LAUSR

Abstract Diet composition and intake are the main determinants of nitrogen (N) use efficiency (NUE) in beef cattle. Accounting for the interactions and comparative effects of different feedstuff types on NUE and N losses in urine and faeces can inform the development of financially and environmentally sustainable feeding protocols for beef cattle. This study aimed to assess the impact of various individual feedstuffs and feedstuff types/groups on NUE and N partitioning to faeces and urine in beef cattle, for diets with contrasting crude protein (CP) concentrations. Partial multivariate redundancy analysis (pRDA) was used to associate the influence of the intakes of individual feedstuffs and feedstuff types/groups on NUE and N partitioning by using results from 59 published trials with growing and finishing beef cattle. The data were split into three sub-sets, according to diet CP concentration (low CP, 47-120 g CP/kg DM, n = 73; medium CP, 121-150 g CP/kg DM, n = 90; high CP, 151-269 g CP/kg DM, n = 74). In low CP diets, the main feedstuffs that improved NUE and shifted N outputs from urine to faeces were grass and legume hay, grass hay, straws, brans and pulps. In medium CP diets, the main feedstuffs that improved NUE were fresh grass and hays, fresh legumes, and straws; while legume and grass hay, straws, pulps and hulls also shifted N excretion from urine to faeces. In high CP diets, the main feedstuffs that improved NUE were grass hay, grass silages, straws, fibre-rich by-products, hulls and meals; while grass silages, straws and meals also shifted N outputs from urine to faeces. The present study highlighted that selection of feedstuffs that provide adequate digestible fibre and energy supply to rumen microbes, as well as minimizing the oversupply of CP, could be used to improve NUE and shift N outputs from urine to faeces in beef cattle; while the effectiveness of providing fibre and energy might be influenced by the overall diet CP concentration.