The effect of sheep genetic merit and feed allowance on nitrogen partitioning and isotopic discrimination

Animal nitrogen (N) partitioning is a key parameter for profitability and sustainability of ruminant production systems, which may be predicted from N isotopic discrimination or fractionation (Delta N-15). Both animal genetics and feeding level may interact and impact on N partitioning. Therefore, this study aimed to assess the interactive effects of genetic merit (G) and feed allowance (F) on N partitioning and Delta N-15 in sheep. The sheep were drawn from two levels of G (high G vs. low G; based on New Zealand Sheep Improvement Limited (http://www.sil.co.nz/) dual (wool and meat) growth index) and allocated to two levels of F (1.7 (high F) vs. 1.1 (low F) times Metabolisable Energy requirement for maintenance) treatments. Twenty-four Coopworth rams were divided into four equal groups for a N balance study: high G x high F, high G x low F, low G x high F, and low G x low F. The main factors (G and F) and the inter-action term were used for 2-way ANOVA and regression analysis. Higher F led to higher N excretions (uri-nary N (UN); faecal N (FN); manure N), retained N, N use efficiency (NUE), and urinary purine derivatives excretion (P < 0.05). On the other hand, higher UN/N intake, and plasma Delta N-15 were observed with the lower F (P < 0.05). Higher G led to increased UN, FN, manure N, apparent N digestibility, and urinary pur-ine derivatives excretion (P < 0.05). Higher F only increased UN in high G sheep, with no effect on low G sheep (P < 0.05). Regression analysis results demonstrated potential to use plasma Delta N-15 to reflect the effects of G and F on NUE and UN/N intake. Further research is urged to study interactive effects of genetic and feeding level on sheep N partitioning. (C) 2021 The Author(s). Published by Elsevier B.V. on behalf of The Animal Consortium.

Automated summary

Animal nitrogen partitioning is influenced by genetic merit and feed allowance, with higher feed levels leading to increased nitrogen excretion and higher genetic merit resulting in elevated urinary nitrogen, faecal nitrogen, and manure nitrogen. However, higher feed only increased urinary nitrogen excretion in sheep with high genetic merit, with no effect observed in sheep with low genetic merit. Additionally, regression analysis suggests the potential use of plasma Delta N-15 to reflect the effects of genetic merit and feed allowance on nitrogen use efficiency and urinary nitrogen intake.