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

The natural abundance of N-15 in animal proteins (delta N-15(animal)) is greater than that in the diet consumed by the animals (delta N-15(diet)), with a discrimination factor (Delta N-15 - delta N-15(animal) - delta N-15(diet)) that is known to vary according to nutritional conditions. The objectives of the present study were to test the hypothesis that Delta N-15 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 Delta N-15 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 Delta N-15 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 Delta N-15 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 Delta N-15 obtained from different N pools in dairy cows were all negatively correlated with ENU, the highest correlation was found when Delta N-15 was calculated from plasma proteins. Delta N-15 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.