Diet-animal fractionation of nitrogen stable isotopes reflects the efficiency of nitrogen assimilation in ruminants

The natural abundance of 15N in animal proteins (delta15Nanimal) is greater than that in the diet consumed by the animals (delta15Ndiet), with a discrimination factor (DELTA15N = delta15Nanimal - delta15Ndiet) that is known to vary according to nutritional conditions. The objectives of the present study were to test the hypothesis that DELTA15N variations depend on the efficiency of nitrogen utilisation (ENU) in growing beef cattle, and to identify some of the physiological mechanisms responsible for this N isotopic fractionation in ruminants. Thus, we performed the regression of the DELTA15N of plasma proteins obtained from thirty-five finishing beef cattle fed standard and non-conventional diets against different feed efficiency indices, including ENU. We also performed the regression of the DELTA15N of different ruminant N pools (plasma and milk proteins, urine and faeces) against different splanchnic N fluxes obtained from multi-catheterised lactating dairy cows. The DELTA15N of plasma proteins was negatively correlated with feed efficiency indices in beef cattle, especially ENU (body protein gain/N intake) and efficiency of metabolisable protein (MP) utilisation (body protein gain/MP intake). Although DELTA15N obtained from different N pools in dairy cows were all negatively correlated with ENU, the highest correlation was found when DELTA15N was calculated from plasma proteins. DELTA15N showed no correlation with urea-N recycling or rumen NH3 absorption, but exhibited a strong correlation with liver urea synthesis and splanchnic amino acid metabolism, which points to a dominant role of splanchnic tissues in the present N isotopic fractionation study.