The Radiocarbon Laboratory of the Universidade Federal Fluminense, in Brazil, has been successfully applying the zinc reduction method for graphitization of carbon samples since the development of its early protocols in 2009. Successive methodological research aiming to improve and, ultimately, optimize the precision and accuracy of our results indicates that graphitization temperatures as low as 460°C promote erratic 13C isotopic fractionation, but an approximately constant fractionation of about –5‰ is achieved at 520°C. In this work, we present isotope ratio mass spectrometry (IRMS) δ13C results for 14C reference materials graphitized at 550°C with variable amounts of zinc. Based on the results obtained from the addition of 20, 35, and 50 mg of zinc, we conclude that a slightly lower variation in 13C isotope fractionation during graphitization is obtained with less zinc. Moreover, the average isotopic fractionation is not altered by increasing the graphitization temperature from 520°C to 550°C.

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.

Fractionation of N isotopes occurs in many metabolic reactions which causes tissue proteins to become enriched in 15N while urea (urine) is depleted in 15N relative to the diet. We investigated 15N enrichment of whole plasma and its relationship with feed conversion efficiency (FCE) in growing beef heifers (n 84) offered 2 kg/d of concentrates with grass silage ad libitum. Heifers were on average 299 (sd 48·3) d old and weighed 311 (sd 48·8) kg. Plasma was obtained on day 79 (n 84) of the experiment and from a subset of animals (n 20) on four occasions between days 16 and 79. Silage DM intake (DMI) averaged 4·1 (sd 0·74) kg/d and concentrate DMI was 1·72 kg/d. Mean mid-test live weight was 333 (sd 47·6) kg, daily gain was 0·53 (sd 0·183) kg, FCE (g live-weight gain/g DMI) was 0·09 (sd 0·028) and residual feed intake (RFI) was 0 (sd 0·428). N isotopic fractionation (Δ15N; plasma δ15N − diet δ15N) averaged 3·58 ‰ on day 79 (n 84) and 3·90 ‰ for the subset of heifers. There was no relationship between Δ15N and RFI. Plasma δ15N and Δ15N were related to both FCE (negative) and animal weight (positive) for the whole population, and repeatable for the subset of animals over four time points. These relationships of Δ15N with FCE and animal weight are consistent with the anticipated negative relationship with N-use efficiency. There is potential to use Δ15N to provide rapid, low-cost estimates of FCE in cattle.