Plasma nitrogen isotopic fractionation and feed efficiency in growing beef heifers

Fractionation of N isotopes occurs in many metabolic reactions which causes tissue proteins to become enriched in N-15 while urea (urine) is depleted in N-15 relative to the diet. We investigated N-15 enrichment of whole plasma and its relationship with feed conversion efficiency (FCE) in growing beef heifers (n 84) offered 2kg/d of concentrates with grass silage ad libitum. Heifers were on average 299 (sd 48 center dot 3)d old and weighed 311 (sd 48 center dot 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 center dot 1 (sd 0 center dot 74)kg/d and concentrate DMI was 1 center dot 72kg/d. Mean mid-test live weight was 333 (sd 47 center dot 6)kg, daily gain was 0 center dot 53 (sd 0 center dot 183)kg, FCE (g live-weight gain/g DMI) was 0 center dot 09 (sd 0 center dot 028) and residual feed intake (RFI) was 0 (sd 0 center dot 428). N isotopic fractionation (Delta N-15; plasma delta N-15-diet delta N-15) averaged 3 center dot 58 parts per thousand on day 79 (n 84) and 3 center dot 90 parts per thousand for the subset of heifers. There was no relationship between Delta N-15 and RFI. Plasma delta N-15 and Delta N-15 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 Delta N-15 with FCE and animal weight are consistent with the anticipated negative relationship with N-use efficiency. There is potential to use Delta N-15 to provide rapid, low-cost estimates of FCE in cattle.