urea N exhibited a positive relationship with urine N excretion (g/l; r=0.52; P=0.003). Δ15N observed a positive correlation with both retained N efficiencies (RNE), RNEChizzotti (r=0.39; P=0.026) and RNEPacheco (r=0.50; P=0.004). Urinary creatinine concentrations (g/l) demonstrated a negative correlation with urine volume (l/d; r=-0.88; P<0.001), and a positive correlation with urine N concentration (g/l; r=0.88; P<0.001). Δ15N presented correlations contradicting predisposed beliefs, likely due to estimation errors in intake back-calculations. Plasma urea N presents as a useful biomarker for predicting urinary N excretion in ruminants; however, care must be taken to account for diurnal variation. Creatinine aligned with previous publications, further validating its use for predicting urine output in ruminats via spot sampling.