Diet–animal fractionation of nitrogen stable isotopes reflects the efficiency of nitrogen assimilation in ruminants
Cantalapiedra-Hijar, G. | Ortigues-Marty, I. | Sepchat, B. | Agabriel, J. | Huneau, J. F. | Fouillet, H.
The natural abundance of ¹⁵N in animal proteins (δ¹⁵Nₐₙᵢₘₐₗ) is greater than that in the diet consumed by the animals (δ¹⁵Ndᵢₑₜ), with a discrimination factor (Δ¹⁵N = δ¹⁵Nₐₙᵢₘₐₗ− δ¹⁵Ndᵢₑₜ) that is known to vary according to nutritional conditions. The objectives of the present study were to test the hypothesis that Δ¹⁵N 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 Δ¹⁵N 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 Δ¹⁵N 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 Δ¹⁵N 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 Δ¹⁵N obtained from different N pools in dairy cows were all negatively correlated with ENU, the highest correlation was found when Δ¹⁵N was calculated from plasma proteins. Δ¹⁵N 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.
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