title: Plasma nitrogen isotopic fractionation and feed efficiency in growing beef heifers.
abstract: 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.
authors.affiliation: Animal and Grassland Research and Innovation Centre, Teagasc, Grange, Dunsany, County Meath, Republic of Ireland.
authors.forename: N M
authors.initials: NM
authors.lastname: Wheadon
authors.affiliation: Animal and Grassland Research and Innovation Centre, Teagasc, Grange, Dunsany, County Meath, Republic of Ireland.
authors.forename: M
authors.initials: M
authors.lastname: McGee
authors.affiliation: Faculty of Agriculture and Life Sciences, PO Box 84, Lincoln University, Lincoln, New Zealand.
authors.forename: G R
authors.initials: GR
authors.lastname: Edwards
authors.affiliation: Animal and Grassland Research and Innovation Centre, Teagasc, Grange, Dunsany, County Meath, Republic of Ireland.
authors.forename: R J
authors.initials: RJ
authors.lastname: Dewhurst
mesh_terms: D000465:Algorithms; D000818:Animals; D017849:Animals, Inbred Strains; D001835:Body Weight; D002417:Cattle / Q000254:growth & development*; D003433:Crosses, Genetic; D002149:Energy Intake; D005260:Female; D007494:Ireland; D013058:Mass Spectrometry; D008954:Models, Biological; D009584:Nitrogen / Q000032:analysis / Q000097:blood*; D009587:Nitrogen Isotopes; D006109:Poaceae / Q000737:chemistry; D015203:Reproducibility of Results; D012820:Silage / Q000032:analysis; D015430:Weight Gain
chemical_list: D009587:Nitrogen Isotopes; D009584:Nitrogen